Rho family of GTPases
Encyclopedia
The Rho family of GTPases is a family of small (~21 kDa) signaling G protein
(more specific, a GTPase
), and is a subfamily of the Ras superfamily
. The members of the Rho GTPase family have been shown to regulate many aspects of intracellular actin
dynamics, and are found in all eukaryotic organisms including yeasts and some plants. Three members of the family have been studied a great deal: Cdc42
, Rac1
, and RhoA
. Rho proteins have been described as "molecular switches" and play a role in cell proliferation, apoptosis
, gene expression
, and multiple other common cellular functions.
As early as 1990, Paterson et al. began injecting active rho protein into Swiss 3T3 cells..
In the 2006 review article released by Bement et al., the spatial zones of rho activation were explained.
As early as the mid-1990s, these processes and the effects of the rho proteins were observed in fibroblasts. Dr. Alan Hall, one of the front-runners in rho protein research, compiled evidence in his 1998 review, which showed that it is not only fibroblasts that formed processes based on rho activation, but virtually all eukaryotic cells.
, RhoB
, and RhoC
), and sometimes refers to all members of the family. This article is about the family as a whole.
In mammals, the Rho family contains 20 members. Almost all research involves the three most common members of the Rho family: Cdc42, Rac1 and RhoA.
The current 20 members of the Rho family include RhoA, RhoB, RhoC, RhoD, Rif, Rnd1, Rnd2, Rnd3/RhoE, RhoH/TTF, Rac1, Rac2, Rac3, RhoG, Cdc42, TC10 (RhoQ), TCL (RhoJ), Wrch1 (RhoV), Chp/Wrch2 (RhoU), RhoBTB1, RhoBTB2.
, GTPase-activating proteins (GAPs) and guanine nucleotide dissociation inhibitors (GDIs)
. GEFs control the release of GDP from the rho protein and the replacement with GTP
. GAPs control the ability of the GTPase to hydrolyze GTP to GDP
, controlling the natural rate of movement from the active conformation to the inactive conformation. GDI proteins form a large complex with the rho protein helping to prevent diffusion within the membrane and into the cytosol, thus acting as an anchor and allowing for very specific spatial control of rho activation.
One example of Rho GTPases' role in cell polarity is seen in the much-studied yeast cell. Before the cell can bud, Cdc42 is used to locate the region of the cell’s membrane which will begin to bulge into the new cell. When Cdc42 is removed from the cell, the cell’s outgrowths still form but form in an unorganized manner.
One of the most obvious changes to cell morphology controlled by rho proteins is the formation of lamellipodia
and filopodia
, the processes that look like fingers or feet, which often propel cells across surfaces. Fibroblasts form processes based on rho activation, but also virtually all eukaryotic cells do so as well.
s. The proteins is made to be constitutively active using recombinant techniques. In essence, by changing one codon of the protein’s DNA
, one amino acid is changed, and, therefore, the conformation of the entire protein is altered into one that resembles the GTP-bound state. After injection into the 3T3 cells, morphological changes ensue - contractions and filopodia
.
Because rho proteins are G-proteins and plasma-membrane-bound, their location can be easily controlled. In each situation, whether it be wound-healing, cytokinesis
, or budding
, the location of the rho activation can be imaged and identified. For example, if a circular hole is inflicted in a spherical cell, Cdc42
and other active rhos are seen in highest concentration around the circumference of the circular injury. One methods of maintaining the spatial zones of activation is, e.g., through anchoring to the actin cytoskeleton, keeping the membrane-bound protein from diffusing away from the region where it is most needed. Another method of maintenance is through the formation of a large complex which is resistant to diffusion and more rigidly bound to the membrane than the rho itself .
that form during axon
al generation and re-generation in the nervous system. Some consider rho proteins to be a potential target for delivery into spinal cord lesions after traumatic injury. Following injury to the spinal cord, the extracellular space becomes inhibitory to the natural efforts neurons undergo to regenerate.
These ‘natural efforts’ include the formation of a growth cone at the proximal end of an injured axon. Newly-formed growth cones subsequently attempt to ‘crawl’ across the lesion and are quite sensitive to chemical cues in the extracellular environment. One of the many inhibitory cues includes chondroitin sulfate proteoglycans
or CSPGs. Neurons growing in culture increase in their ability to cross over inhibitory CSPG lanes after administration of constitutively-active Cdc42, Rac1 and RhoA. This is partly due to the exogenous rho proteins driving cellular locomotion despite the extracellular cues promoting apoptosis and growth cone collapse. It is situations like these that make intracellular modulation of rho proteins the subject of a significant amount of spinal cord research.
