M-CSFR encoded by the proto-oncogene c-fms is the receptor for colony stimulating factor 1 (CSF1R) , a cytokine involved in the proliferation , differentiation , and activation of macrophages. This cell surface glycoprotein is consisted by an extracellular ligand-binding domain , a single membrane-spanning segment , and an intracellular tyrosine kinase domain. Binding of CSF1 activates the receptor kinase , leading to "autophosphorylation" of receptor subunits and the concomitant phosphorylation of a series of cellular proteins on tyrosine residues. CSF1R is a tyrosine kinase receptor that is absolutely required for macrophage differentiation and thus occupies a central role in hematopoiesis. CSF1 and its receptor (CSF1R , product of c-fms proto-oncogene) were initially implicated as essential for normal monocyte development as well as for trophoblastic implantation. This apparent role for CSF1/CSF1R in normal mammary gland development is very intriguing because this receptor/ligand pair has also been found to be important in the biology of breast cancer in which abnormal expression of CSF1 and its receptor correlates with tumor cell invasiveness and adverse clinical prognosis. Tumor cell expression of CSF1R is under the control of several steroid hormones (glucocorticoids and progestins) and the binding of several bHLH transcription factors , while tumor cell expression of CSF-1 appears to be regulated by other hormones , some of which are involved in normal lactogenic differentiation. However , studies have demonstrated that CSF1 and CSF1R have additional roles in mammary gland development during pregnancy and lactation. The role of CSF1 and CSF1R in normal and neoplastic mammary development that may elucidate potential relationships of growth factor-induced biological changes in the breast during pregnancy and tumor progression.
MD-1 and MD-2 are secretory glycoproteins that exist on the cell surface in complexes with transmembrane proteins. MD-1 is anchored by radioprotective 105 (RP105) which is a molecule containing leucine-rich repeats and is expressed on B cells, dentritic cells and macrophages, while MD-2 is associated with TLR4. MD-1 is required for efficient RP105 cell surface expression and function. It is indicated that the RP105/MD1 complex, in conjunction with TLR4, mediates the innate immune response to LPS in B cells, and also plays a role in protecting against apoptosis, B-cell proliferation, etc. Mouse MD-1 cDNA encodes a 162 amino acid precursor protein with a putative 19 aa signal peptide and two potential N-linked glycosylation sites. It shares 40% and 66% amino acid sequence identity with chicken and human MD-1 respectively. MD-1 is mainly expressed in spleen, and also detectable in liver, brain, thymus, and kidney.
MAM domain-containing glycosylphosphatidylinositol anchor protein 2, also known as MAM domain-containing protein 1, MDGA2 and MAMDC1, is a cell membrane protein which contains sixIg-like (immunoglobulin-like) domains and oneMAM domain. Analyses of the full-length coding region of MDGA1 and MDGA2 indicate that they encode proteins that comprise a novel subgroup of the Ig superfamily and have a unique structural organization consisting of six immunoglobulin (Ig)-like domains followed by a single MAM domain. Biochemical characterization demonstrates that MDGA1 and MDGA2 proteins are highly glycosylated, and that MDGA1 is tethered to the cell membrane by a GPI anchor. The MDGAs are differentially expressed by subpopulations of neurons in both the central and peripheral nervous systems, including neurons of the basilar pons, inferior olive, cerebellum, cerebral cortex, olfactory bulb, spinal cord, and dorsal root and trigeminal ganglia. The similarity of MDGAs to other Ig-containing molecules and their temporal-spatial patterns of expression within restricted neuronal populations, for example migrating pontine neurons and D1 spinal interneurons, suggest a role for these novel proteins in regulating neuronal migration, as well as other aspects of neural development, including axon guidance.
MEK1, also known as MAP2K1 and MKK1, is a member of the dual specificity protein kinase family, which acts as a mitogen-activated protein (MAP) kinase kinase. MAP kinases, also known as extracellular signal-regulated kinases (ERKs), act as an integration point for multiple biochemical signals. MEK1 is widely expressed, with extremely low levels in brain. It lies upstream of MAP kinases and stimulates the enzymatic activity of MAP kinases upon wide variety of extra- and intracellular signals. As an essential component of MAP kinase signal transduction pathway, MEK1 is involved in many cellular processes such as proliferation, differentiation, transcription regulation and development. Binding extracellular ligands such as growth factors, cytokines and hormones to their cell-surface receptors activates RAS and this initiates RAF1 activation. RAF1 then further activates the dual-specificity protein kinases MAP2K1 and MEK2. MEK1 has been shown to export PPARG from the nucleus. The MAPK cascade is also involved in the regulation of endosomal dynamics, including lysosome processing and endosome cycling through the perinuclear recycling compartment (PNRC), as well as in the fragmentation of the Golgi apparatus during mitosis. MKK1 catalyzes the concomitant phosphorylation of a threonine and a tyrosine residue in a Thr-Glu-Tyr sequence located in MAP kinases. Defects in MEK1 can cause cardiofaciocutaneous syndrome.
