Scramblase
Encyclopedia
Scramblase is a protein
responsible for the translocation of phospholipid
s between the two monolayers of a lipid bilayer
of a cell membrane
. In humans, phospholipid scramblases (PLSCRs) constitute a family of five homologous proteins that are named as hPLSCR1–hPLSCR5. Scramblases are members of the general family of transmembrane lipid transporters known as flippase
s. The inner-leaflet facing the inside of the cell contains negatively-charged amino-phospholipids and phosphatidylethanolamine
. The outer-leaflet, facing the outside environment, contains phosphatidylcholine and sphingomyelin
. Scramblase is an enzyme
, present in the cell membrane, that can transport (scramble) the negatively-charged phospholipids from the inner-leaflet to the outer-leaflet, and vice versa.
s, whereas hPLSCR1 and -3 were not detected in the brain. However, the functional significance of this differential gene expression is not yet understood. While the gene
and the mRNA of hPLSCR5 provide evidence of its existence, the protein has yet to be described in the literature.
surrounding a central alpha helix
. This structure shows similarity to the Tubby protein
.
concentration present inside the cell. The calcium concentration inside cells is, under normal conditions, very low; therefore, scramblase has a low activity under resting conditions. Phospholipid redistribution is triggered by increased cytosolic calcium and seems to be scramblase-dependent, resulting in a symmetric distribution of negatively-charged phospholipids between both leaflets of the lipid bilayer. All scramblases contain an EF hand
-like Ca2+binding domain
that is probably responsible for the calcium activation of the enzyme. The activity of scramblase does not require energy, meaning that there is no contribution of adenosine triphosphate
in the process.
Scramblases are cysteine
-rich protein
s, possessing many cysteinyl sulfhydryl groups that are prone to modifications. Oxidation
, nitrosylation
, and blockage of these sulfhydril groups produce an enhanced scramblase activity. Patients with sickle cell disease exhibit a fraction of erythrocytes with an aberrantly enhanced exposure of phosphotidyl serine on their surface. As the erythrocytes of these patients have an enhanced oxidative stress, it is probable that increased scramblase activity might play a role in the etiology of the disease. Furthermore, it is well recognized that both reactive oxygen species and intracellular Ca2+ fluxes affect mitochondria at the beginning of the apoptotic program. Sulfhydryl modification of PLSCR3 in mitochondria during apoptosis may be a key regulator initiating the intrinsic apoptotic pathways.
), a lipid-binding protein that enters the nucleus
via the nonclassical NLS
(257)GKISKHWTGI(266). The structure of the nuclear localisation sequence of scramblase PLSCR1 complexed to importin was determined using X-ray diffraction with a resolution of 2.20 Ångströms. It is found in most mammals including humans. The import sequence lacks a continuous stretch of positively charged residues, and it is enriched in hydrophobic residues. Thus, Scramblase can transport negatively-charged phospholipids from the inside of the cell to the outside of the cell. The importin structure is composed of many alpha helices that integrate the protein into membranes. The role of importin is to move proteins such as scramblase into the nucleus.
in mitochondria
, and its overexpression in cultured cells resulted in increased cardiolipin synthase, activity. As cardiolipin is synthesized in the luminal side of inner mitochondrial membrane, a major fraction of this newly-synthesized pool of cardiolipin has to be translocated from the inner to the outer mitochondrial membrane. PLSCR3 has been proposed to be involved in this translocation from the inner to the outer membrane that is essential for maintaining the mitochondrial architecture, mass, and transmembrane potential.
. PLSCR3 knockout mice showed an aberrant abdominal fat accumulation, glucose intolerance, insulin resistance, and dyslipidema as compared to controlled mice. Cultured fat cells from PLSCR3 knockout mice were engorged with neutral lipid
s. Blood plasma of these mice showed elevated levels of non-high-density lipoprotein
s, cholesterol
, triglyceride
s, non-esterified fatty acid
s, and leptin
, but low adiponectin
content. Abdominal fat accumulation with the formation of enlarged lipid engorged adipocyte
s has emerged as the key risk factor for the onset of type 2 diabetes, which is often a manifestation of a broader underlying metabolic disorder termed as metabolic syndrome. Further studies on the regulation of lipid metabolism by PLSCRs are required to understand the risk for development of similar diseases in humans when PLSCR genes are mutated, leading to a defective expression and/or function of PLSCR proteins.