. Mental retardation occurs in approximately 3% of the population and is characterized by having an IQ
of less than 70. Multiple sources have noticed that mental retardation in some cases shows malformation of the dendritic spines
, which form the post-synaptic connections between neuron
s. As expected, the misshapen dendritic spines are sometimes the result of rho protein-signaling modulation. After cloning of various genes implicated in X-linked
mental retardation, three genes that have effects on rho signaling were identified, including oligophrenin-1 (GAP protein that stimulates GTPase activity of Rac1, Cdc42, and RhoA), PAK3 (involved with the effects of Rac and Cdc42 on the actin cytoskeleton) and αPIX (a GEF that helps activate Rac1 and Cdc42). Because of the effect of rho signaling on the actin cytoskeleton, genetic malfunctions of a rho protein could explain the irregular morphology of neuronal dendrites seen in many cases of mental retardation.
One way to explain altered signaling in the absence of mutation is through increased expression. Overexpression of RhoA, RhoB, RhoC, Rac1, Rac2, Rac3, RhoE, RhoG, RhoH, and Cdc42 has been shown in multiple types of cancer. This increased presence of so many signaling molecules implies that these proteins promote the cellular functions that become overly active in cancerous cells.
A second target to explain the role of the rho proteins in cancer is their regulatory proteins. Rho proteins are very tightly controlled by a wide variety of sources, and over 60 activators and 70 inactivators have been identified. Multiple GAPs, GDIs, and GEFs have been shown to undergo overexpression, downregulation, or mutation in different types of cancer. As one can imagine, once an upstream signal is changed, the activity of its targets downstream, i.e. the rho proteins, will change in activity.
Ellenbroek et al. outlined a number of different effects of rho activation in cancerous cells. First, in the initiation of the tumor modification of rho activity can suppress apoptosis
and therefore contribute to artificial cell longevity. After natural apoptosis is suppressed, abnormal tumor growth can be observed through the loss of polarity in which rho proteins play an integral role. Next, the growing mass can invade across its normal boundaries through the alteration of adhesion proteins potentially caused by rho proteins. Finally, after inhibition of apoptosis, cell polarity and adhesion molecules, the cancerous mass is free to metastasize
and spread to other regions of the body.
G protein
G proteins are a family of proteins involved in transmitting chemical signals outside the cell, and causing changes inside the cell. They communicate signals from many hormones, neurotransmitters, and other signaling factors. G protein-coupled receptors are transmembrane receptors...
(more specific, a GTPase
GTPase
GTPases are a large family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate . The GTP binding and hydrolysis takes place in the highly conserved G domain common to all GTPases.-Functions:...
), and is a subfamily of the Ras superfamily
Ras superfamily
The Ras superfamily is a protein superfamily of small GTPases, which are all related, to a degree, to the Ras protein subfamily .There are more than a hundred proteins in the Ras superfamily...
. The members of the Rho GTPase family have been shown to regulate many aspects of intracellular actin
Actin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
dynamics, and are found in all eukaryotic organisms including yeasts and some plants. Three members of the family have been studied a great deal: Cdc42
CDC42
Cell division control protein 42 homolog also known as CDC42 is a protein involved in regulation of the cell cycle. In humans, CDC42 is encoded by the CDC42 gene.- Function :...
, Rac1
RAC1
Ras-related C3 botulinum toxin substrate 1 also known as Rac1 is a protein that in humans is encoded by the RAC1 gene. Several alternatively spliced transcript variants of this gene have been described, but the full-length nature of some of these variants has not been determined.- Function :Rac1 is...
, and RhoA
RHOA
Ras homolog gene family, member A is a small GTPase protein known to regulate the actin cytoskeleton in the formation of stress fibers. In humans, it is encoded by the gene RHOA....
. Rho proteins have been described as "molecular switches" and play a role in cell proliferation, apoptosis
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
, gene expression
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
, and multiple other common cellular functions.
History
Identification of the Rho family of GTPases began in the mid 1980s. The first identified Rho member was RhoA, isolated serendipitously in 1985 from a low stringency cDNA screening. Rac1 and Rac2 were next identified in 1989 followed by Cdc42 in 1990 . Eight additional mammalian Rho members were identified from biological screenings until the late 1990s, a turning point in biology where availability of complete genome sequences allowed full identification of gene families. In mammals, the Rho family is thus made of 20 members distributed into eight subfamilies : Rho, Rnd, RhoD/F, RhoH, Rac, Cdc42, RhoU/V and RhoBTB .As early as 1990, Paterson et al. began injecting active rho protein into Swiss 3T3 cells..
In the 2006 review article released by Bement et al., the spatial zones of rho activation were explained.
As early as the mid-1990s, these processes and the effects of the rho proteins were observed in fibroblasts. Dr. Alan Hall, one of the front-runners in rho protein research, compiled evidence in his 1998 review, which showed that it is not only fibroblasts that formed processes based on rho activation, but virtually all eukaryotic cells.
