Rap1
Encyclopedia
Rap1 is a small GTPase
, which are small cytosolic proteins that act like cellular switches and are vital for effective signal transduction
. There are two isoforms of the Rap1 protein, each encoded by a separate gene, RAP1A
and RAP1B
. Rap1 belongs to Ras-related protein
family.
GTPases are inactive when in their GDP-bound form, and become active when they bind to GTP. GTPase activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs) regulate small GTPases, with GAPs promoting the GDP-bound (inactive) form, and GEFs promoting the GTP-bound (active) form. When bound to GTP, small GTPases regulate myriad cellular processes. These proteins are divided into families depending on their protein structure, and the most well studied is the Ras superfamily
, of which Rap1 is a member. Whereas Ras is known for its role in cell proliferation
and survival, Rap1 is predominantly involved in cell adhesion
and cell junction
formation. Ras and Rap are regulated by different sets of guanine nucleotide exchange factors and GTPase-activating proteins, thus providing one level of specificity.
(TCR) signaling to integrins. A constitutively active Rap1 construct, Rap1G12V, was used as a bait in a yeast two-hybrid screen
to identify RAPL
as a Rap1-binding protein.
Overexpression of RAPL enhances LFA-1 clustering and adhesion, and RAPL-deficient lymphocytes and dendritic cell
s exhibit impaired adhesion and migration. RAPL is also an integrin-associated protein as RAPL polarizes to the immunological synapse
following antigen stimulation of T cell
s, colocalizes with LFA-1 following TCR or chemokine stimulation, and co-immunoprecipitates with LFA-1 in a Rap1-dependent manner (108). This interaction between RAPL and LFA-1 is dependent on lysine residues at positions 1097 and 1099 in the juxtamembrane region of the αL-subunit cytoplasmic domain. This is a functionally significant region of the αL cytoplasmic domain as deletion of the adjacent GFFKR motif results in a constitutively active LFA-1 integrin (124, 125). While lysines 1097 and 1099 are critical for Rap1-dependent activation of LFA-1, the β2-subunit cytoplasmic domain appears to be dispensable for activation of LFA-1 by Rap1 (126). Mutation of these lysine residues to alanine impairs the ability of LFA-1 to redistribute to the leading edge induced by Rap1 activation or overexpression of RAPL. Because RAPL localizes to the leading edge properly in cells expressing this mutant LFA-1, this finding suggests that RAPL may play a critical role in localizing LFA-1 to discrete regions of the plasma membrane
.
, a member of a family of kinases homologous to the Ste20
kinase in yeast , has recently been identified as a RAPL effector. TCR-mediated activation of Mst1 is dependent on RAPL, and TCR-mediated adhesion to ICAM-1
and antigen-dependent conjugate formation are impaired following RNAi-mediated knockdown of Mst1 expression. Although Rap1 and RAPL have been shown to regulate both LFA-1 affinity and clustering, overexpression of Mst1 only enhances LFA-1 clustering. This finding suggests that LFA-1 clustering is critical for TCR signaling to integrins that is mediated by Rap1. It also implies the existence of Mst1-independent mechanisms by which Rap1 regulates LFA-1 affinity.
, RAPL, and Mst1 is their localization to membranes where integrins are found. This provides a mechanism by which Rap1 can act directly on integrins and modulate integrin affinity and/or clustering. PKD, RAPL, and Mst1 have also all been proposed to play a role in movement of receptors to the plasma membrane. PKD-dependent regulation of vesicular transport requires PKD kinase activity, while PKD-dependent regulation of TCR signaling to integrins does not appear to require PKD kinase activity. Thus, PKD may play a distinct role in regulating Rap1-dependent integrin regulation. For example, the PKD-dependent association of Rap1 with C3G suggests that PKD may be critical for localizing Rap1 not only with integrins but also with Rap1 GEFs. The PKD–Rap1 interaction may thus be central to the subsequent activation of Rap1 and triggering of downstream effectors such as RAPL and Mst1.
(Rap1–GTP-interacting adapter molecule) is a broadly expressed adapter protein
that contains an RA (Ras association)-like domain, a PH domain
, and several proline-rich sequences. Like RAPL, RIAM interacts preferentially with active Rap1, and overexpression of RIAM enhances integrin-mediated adhesion. In addition, knockdown of RIAM inhibits adhesion induced by active Rap1 and inhibits the localization of active Rap1 at the plasma membrane. The ability of RIAM to associate with profilin, Ena/VASP proteins, and talin suggests that RIAM promotes Rap1-dependent integrin activation through effects on the actin cytoskeleton, particularly the interaction of talin with integrin cytoplasmic tails. Given the known role of talin in regulating integrin affinity, RIAM may provide an Mst1-independent mechanism by which Rap1 regulates integrin affinity.