phosphatidylserine
on their surface and act as catalysts to induce the coagulation cascade. Surface exposure of phosphatidylserine is thought to be brought about by the activation of scramblases. Several enzyme complexes of blood coagulation cascade such as tenase
and prothrombinase
are activated by the cell surface exposure of the phosphatidylserine. However, the most studied member of the scramblase family PLSCR1 was shown to be defective in translocation of phospholipids when reconstituted into proteoliposomes in vitro. Although recent studies show that PLSCR1 is neither sufficient nor necessary for the phosphatidylserine externalization, the involvement of PLSCR1 in blood coagulation remains elusive, raising the question of additional membrane components in the externalization pathway. To date, no report is available on the involvement of any other identified member of PLSCRs in blood clotting.
cell death is characterized by a proteolytic
caspase
cascade that emanates from either an extrinsic or an intrinsic pathway. The extrinsic pathway is initiated by membrane bound death receptors, leading to activation of caspase 8
, whereas the intrinsic pathway is triggered by DNA damaging drugs and UV radiation, leading to mitochondrial depolarization and subsequent activation of caspase 9. PLSCRs are supposed to play an important role in both intrinsic and extrinsic apoptotic responses that are linked to each other via the activation of caspase 8. Activated caspase 8 causes the cleavage of the amino terminal portion of the cytosolic protein Bid
to generate t-Bid that is translocated into mitochondria during apoptosis. hPLSCR1 and its mitochondrial counterpart hPLSCR3 are phosphorylated by PKCδ
during PKC-δ-induced apoptosis. While the consequence of hPLSCR1 phosphorylation and its mechanism of action during cellular apoptotic response remain unclear, phosphorylated PLSCR3 is thought to facilitate mitochondrial targeting of t-Bid that is an essential requirement in caspase 8-mediated apoptosis. The active t-Bid fragment is shown to localize to mitochondria through a positive interaction with cardiolipin. This activated t-Bid induces activation of Bax
and Bak proteins to form cytochrome c
channels that facilitate the release of cytochrome c during apoptosis.
An early morphological event in both the extrinsic and the intrinsic apoptotic pathways is the surface exposure of the phospholipid
phosphatidylserine
, about 96% of which normally reside in the cytosolic leaflet of the plasma membrane. Phosphotidyl serine is translocated to the exoplasmic leaflet by the activation of scramblases, leading to pro-coagulant properties and providing a phagocytic signal to the macrophage
s that engulf and clear the apoptotic cells. The involvement of other associated proteins aiding scrambling activity cannot be ruled out.
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
responsible for the translocation of phospholipid
Phospholipid
Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from...
s between the two monolayers of a lipid bilayer
Lipid bilayer
The lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus...
of a cell 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...
. In humans, phospholipid scramblases (PLSCRs) constitute a family of five homologous proteins that are named as hPLSCR1–hPLSCR5. Scramblases are members of the general family of transmembrane lipid transporters known as flippase
Flippase
Flippases are a family of transmembrane lipid transporter enzymes located in the membrane responsible for aiding the movement of phospholipid molecules between the two leaflets that compose a cell's membrane...
s. The inner-leaflet facing the inside of the cell contains negatively-charged amino-phospholipids and phosphatidylethanolamine
Phosphatidylethanolamine
Phosphatidylethanolamine is a lipid found in biological membranes. It is synthesized by the addition of CDP-ethanolamine to diglyceride, releasing CMP. S-adenosyl methionine can subsequently methylate the amine of phosphatidyl ethanolamine to yield phosphatidyl choline.Cephalin is a phospholipid,...
. The outer-leaflet, facing the outside environment, contains phosphatidylcholine and sphingomyelin
Sphingomyelin
Sphingomyelin is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath that surrounds some nerve cell axons. It usually consists of phosphorylcholine and ceramide...
. Scramblase is an enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
, present in the cell membrane, that can transport (scramble) the negatively-charged phospholipids from the inner-leaflet to the outer-leaflet, and vice versa.