Categorization
The Rho family of GTPases belong to the superfamily named ‘Ras-like’ proteins, which consists of over 150 varieties in mammals. Rho proteins sometimes denote some members of the Rho family (RhoARHOA
Ras homolog gene family, member A is a small GTPase protein known to regulate the actin cytoskeleton in the formation of stress fibers. In humans, it is encoded by the gene RHOA....
, RhoB
RHOB
Ras homolog gene family, member B, also known as RHOB, is a protein which in humans is encoded by the RHOB gene.RHOB is a member of the Rho GTP-binding protein family.-Interactions:...
, and RhoC
RhoC
RhoC is a small signaling G protein , and is a member of the Rac subfamily of the family Rho family of GTPases. It is encoded by the gene RHOC. -Mechanism and function:...
), and sometimes refers to all members of the family. This article is about the family as a whole.
In mammals, the Rho family contains 20 members. Almost all research involves the three most common members of the Rho family: Cdc42, Rac1 and RhoA.
| Rho family member | Action on actin filaments |
|---|---|
| Cdc42 CDC42 Cell division control protein 42 homolog also known as CDC42 is a protein involved in regulation of the cell cycle. In humans, CDC42 is encoded by the CDC42 gene.- Function :... |
affects filopodia Filopodia Filopodia are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. They contain actin filaments cross-linked into bundles by actin-binding proteins, e.g. fascin and fimbrin. Filopodia form focal adhesions with the substratum, linking it to the... |
| Rac1 RAC1 Ras-related C3 botulinum toxin substrate 1 also known as Rac1 is a protein that in humans is encoded by the RAC1 gene. Several alternatively spliced transcript variants of this gene have been described, but the full-length nature of some of these variants has not been determined.- Function :Rac1 is... |
affects lamellipodia Lamellipodia The lamellipodium is a cytoskeletal protein actin projection on the mobile edge of the cell. It contains a quasi-two-dimensional actin mesh; the whole structure propels the cell across a substrate... |
| RhoA RHOA Ras homolog gene family, member A is a small GTPase protein known to regulate the actin cytoskeleton in the formation of stress fibers. In humans, it is encoded by the gene RHOA.... |
affects stress fibres |
The current 20 members of the Rho family include RhoA, RhoB, RhoC, RhoD, Rif, Rnd1, Rnd2, Rnd3/RhoE, RhoH/TTF, Rac1, Rac2, Rac3, RhoG, Cdc42, TC10 (RhoQ), TCL (RhoJ), Wrch1 (RhoV), Chp/Wrch2 (RhoU), RhoBTB1, RhoBTB2.
| Subclass | Cytoskeletal effect | Rho family members |
|---|---|---|
| Cdc42 subclass | filopodia Filopodia Filopodia are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. They contain actin filaments cross-linked into bundles by actin-binding proteins, e.g. fascin and fimbrin. Filopodia form focal adhesions with the substratum, linking it to the... |
Cdc42 CDC42 Cell division control protein 42 homolog also known as CDC42 is a protein involved in regulation of the cell cycle. In humans, CDC42 is encoded by the CDC42 gene.- Function :... |
| TC10 TC10 protein TC10 is a small signaling G protein , and is a member of the Rho family of GTPases.Further reading: Rho family of GTPases... |
||
| TCL TCL (GTPase) TCL is a small signaling G protein , and is a member of the Rho family of GTPases.,.TCL shares 85% and 78% amino acid similarity to TC10 and Cdc42, respectively. TCL mRNA is 2.5 kb long and is mainly expressed in heart. In vitro, TCL shows rapid GDP/GTP exchange and displays higher GTP... |
||
| RhoUV subclass | filopodia Filopodia Filopodia are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. They contain actin filaments cross-linked into bundles by actin-binding proteins, e.g. fascin and fimbrin. Filopodia form focal adhesions with the substratum, linking it to the... and lamellipodia Lamellipodia The lamellipodium is a cytoskeletal protein actin projection on the mobile edge of the cell. It contains a quasi-two-dimensional actin mesh; the whole structure propels the cell across a substrate... |
RhoU Wrch1 RhoU is a small signaling G protein , and is a member of the Rho family of GTPases.Wrch1 was identified in 2001 as encoded by a non-canonical Wnt induced gene .... (Wrch) |
| RhoV Chp (GTPase) RhoV is a small signaling G protein , and is a member of the Rho family of GTPases.Chp was identified in 1998 as a GTPase interacting with the p21 activated kinase PAK2 .... (Chp) |
||
| Rac Rac (GTPase) Rac is a subfamily of the Rho family of GTPases, small signaling G proteins .The subgroup include:*Rac1*Rac2*Rac3*RhoG... |
lamellipodia Lamellipodia The lamellipodium is a cytoskeletal protein actin projection on the mobile edge of the cell. It contains a quasi-two-dimensional actin mesh; the whole structure propels the cell across a substrate... |