Small GTPase
Small GTPases are a family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate . They are a form of G-proteins found in the cytosol which are homologous to the alpha subunit of heterotrimeric G-proteins, but unlike the alpha subunit of G proteins, a small GTPase can function...
, which are small cytosolic proteins that act like cellular switches and are vital for effective signal transduction
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
. There are two isoforms of the Rap1 protein, each encoded by a separate gene, RAP1A
RAP1A
Ras-related protein Rap-1A is a protein that in humans is encoded by the RAP1A gene.- Function :The product of this gene belongs to the family of Ras-related proteins. These proteins share approximately 50% amino acid identity with the classical RAS proteins and have numerous structural features in...
and RAP1B
RAP1B
Ras-related protein Rap-1b, also known as GTP-binding protein smg p21B, is a protein that in humans is encoded by the RAP1B gene....
. Rap1 belongs to Ras-related protein
Rap GTP-binding protein
Rap GTP-binding protein also known as Ras-related proteins or simply RAP is a type of small GTPase, similar in structure to Ras.These proteins share approximately 50% amino acid identity with the classical RAS proteins and have numerous structural features in common...
family.
GTPases are inactive when in their GDP-bound form, and become active when they bind to GTP. GTPase activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs) regulate small GTPases, with GAPs promoting the GDP-bound (inactive) form, and GEFs promoting the GTP-bound (active) form. When bound to GTP, small GTPases regulate myriad cellular processes. These proteins are divided into families depending on their protein structure, and the most well studied is 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...
, of which Rap1 is a member. Whereas Ras is known for its role in cell proliferation
Cell growth
The term cell growth is used in the contexts of cell development and cell division . When used in the context of cell division, it refers to growth of cell populations, where one cell grows and divides to produce two "daughter cells"...
and survival, Rap1 is predominantly involved in cell adhesion
Cell adhesion
Cellular adhesion is the binding of a cell to a surface, extracellular matrix or another cell using cell adhesion molecules such as selectins, integrins, and cadherins. Correct cellular adhesion is essential in maintaining multicellular structure...
and cell junction
Cell junction
A cell junction is a type of structure that exists within the tissue of a some multicellular organism . Cell junctions consist of protein complexes and provide contact between neighbouring cells or between a cell and the extracellular matrix...
formation. Ras and Rap are regulated by different sets of guanine nucleotide exchange factors and GTPase-activating proteins, thus providing one level of specificity.
RAPL
The identification of Rap1 effector proteins has provided important insights into mechanisms by which Rap1 regulates T-cell receptorT cell receptor
The T cell receptor or TCR is a molecule found on the surface of T lymphocytes that is responsible for recognizing antigens bound to major histocompatibility complex molecules...
(TCR) signaling to integrins. A constitutively active Rap1 construct, Rap1G12V, was used as a bait in a yeast two-hybrid screen
Two-hybrid screening
Two-hybrid screening is a molecular biology technique used to discover protein–protein interactions and protein–DNA interactions by testing for physical interactions between two proteins or a single protein and a DNA molecule, respectively.The premise behind the test is the activation of...
to identify RAPL
RASSF5
Ras association domain-containing protein 5 is a protein that in humans is encoded by the RASSF5 or F5 gene.-Interactions:RASSF5 has been shown to interact with RRAS, RAP2A, MRAS and RASSF1.-Further reading:...
as a Rap1-binding protein.
Overexpression of RAPL enhances LFA-1 clustering and adhesion, and RAPL-deficient lymphocytes and dendritic cell
Dendritic cell
Dendritic cells are immune cells forming part of the mammalian immune system. Their main function is to process antigen material and present it on the surface to other cells of the immune system. That is, dendritic cells function as antigen-presenting cells...
s exhibit impaired adhesion and migration. RAPL is also an integrin-associated protein as RAPL polarizes to the immunological synapse
Immunological synapse
In immunology, an immunological synapse is the interface between an antigen-presenting cell and a lymphocyte. It was first discovered by Abraham Kupfer at the National Jewish Medical and Research Center in Denver and the term was coined by Michael Dustin at NYU who studied it in further detail...
following antigen stimulation of T cell
T cell
T cells or T lymphocytes belong to a group of white blood cells known as lymphocytes, and play a central role in cell-mediated immunity. They can be distinguished from other lymphocytes, such as B cells and natural killer cells , by the presence of a T cell receptor on the cell surface. They are...
s, colocalizes with LFA-1 following TCR or chemokine stimulation, and co-immunoprecipitates with LFA-1 in a Rap1-dependent manner (108). This interaction between RAPL and LFA-1 is dependent on lysine residues at positions 1097 and 1099 in the juxtamembrane region of the αL-subunit cytoplasmic domain. This is a functionally significant region of the αL cytoplasmic domain as deletion of the adjacent GFFKR motif results in a constitutively active LFA-1 integrin (124, 125). While lysines 1097 and 1099 are critical for Rap1-dependent activation of LFA-1, the β2-subunit cytoplasmic domain appears to be dispensable for activation of LFA-1 by Rap1 (126). Mutation of these lysine residues to alanine impairs the ability of LFA-1 to redistribute to the leading edge induced by Rap1 activation or overexpression of RAPL. Because RAPL localizes to the leading edge properly in cells expressing this mutant LFA-1, this finding suggests that RAPL may play a critical role in localizing LFA-1 to discrete regions of the plasma membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
.