Expression
Whereas hPLSCR1, -3, and -4 are expressed in a variety of tissues with few exceptions, expression of hPLSCR2 is restricted only to the testis. hPLSCR4 is not expressed in peripheral blood lymphocyteLymphocyte
A lymphocyte is a type of white blood cell in the vertebrate immune system.Under the microscope, lymphocytes can be divided into large lymphocytes and small lymphocytes. Large granular lymphocytes include natural killer cells...
s, whereas hPLSCR1 and -3 were not detected in the brain. However, the functional significance of this differential gene expression is not yet understood. While the gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
and the mRNA of hPLSCR5 provide evidence of its existence, the protein has yet to be described in the literature.
Structure
Scramblase proteins contain a region of conservation that possesses a 12-stranded beta barrelBeta barrel
A beta barrel is a large beta-sheet that twists and coils to form a closed structure in which the first strand is hydrogen bonded to the last.Beta-strands in beta-barrels are typically arranged in an antiparallel fashion...
surrounding a central alpha helix
Alpha helix
A common motif in the secondary structure of proteins, the alpha helix is a right-handed coiled or spiral conformation, in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues earlier...
. This structure shows similarity to the Tubby protein
Tubby protein
The tubby protein is encoded by the TUB gene. It is an upstream cell signaling protein common to multicellular eukaryotes. The first tubby gene was identified in mice, and proteins that are homologous to tubby are known as "tubby-like proteins"...
.
Enzyme activation
The enzymatic activity of scramblase depends on the calciumCalcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
concentration present inside the cell. The calcium concentration inside cells is, under normal conditions, very low; therefore, scramblase has a low activity under resting conditions. Phospholipid redistribution is triggered by increased cytosolic calcium and seems to be scramblase-dependent, resulting in a symmetric distribution of negatively-charged phospholipids between both leaflets of the lipid bilayer. All scramblases contain an EF hand
EF hand
The EF hand is a helix-loop-helix structural domain found in a large family of calcium-binding proteins. The EF-hand motif contains a helix-loop-helix topology, much like the spread thumb and forefinger of the human hand, in which the Ca2+ ions are coordinated by ligands within the loop...
-like Ca2+binding domain
Protein domain
A protein domain is a part of protein sequence and structure that can evolve, function, and exist independently of the rest of the protein chain. Each domain forms a compact three-dimensional structure and often can be independently stable and folded. Many proteins consist of several structural...
that is probably responsible for the calcium activation of the enzyme. The activity of scramblase does not require energy, meaning that there is no contribution of adenosine triphosphate
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
in the process.
Scramblases are cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
-rich protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s, possessing many cysteinyl sulfhydryl groups that are prone to modifications. Oxidation
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
, nitrosylation
Nitrosylation
Nitrosylation is a protein modification in which a nitrosyl group is post-translationally added to a protein.There is a range of enzymes that produce nitric oxide, and the frequent consequence of this production is nitrosylation....
, and blockage of these sulfhydril groups produce an enhanced scramblase activity. Patients with sickle cell disease exhibit a fraction of erythrocytes with an aberrantly enhanced exposure of phosphotidyl serine on their surface. As the erythrocytes of these patients have an enhanced oxidative stress, it is probable that increased scramblase activity might play a role in the etiology of the disease. Furthermore, it is well recognized that both reactive oxygen species and intracellular Ca2+ fluxes affect mitochondria at the beginning of the apoptotic program. Sulfhydryl modification of PLSCR3 in mitochondria during apoptosis may be a key regulator initiating the intrinsic apoptotic pathways.
Nuclear localisation sequence
Phospholipid scramblase 1 (PLSCR1PLSCR1
Phospholipid scramblase 1 is an enzyme that in humans is encoded by the PLSCR1 gene.-Interactions:PLSCR1 has been shown to interact with NEU4, SLPI, SHC1, CPSF6, Epidermal growth factor receptor and TFG.-Further reading:...
), a lipid-binding protein that enters the nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
via the nonclassical NLS
Nuclear localization signal
A nuclear localization signal or sequence is an amino acid sequence which 'tags' a protein for import into the cell nucleus by nuclear transport. Typically, this signal consists of one or more short sequences of positively charged lysines or arginines exposed on the protein surface. Different...