Rac1 RAC1 Ras-related C3 botulinum toxin substrate 1 also known as Rac1 is a protein that in humans is encoded by the RAC1 gene. Several alternatively spliced transcript variants of this gene have been described, but the full-length nature of some of these variants has not been determined.- Function :Rac1 is... |
| Rac2 Rac2 Rac2 is a small signaling G protein , and is a member of the Rac subfamily of the family Rho family of GTPases... |
||
| Rac3 Rac3 Rac3 is a small monomeric GTP-binding protein G protein and is an important component of intracellular signalling pathways. It is a member of the Rac subfamily of the Rho family of small G proteins and is encoded by the gene RAC3.... |
||
| RhoG RhoG RhoG is a small monomeric GTP-binding protein , and is an important component of many intracellular signalling pathways... |
||
| RhoBTB RhoBTB The RhoBTB family is a subgroup of the Rho family of small GTPases. They are a highly divergent class and are all characterized by an N-terminal Rho-related domain followed by at least one C-terminal BTB domain.- Discovery :... |
protein stability | RhoBTB1 RHOBTB1 Rho-related BTB domain-containing protein 1 is a protein that in humans is encoded by the RHOBTB1 gene.-Further reading:... |
| RhoBTB2 RHOBTB2 Rho-related BTB domain-containing protein 2 is a protein that in humans is encoded by the RHOBTB2 gene.-Further reading:... |
||
| RhoBTB3 RHOBTB3 Rho-related BTB domain-containing protein 3 is a protein that in humans is encoded by the RHOBTB3 gene.-Further reading:... |
||
| RhoH RhoH RhoH is a small signaling G protein , and is a member of the Rac subfamily of the family Rho family of GTPases. It is encoded by the gene RHOH.-Gene:... |
Rac agonist? | RhoH RhoH RhoH is a small signaling G protein , and is a member of the Rac subfamily of the family Rho family of GTPases. It is encoded by the gene RHOH.-Gene:... |
| Rho (subclass) Rho (subclass) Rho as a subclass may refer to:*Rho family of GTPases*RhoA*RhoB*RhoC... |
↑stress fibres and ↑focal adhesion Focal adhesion In cell biology, focal adhesions are specific types of large macromolecular assemblies through which both mechanical force and regulatory signals are transmitted. More precisely, they can be considered as sub-cellular macromolecules that mediate the regulatory effects In cell biology, focal... s |
RhoA RHOA Ras homolog gene family, member A is a small GTPase protein known to regulate the actin cytoskeleton in the formation of stress fibers. In humans, it is encoded by the gene RHOA.... |
| RhoB RHOB Ras homolog gene family, member B, also known as RHOB, is a protein which in humans is encoded by the RHOB gene.RHOB is a member of the Rho GTP-binding protein family.-Interactions:... |
||
| RhoC RhoC RhoC is a small signaling G protein , and is a member of the Rac subfamily of the family Rho family of GTPases. It is encoded by the gene RHOC. -Mechanism and function:... |
||
| Rnd Rnd (GTPase) Rnd is a subclass of the Rho family of GTPases, and includes:*Rnd1*Rnd2*Rnd3 Functions include downregulation of stress fibres and focal adhesions.... |
↓stress fibres and ↓focal adhesion Focal adhesion In cell biology, focal adhesions are specific types of large macromolecular assemblies through which both mechanical force and regulatory signals are transmitted. More precisely, they can be considered as sub-cellular macromolecules that mediate the regulatory effects In cell biology, focal... s |
Rnd1 Rnd1 Rnd1 is a small signaling G protein , and is a member of the Rnd subgroup of the Rho family of GTPases.. It is encoded by the gene RND1.... |
| Rnd2 Rnd2 Rnd2 is a small signaling G protein , and is a member of the Rnd subgroup of the Rho family of GTPases.. It is encoded by the gene RND2.... |
||
| Rnd3 Rnd3 Rnd3 is a small signaling G protein , and is a member of the Rnd subgroup of the Rho family of GTPases. It is encoded by the gene RND3.... (RhoE) |
||
| RhoF Rif (GTPase) Rif is a small signaling G protein , and is a member of the Rho family of GTPases.Further reading: Rho family of GTPases... |
Vesicle transport, filopodia Filopodia Filopodia are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. They contain actin filaments cross-linked into bundles by actin-binding proteins, e.g. fascin and fimbrin. Filopodia form focal adhesions with the substratum, linking it to the... |
RhoD RhoD RhoD is a small signaling G protein , and is a member of the Rac subfamily of the family Rho family of GTPases. It is encoded by the gene RHOD... |
| RhoF Rif (GTPase) Rif is a small signaling G protein , and is a member of the Rho family of GTPases.Further reading: Rho family of GTPases... (Rif) |
Regulators
Three general classes of regulators of rho protein signaling have been identified: guanine nucleotide exchange factor (GEFs)Guanine nucleotide exchange factor
Guanine nucleotide exchange factors activate monomeric GTPases by stimulating the release of guanosine diphosphate to allow binding of guanosine triphosphate . A variety of unrelated structural domains have been shown to exhibit guanine nucleotide exchange activity...