Mst1
The serine–threonine kinase Mst1MST1
Hepatocyte growth factor-like protein is a protein that in humans is encoded by the MST1 gene.-Further reading:...
, a member of a family of kinases homologous to the Ste20
STK24
Serine/threonine-protein kinase 24 is an enzyme that in humans is encoded by the STK24 gene.-Interactions:STK24 has been shown to interact with PDCD10, TRAF3IP3, STRN3, MOBKL3, STRN, SLMAP, PPP2R1A, CTTNBP2NL, FAM40A and STRN4.-Further reading:...
kinase in yeast , has recently been identified as a RAPL effector. TCR-mediated activation of Mst1 is dependent on RAPL, and TCR-mediated adhesion to ICAM-1
Intercellular adhesion molecule
Intercellular adhesion molecules are members of the family of cell adhesion molecules. They include the following:* ICAM-1 * ICAM2* ICAM3* ICAM4* ICAM5...
and antigen-dependent conjugate formation are impaired following RNAi-mediated knockdown of Mst1 expression. Although Rap1 and RAPL have been shown to regulate both LFA-1 affinity and clustering, overexpression of Mst1 only enhances LFA-1 clustering. This finding suggests that LFA-1 clustering is critical for TCR signaling to integrins that is mediated by Rap1. It also implies the existence of Mst1-independent mechanisms by which Rap1 regulates LFA-1 affinity.
PKD
A striking feature of Rap1 and the Rap1-associated signaling proteins PKDPKD1
Polycystin-1 is a protein that in humans is encoded by the PKD1 gene.-Gene product:-Function:Polycystin-1 is a glycoprotein which contains a large N-terminal extracellular region, multiple transmembrane domains and a cytoplasmic C-tail...
, RAPL, and Mst1 is their localization to membranes where integrins are found. This provides a mechanism by which Rap1 can act directly on integrins and modulate integrin affinity and/or clustering. PKD, RAPL, and Mst1 have also all been proposed to play a role in movement of receptors to the plasma membrane. PKD-dependent regulation of vesicular transport requires PKD kinase activity, while PKD-dependent regulation of TCR signaling to integrins does not appear to require PKD kinase activity. Thus, PKD may play a distinct role in regulating Rap1-dependent integrin regulation. For example, the PKD-dependent association of Rap1 with C3G suggests that PKD may be critical for localizing Rap1 not only with integrins but also with Rap1 GEFs. The PKD–Rap1 interaction may thus be central to the subsequent activation of Rap1 and triggering of downstream effectors such as RAPL and Mst1.
RIAM
An additional Rap1 effector provides a link between Rap1 and the actin cytoskeleton. RIAMAPBB1IP
Amyloid beta A4 precursor protein-binding family B member 1-interacting protein , also known as APBB1-interacting protein 1 or Rap1-GTP-interacting adapter molecule is a protein that in humans is encoded by the APBB1IP gene....
(Rap1–GTP-interacting adapter molecule) is a broadly expressed adapter protein
Adaptor protein
Signal transducing adaptor proteins are proteins which are accessory to main proteins in a signal transduction pathway. These proteins tend to lack any intrinsic enzymatic activity themselves but instead mediate specific protein–protein interactions that drive the formation of protein complexes...
that contains an RA (Ras association)-like domain, a PH domain
Pleckstrin homology domain
Pleckstrin homology domain is a protein domain of approximately 120 amino acids that occurs in a wide range of proteins involved in intracellular signaling or as constituents of the cytoskeleton....
, and several proline-rich sequences. Like RAPL, RIAM interacts preferentially with active Rap1, and overexpression of RIAM enhances integrin-mediated adhesion. In addition, knockdown of RIAM inhibits adhesion induced by active Rap1 and inhibits the localization of active Rap1 at the plasma membrane. The ability of RIAM to associate with profilin, Ena/VASP proteins, and talin suggests that RIAM promotes Rap1-dependent integrin activation through effects on the actin cytoskeleton, particularly the interaction of talin with integrin cytoplasmic tails. Given the known role of talin in regulating integrin affinity, RIAM may provide an Mst1-independent mechanism by which Rap1 regulates integrin affinity.