(257)GKISKHWTGI(266). The structure of the nuclear localisation sequence of scramblase PLSCR1 complexed to importin was determined using X-ray diffraction with a resolution of 2.20 Ångströms. It is found in most mammals including humans. The import sequence lacks a continuous stretch of positively charged residues, and it is enriched in hydrophobic residues. Thus, Scramblase can transport negatively-charged phospholipids from the inside of the cell to the outside of the cell. The importin structure is composed of many alpha helices that integrate the protein into membranes. The role of importin is to move proteins such as scramblase into the nucleus.
Mitochondrial membrane maintenance
Recent findings suggest that PLSCR3 is involved in regulation of biosynthesis of cardiolipinCardiolipin
Cardiolipin is an important component of the inner mitochondrial membrane, where it constitutes about 20% of the total lipid composition. The only other place that cardiolipin can be found is in the membranes of most bacteria. The name ‘cardiolipin’ is derived from the fact that it was first...
in mitochondria
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
, and its overexpression in cultured cells resulted in increased cardiolipin synthase, activity. As cardiolipin is synthesized in the luminal side of inner mitochondrial membrane, a major fraction of this newly-synthesized pool of cardiolipin has to be translocated from the inner to the outer mitochondrial membrane. PLSCR3 has been proposed to be involved in this translocation from the inner to the outer membrane that is essential for maintaining the mitochondrial architecture, mass, and transmembrane potential.
Lipid metabolism
Recent findings suggest that PLSCR3 and, to a lesser degree, PLSCR1 are critical to the normal regulation of fat accumulation in mice. In addition to blood cells, PLSCR3 is expressed to a significantly higher level in fat and muscle cells, which are actively involved in fat metabolismMetabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
. PLSCR3 knockout mice showed an aberrant abdominal fat accumulation, glucose intolerance, insulin resistance, and dyslipidema as compared to controlled mice. Cultured fat cells from PLSCR3 knockout mice were engorged with neutral lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
s. Blood plasma of these mice showed elevated levels of non-high-density lipoprotein
Lipoprotein
A lipoprotein is a biochemical assembly that contains both proteins and lipids water-bound to the proteins. Many enzymes, transporters, structural proteins, antigens, adhesins, and toxins are lipoproteins...
s, cholesterol
Cholesterol
Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
, triglyceride
Triglyceride
A triglyceride is an ester derived from glycerol and three fatty acids. There are many triglycerides, depending on the oil source, some are highly unsaturated, some less so....
s, non-esterified fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
s, and leptin
Leptin
Leptin is a 16 kDa protein hormone that plays a key role in regulating energy intake and energy expenditure, including appetite and metabolism. It is one of the most important adipose derived hormones...
, but low adiponectin
Adiponectin
Adiponectin is a protein which in humans is encoded by the ADIPOQ gene.- Structure :...
content. Abdominal fat accumulation with the formation of enlarged lipid engorged adipocyte
Adipocyte
However, in some reports and textbooks, the number of fat cell increased in childhood and adolescence. The total number is constant in both obese and lean adult...
s has emerged as the key risk factor for the onset of type 2 diabetes, which is often a manifestation of a broader underlying metabolic disorder termed as metabolic syndrome. Further studies on the regulation of lipid metabolism by PLSCRs are required to understand the risk for development of similar diseases in humans when PLSCR genes are mutated, leading to a defective expression and/or function of PLSCR proteins.
Thrombosis
Upon activation (in platelets) or injury (in erythrocytes, platelets, endothelium, and other cells), certain cells expose the phospholipidPhospholipid
Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from...
phosphatidylserine
Phosphatidylserine
Phosphatidylserine is a phospholipid component, usually kept on the inner-leaflet of cell membranes by an enzyme called flippase...
on their surface and act as catalysts to induce the coagulation cascade. Surface exposure of phosphatidylserine is thought to be brought about by the activation of scramblases. Several enzyme complexes of blood coagulation cascade such as tenase
Tenase
In coagulation, the procoagulant protein factor X can be activated into factor Xa two ways. Extrinsic and intrinsic ways.The activating complexes are called tenase...
and prothrombinase
Prothrombinase
The prothrombinase complex consists of the serine protease, Factor Xa, and the protein cofactor, Factor Va. The complex assembles on negatively charged phospholipid membranes in the presence of calcium ions. The prothrombinase complex catalyzes the conversion of prothrombin , an inactive zymogen,...
are activated by the cell surface exposure of the phosphatidylserine. However, the most studied member of the scramblase family PLSCR1 was shown to be defective in translocation of phospholipids when reconstituted into proteoliposomes in vitro. Although recent studies show that PLSCR1 is neither sufficient nor necessary for the phosphatidylserine externalization, the involvement of PLSCR1 in blood coagulation remains elusive, raising the question of additional membrane components in the externalization pathway. To date, no report is available on the involvement of any other identified member of PLSCRs in blood clotting.