, GTPase-activating proteins (GAPs) and guanine nucleotide dissociation inhibitors (GDIs)
Guanosine nucleotide dissociation inhibitors
The Guanosine nucleotide dissociation inhibitors , bind to the GDP-bound form of Rho and Rab small GTPases and not only prevent exchange , but also prevent the small GTPase from localizing at the membrane, which is their place of action. This inhibition can be removed by the action of a Guanine...
. GEFs control the release of GDP from the rho protein and the replacement with GTP
Guanosine triphosphate
Guanosine-5'-triphosphate is a purine nucleoside triphosphate. It can act as a substrate for the synthesis of RNA during the transcription process...
. GAPs control the ability of the GTPase to hydrolyze GTP to GDP
Guanosine diphosphate
Guanosine diphosphate, abbreviated GDP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside guanosine. GDP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase guanine....
, controlling the natural rate of movement from the active conformation to the inactive conformation. GDI proteins form a large complex with the rho protein helping to prevent diffusion within the membrane and into the cytosol, thus acting as an anchor and allowing for very specific spatial control of rho activation.
Effectors
Each Rho protein affects numerous proteins downstream, all of which have roles in various cell processes. In fact, over 60 targets of the three common Rho GTPases have been found. Two molecules that directly stimulate actin polymerization are the Arp2/3 proteins and the Diaphanous-related formins.| GTPase | Effector |
|---|---|
| RhoA RHOA Ras homolog gene family, member A is a small GTPase protein known to regulate the actin cytoskeleton in the formation of stress fibers. In humans, it is encoded by the gene RHOA.... |
Cit CIT (gene) Citron Rho-interacting kinase is an enzyme that in humans is encoded by the CIT gene.-Interactions:CIT has been shown to interact with RHOB and RHOA.-Further reading:... , Cnksr1 CNKSR1 Connector enhancer of kinase suppressor of ras 1 is an enzyme that in humans is encoded by the CNKSR1 gene.-Interactions:CNKSR1 has been shown to interact with RhoD and RASSF1.-Further reading:... , Diaph1 DIAPH1 Protein diaphanous homolog 1 is a protein that in humans is encoded by the DIAPH1 gene.- Function :This gene is a homolog of the Drosophila diaphanous gene and belongs to the protein family of the formins, characterized by the formin homology 2 domain... , Diaph2 DIAPH2 Protein diaphanous homolog 2 is a protein that in humans is encoded by the DIAPH2 gene.-Further reading:... , DgkQ DGKQ Diacylglycerol kinase theta is an enzyme that in humans is encoded by the DGKQ gene.-Further reading:... , FlnA FLNA Filamin-A is a protein that in humans is encoded by the FLNA gene.- Function :Actin-binding protein, or filamin, is a 280-kD protein that crosslinks actin filaments into orthogonal networks in cortical cytoplasm and participates in the anchoring of membrane proteins for the actin cytoskeleton.... , KcnA2 KCNA2 Potassium voltage-gated channel subfamily A member 2 also known as Kv1.2 is a protein that in humans is encoded by the KCNA2 gene.-Interactions:KCNA2 has been shown to interact with KCNA4, DLG4, PTPRA, KCNAB2, RHOA and Cortactin.... , Ktn1 KTN1 Kinectin is a protein that in humans is encoded by the KTN1 gene.-Interactions:KTN1 has been shown to interact with EEF1D, RhoG and RHOA.-Further reading:... , Rtkn1, Rtkn2, Rhpn1 RHPN1 Rhophilin-1 is a protein that in humans is encoded by the RHPN1 gene.-Further reading:... , Rhpn2 RHPN2 Rhophilin-2 is a protein that in humans is encoded by the RHPN2 gene.-Further reading:... , Itpr1 ITPR1 Inositol 1,4,5-trisphosphate receptor type 1 is a protein that in humans is encoded by the ITPR1 gene.-Interactions:ITPR1 has been shown to interact with CA8, PRKG1, FKBP1A, MRVI1, RHOA, AHCYL1, EPB41L1 and TRPC4.-External Links:*... , PlcG1 PLCG1 Phospholipase C, gamma 1, also known as PLCG1, is a protein that in humans is encoded by the PLCG1 gene.-Function:The protein encoded by this gene catalyzes the formation of inositol 1,4,5-trisphosphate and diacylglycerol from phosphatidylinositol 4,5-bisphosphate... , PI-5-p5K, Pld1, Pkn1, Pkn2 PKN2 Serine/threonine-protein kinase N2 is an enzyme that in humans is encoded by the PKN2 gene.-Interactions:PKN2 has been shown to interact with Phosphoinositide-dependent kinase-1, AKT1, NCK1, PTPN13 and RHOA.-Further reading:... , Rock1 ROCK1 Rho-associated, coiled-coil containing protein kinase 1, also known as ROCK1, is a protein serine/threonine kinase, but also denotes the human gene encoding it.... , Rock2, PrkcA, Ppp1r12A PPP1R12A Protein phosphatase 1 regulatory subunit 12A is an enzyme that in humans is encoded by the PPP1R12A gene.-Interactions:PPP1R12A has been shown to interact with Interleukin 16.-External links:* Info with links in the... |
| Rac1 RAC1 Ras-related C3 botulinum toxin substrate 1 also known as Rac1 is a protein that in humans is encoded by the RAC1 gene. Several alternatively spliced transcript variants of this gene have been described, but the full-length nature of some of these variants has not been determined.- Function :Rac1 is... |
Sra1 SRA1 Steroid receptor RNA activator 1 also known as steroid receptor RNA activator protein is a protein that in humans is encoded by the SRA1 gene. The mRNA transcribed from the SRA1 gene is a component of the ribonucleoprotein complex containing NCOA1... , IRSp53, PAK1 PAK1 Serine/threonine-protein kinase PAK 1 is an enzyme that in humans is encoded by the PAK1 gene.-Interactions:PAK1 has been shown to interact with C-Raf, NCK1, LIMK1, RAC1, ARHGEF2, BMX, ARPC1B, DYNLL1, Cyclin-dependent kinase 5, PAK1IP1 and CDC42.... , PAK2 PAK2 Serine/threonine-protein kinase PAK 2 is an enzyme that in humans is encoded by the PAK2 gene.-Interactions:PAK2 has been shown to interact with SH3KBP1, CDC42 and Abl gene.-External links:* Info with links in the... , PAK3 PAK3 Serine/threonine-protein kinase PAK 3 is an enzyme that in humans is encoded by the PAK3 gene.-External links:* Info with links in the... |
| Cdc42 CDC42 Cell division control protein 42 homolog also known as CDC42 is a protein involved in regulation of the cell cycle. In humans, CDC42 is encoded by the CDC42 gene.- Function :... |
Wiskott-Aldrich syndrome protein Wiskott-Aldrich syndrome protein The Wiskott–Aldrich Syndrome Protein is a 502-amino acid protein that is expressed in cells of the hematopoietic system. In the inactive state, WASp exists in an auto-inhibited conformation with sequences near its C-terminus binding to a region near its N-terminus... , N-WASP, IRSp53, Dia2, Dia3, ROCK1 ROCK1 Rho-associated, coiled-coil containing protein kinase 1, also known as ROCK1, is a protein serine/threonine kinase, but also denotes the human gene encoding it.... , ROCK2 |
Functions
Rho/Rac proteins are involved in a wide variety of cellular functions such as cell polarity, vesicular trafficking, the cell cycle and transcriptomal dynamics .Morphology
Animal cells form many different shapes based on their function and location in the body. Rho proteins help cells regulate changes in shape throughout their life-cycle. Before cells can undergo key processes such as budding, mitosis, or locomotion, a certain degree of polarity is required. A ‘polar’ cell is one that has some sort of shape or direction rather than existing as an amorphous, symmetrical shape. For instance, an amoeba becomes polar when it undergoes locomotion and travels from one point to another.One example of Rho GTPases' role in cell polarity is seen in the much-studied yeast cell. Before the cell can bud, Cdc42 is used to locate the region of the cell’s membrane which will begin to bulge into the new cell. When Cdc42 is removed from the cell, the cell’s outgrowths still form but form in an unorganized manner.
One of the most obvious changes to cell morphology controlled by rho proteins is the formation of lamellipodia
Lamellipodia
The lamellipodium is a cytoskeletal protein actin projection on the mobile edge of the cell. It contains a quasi-two-dimensional actin mesh; the whole structure propels the cell across a substrate...
and filopodia
Filopodia
Filopodia are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. They contain actin filaments cross-linked into bundles by actin-binding proteins, e.g. fascin and fimbrin. Filopodia form focal adhesions with the substratum, linking it to the...
, the processes that look like fingers or feet, which often propel cells across surfaces. Fibroblasts form processes based on rho activation, but also virtually all eukaryotic cells do so as well.