Apoptosis
ApoptoticApoptosis
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...
cell death is characterized by a proteolytic
Proteolysis
Proteolysis is the directed degradation of proteins by cellular enzymes called proteases or by intramolecular digestion.-Purposes:Proteolysis is used by the cell for several purposes...
caspase
Caspase
Caspases, or cysteine-aspartic proteases or cysteine-dependent aspartate-directed proteases are a family of cysteine proteases that play essential roles in apoptosis , necrosis, and inflammation....
cascade that emanates from either an extrinsic or an intrinsic pathway. The extrinsic pathway is initiated by membrane bound death receptors, leading to activation of caspase 8
Caspase 8
Caspase 8 is a caspase protein, encoded by the CASP8 gene. It most likely acts upon caspase 3.CASP8 orthologs have been identified in numerous mammals for which complete genome data are available...
, whereas the intrinsic pathway is triggered by DNA damaging drugs and UV radiation, leading to mitochondrial depolarization and subsequent activation of caspase 9. PLSCRs are supposed to play an important role in both intrinsic and extrinsic apoptotic responses that are linked to each other via the activation of caspase 8. Activated caspase 8 causes the cleavage of the amino terminal portion of the cytosolic protein Bid
BH3 interacting domain death agonist
The BH3 interacting-domain death agonist, or BID, gene is a pro-apoptotic member of the Bcl-2 protein family. Bcl-2 family members share one or more of the four characteristic domains of homology entitled the Bcl-2 homology domains , and can form hetero- or homodimers...
to generate t-Bid that is translocated into mitochondria during apoptosis. hPLSCR1 and its mitochondrial counterpart hPLSCR3 are phosphorylated by PKCδ
PRKCD
Protein kinase C delta type is an enzyme that in humans is encoded by the PRKCD gene.- Function :Protein kinase C is a family of serine- and threonine-specific protein kinases that can be activated by calcium and the second messenger diacylglycerol...
during PKC-δ-induced apoptosis. While the consequence of hPLSCR1 phosphorylation and its mechanism of action during cellular apoptotic response remain unclear, phosphorylated PLSCR3 is thought to facilitate mitochondrial targeting of t-Bid that is an essential requirement in caspase 8-mediated apoptosis. The active t-Bid fragment is shown to localize to mitochondria through a positive interaction with cardiolipin. This activated t-Bid induces activation of Bax
Bcl-2-associated X protein
The Bcl-2–associated X protein, or Bax is a protein of the Bcl-2 gene family. It promotes apoptosis by competing with Bcl-2 proper.The BAX gene was the first identified pro-apoptotic member of the Bcl-2 protein family....
and Bak proteins to form cytochrome c
Cytochrome c
The Cytochrome complex, or cyt c is a small heme protein found loosely associated with the inner membrane of the mitochondrion. It belongs to the cytochrome c family of proteins. Cytochrome c is a highly soluble protein, unlike other cytochromes, with a solubility of about 100 g/L and is an...
channels that facilitate the release of cytochrome c during apoptosis.
An early morphological event in both the extrinsic and the intrinsic apoptotic pathways is the surface exposure of the phospholipid
Phospholipid
Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from...
phosphatidylserine
Phosphatidylserine
Phosphatidylserine is a phospholipid component, usually kept on the inner-leaflet of cell membranes by an enzyme called flippase...
, about 96% of which normally reside in the cytosolic leaflet of the plasma membrane. Phosphotidyl serine is translocated to the exoplasmic leaflet by the activation of scramblases, leading to pro-coagulant properties and providing a phagocytic signal to the macrophage
Macrophage
Macrophages are cells produced by the differentiation of monocytes in tissues. Human macrophages are about in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense as well as help initiate specific defense mechanisms of vertebrate animals...
s that engulf and clear the apoptotic cells. The involvement of other associated proteins aiding scrambling activity cannot be ruled out.