Study techniques
Much of what is known about cellular morphology changes, and the effects of Rho proteins comes from the creation of a constitutively-active mutation of the protein, e.g., by injecting active rho protein into Swiss 3T3 cell3T3 cells
3T3 cells come from a cell line established in 1962 by two scientists then at the Department of Pathology in the New York University School of Medicine, George Todaro and Howard Green. The 3T3 cell line has become the standard fibroblast cell line...
s. The proteins is made to be constitutively active using recombinant techniques. In essence, by changing one codon of the protein’s DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
, one amino acid is changed, and, therefore, the conformation of the entire protein is altered into one that resembles the GTP-bound state. After injection into the 3T3 cells, morphological changes ensue - contractions and filopodia
Filopodia
Filopodia are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. They contain actin filaments cross-linked into bundles by actin-binding proteins, e.g. fascin and fimbrin. Filopodia form focal adhesions with the substratum, linking it to the...
.
Because rho proteins are G-proteins and plasma-membrane-bound, their location can be easily controlled. In each situation, whether it be wound-healing, cytokinesis
Cytokinesis
Cytokinesis is the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation...
, or budding
Budding
Budding is a form of asexual reproduction in which a new organism grows on another one. The new organism remains attached as it grows, separating from the parent organism only when it is mature. Since the reproduction is asexual, the newly created organism is a clone and is genetically identical...
, the location of the rho activation can be imaged and identified. For example, if a circular hole is inflicted in a spherical cell, Cdc42
CDC42
Cell division control protein 42 homolog also known as CDC42 is a protein involved in regulation of the cell cycle. In humans, CDC42 is encoded by the CDC42 gene.- Function :...
and other active rhos are seen in highest concentration around the circumference of the circular injury. One methods of maintaining the spatial zones of activation is, e.g., through anchoring to the actin cytoskeleton, keeping the membrane-bound protein from diffusing away from the region where it is most needed. Another method of maintenance is through the formation of a large complex which is resistant to diffusion and more rigidly bound to the membrane than the rho itself .
Movement
In addition to the formation of lamellipodia and filopodia, it has been shown that intracellular concentration and cross-talk between different rho proteins drives the extensions and contractions that cause cellular locomotion. Sakumura et al. proposed a model based on differential equations, which helps explain the activity of rhos and their relationship to motion. This model encompassed the three proteins Cdc42, RhoA, and Rac. Cdc42 was assumed to encourage filopodia elongation and block actin depolymerization. RhoA was considered to encourage actin retraction. Rac was treated to encourage lamellipodia exentsion but block actin depolymerization. These three proteins, although significantly simplified, covered the key steps in cellular locomotion. Through various mathematical techniques, solutions to the differential equations that described various regions of activity based on intracellular activity were found. The paper concludes by showing that the model predicts that there are a few threshold concentrations that cause interesting effects on the activity of the cell. Below a certain concentration, there is very little activity, causing no extension of the arms and feet of the cell. Above a certain concentration, the rho protein causes a sinusoidal oscillation to occur, much like the extensions and contractions of the lamellipodia and filopodia. In essence, this model predicts that increasing the intracellular concentration of these three key active rho proteins causes an out-of-phase activity of the cell, resulting in extensions and contractions that are also out of phase.Wound healing
One example of behavior that is modulated by Rho GTPase proteins is in the healing of wounds. Wounds heal differently between young chicks and adult chickens. In young chicks, wounds heal by contraction, much like a draw-string being pulled to close a bag. In older chickens, cells crawl across the wound through locomotion. The actin formation required to close the wounds in young chicks is controlled by Rho GTPase proteins, since, after injection of a bacterial exoenzyme used to block rho and rac activity, the actin polymers do not form, and thus the healing completely fails.Phagocytosis
Another cellular behavior that is affected by rho proteins is phagocytosis. As with most other types of cell membrane modulation, phagocytosis requires the actin cytoskeleton in order to engulf other items. The actin filaments control the formation of the phagocytic cup, and active Rac1 and Cdc42 have been implicated in this signaling cascade.Mitosis
Yet another major aspect of cellular behavior that is thought to include rho protein signaling is the process of cell division, mitosis. While it was thought for years that rho GTPase activity is restricted only to actin polymerization and therefore only to cytokinesis, new evidence that shows some activity in microtubule formation and the overall process of mitosis has arisen. This topic is still debated, and there is evidence both for and against for the importance of rho in mitosis.Nervous System Regeneration
Because of their implications in cellular motility and shape, rho proteins became a clear target in the study of the growth conesGrowth cone
A growth cone is a dynamic, actin-supported extension of a developing axon seeking its synaptic target. Their existence was originally proposed by Spanish histologist Santiago Ramón y Cajal based upon stationary images he observed under the microscope...
that form during axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
al generation and re-generation in the nervous system. Some consider rho proteins to be a potential target for delivery into spinal cord lesions after traumatic injury. Following injury to the spinal cord, the extracellular space becomes inhibitory to the natural efforts neurons undergo to regenerate.
These ‘natural efforts’ include the formation of a growth cone at the proximal end of an injured axon. Newly-formed growth cones subsequently attempt to ‘crawl’ across the lesion and are quite sensitive to chemical cues in the extracellular environment. One of the many inhibitory cues includes chondroitin sulfate proteoglycans
Chondroitin sulfate
Chondroitin sulfate is a sulfated glycosaminoglycan composed of a chain of alternating sugars . It is usually found attached to proteins as part of a proteoglycan. A chondroitin chain can have over 100 individual sugars, each of which can be sulfated in variable positions and quantities...
or CSPGs. Neurons growing in culture increase in their ability to cross over inhibitory CSPG lanes after administration of constitutively-active Cdc42, Rac1 and RhoA. This is partly due to the exogenous rho proteins driving cellular locomotion despite the extracellular cues promoting apoptosis and growth cone collapse. It is situations like these that make intracellular modulation of rho proteins the subject of a significant amount of spinal cord research.
Mental Retardation
Rho proteins have also been implicated in mental retardationMental retardation
Mental retardation is a generalized disorder appearing before adulthood, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors...
. Mental retardation occurs in approximately 3% of the population and is characterized by having an IQ
Intelligence quotient
An intelligence quotient, or IQ, is a score derived from one of several different standardized tests designed to assess intelligence. When modern IQ tests are constructed, the mean score within an age group is set to 100 and the standard deviation to 15...
of less than 70. Multiple sources have noticed that mental retardation in some cases shows malformation of the dendritic spines
Dendrite
Dendrites are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project...
, which form the post-synaptic connections between neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s. As expected, the misshapen dendritic spines are sometimes the result of rho protein-signaling modulation. After cloning of various genes implicated in X-linked
X chromosome
The X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals and is common in both males and females. It is a part of the XY sex-determination system and X0 sex-determination system...
mental retardation, three genes that have effects on rho signaling were identified, including oligophrenin-1 (GAP protein that stimulates GTPase activity of Rac1, Cdc42, and RhoA), PAK3 (involved with the effects of Rac and Cdc42 on the actin cytoskeleton) and αPIX (a GEF that helps activate Rac1 and Cdc42). Because of the effect of rho signaling on the actin cytoskeleton, genetic malfunctions of a rho protein could explain the irregular morphology of neuronal dendrites seen in many cases of mental retardation.
Cancer
After finding that Ras proteins are mutated in 30% of human cancers, it was suspected that mutated rho proteins are also involved in cancer reproduction, as the signaling pathways involving rho proteins are widely known to play an important role in cancer development. However, Ellenbroek et al. reported in their review that, as of August 2007, no mutations have been found in rho proteins, and only one has been found to be genetically altered. To explain the role of rho pathways without mutation, researchers have now turned to the regulators of rho activity and the levels of expression of the rho proteins for answers.One way to explain altered signaling in the absence of mutation is through increased expression. Overexpression of RhoA, RhoB, RhoC, Rac1, Rac2, Rac3, RhoE, RhoG, RhoH, and Cdc42 has been shown in multiple types of cancer. This increased presence of so many signaling molecules implies that these proteins promote the cellular functions that become overly active in cancerous cells.
A second target to explain the role of the rho proteins in cancer is their regulatory proteins. Rho proteins are very tightly controlled by a wide variety of sources, and over 60 activators and 70 inactivators have been identified. Multiple GAPs, GDIs, and GEFs have been shown to undergo overexpression, downregulation, or mutation in different types of cancer. As one can imagine, once an upstream signal is changed, the activity of its targets downstream, i.e. the rho proteins, will change in activity.
Ellenbroek et al. outlined a number of different effects of rho activation in cancerous cells. First, in the initiation of the tumor modification of rho activity can suppress apoptosis
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
and therefore contribute to artificial cell longevity. After natural apoptosis is suppressed, abnormal tumor growth can be observed through the loss of polarity in which rho proteins play an integral role. Next, the growing mass can invade across its normal boundaries through the alteration of adhesion proteins potentially caused by rho proteins. Finally, after inhibition of apoptosis, cell polarity and adhesion molecules, the cancerous mass is free to metastasize
Metastasis
Metastasis, or metastatic disease , is the spread of a disease from one organ or part to another non-adjacent organ or part. It was previously thought that only malignant tumor cells and infections have the capacity to metastasize; however, this is being reconsidered due to new research...
and spread to other regions of the body.