Dual specificity mitogen-activated protein kinase kinase 4, also known as MAP kinase kinase 4, MAPKK4, JNK-activating kinase 1, MAPK/ERK kinase 4, SAPK/ERK kinase 1, c-Jun N-terminal kinase kinase 1, JNKK and MAP2K4, is a protein which belongs to theprotein kinase superfamily, STE Ser/Thr protein kinase family and MAP kinase kinase subfamily. MAP2K4 / JNKK1 is a protein kinase which is a direct activator of MAP kinases in response to various environmental stresses or mitogenic stimuli. MAP2K4 / JNKK1 has been shown to activate MAPK8 / JNK1, MAPK9 / JNK2, and MAPK14 / p38, but not MAPK1 / ERK2 or MAPK3 / ERK1. MAP2K4 / JNKK1 is phosphorylated, and thus activated by MAP3K1 / MEKK. The stress-activated protein kinase (SAPK) pathways represent phosphorylation cascades that convey pro-apoptotic signals. The mitogen-activated protein kinase kinase (MAPKK) homolog MAP2K4 ( MKK4, SEK, JNKK1 ) is a centrally-placed mediator of the SAPK pathways.
Meprin A subunit alpha, also known as MEP1A, and Endopeptidase-2, is a single-pass type I membrane protein which belongs to thepeptidase M12A family. MEP1A contains oneEGF-like domain, oneMAM domain, and oneMATH domain. Meprins are unique plasma membrane and secreted metalloproteinases that are highly regulated at the transcriptional and post-translational levels. Meprin alpha and beta subunits are abundantly expressed in kidney and intestinal epithelial cells, are secreted into the urinary tract and intestinal lumen, and are found in leukocytes and cancer cells under certain conditions. Meprins are capable of proteolytically degrading extracellular matrix proteins, proteolytically processing bioactive proteins, and play a role in inflammatory processes. Meprin A and B are highly regulated, secreted and cell-surface homo- and hetero-oligomeric enzymes. Meprins are abundantly expressed in kidney and intestine. The multidomain alpha and beta subunits have high sequence identity. They have very different substrate specificities, oligomerization potentials and are differentially regulated. Meprin A appears to be an important therapeutic target and urinary excretion appears to be a potential biomarker of acute kidney injury ( AKI ).
Proto-oncogene tyrosine-protein kinase MER (MERTK) is a member of the MER/AXL/TYRO3 receptor kinase family and encodes a transmembrane protein with two fibronectin type-III domains, two Ig-like C2-type (immunoglobulin-like) domains, and one tyrosine kinase domain. MERTK is localized in membrane and is no expressed in normal B- and T-lymphocytes but is expressed in numerous neoplastic B- and T-cell lines. This protein is highly expressed in testis, ovary, prostate, lung, and kidney, with lower expression in spleen, small intestine, colon, and liver. MERTK regulates many physiological processes including cell survival, migration, differentiation, and phagocytosis of apoptotic cells (efferocytosis). Ligand binding at the cell surface induces autophosphorylation of MERTK on its intracellular domain that provides docking sites for downstream signaling molecules. MERTK signaling plays a role in various processes such as macrophage clearance of apoptotic cells, platelet aggregation, cytoskeleton reorganization and engulfment. MERTK plays also an important role in inhibition of Toll-like receptors (TLRs)-mediated innate immune response by activating STAT1, which selectively induces production of suppressors of cytokine signaling SOCS1 and SOCS3. Defects in MERTK are the cause of retinitis pigmentosa type 38.
Mesothelin is derived from a 70 kDa precursor that also includes Megakaryocyte Potentiating Factor (MPF).The 70 kDa precursor is expressed on the cell surface where it is cleaved at a dibasic proteolytic site to release the 32 kDa glycosylated MPF. MPF is a cytokine that potentiates IL3 induced megakaryocyte colony formation. The term Mesothelin refers to the 40 kDa glycosylated protein which remains attached to the cell surface via a GPI linkage. Mesothelin is over expressed in several human tumors, including mesothelioma and ovarian and pancreatic adenocarcinoma. The interaction between mesothelin and MUC16 (also known as CA125) may facilitate the implantation and peritoneal spread of tumors by cell adhesion. The region (296-359) consisting of 64 amino acids at the N-terminal of cell surface mesothelin is the functional binding domain for MUC16. Mesothelin is a tumour differentiation antigen that is normally present on the mesothelial cells lining the pleura, peritoneum and pericardium. A soluble form of Mesothelin, with its GPI anchor intact, is released from tumor cells and binds to MMR/CD206 on macrophages.
METAP2 (Methionine aminopeptidase 2), also known as MAP2 is a a protein which belongs to the peptidase M24A family. MAP2 binds 2 cobalt or manganese ions and contains approximately 12 O-linked N-acetylglucosamine (GlcNAc) residues. It is found in all organisms and is especially important because of its critical role in tissue repair and protein degradation. The catalytic activity of human MAP2 toward Met-Val peptides is consistently two orders of magnitude higher than that of METAP1, suggesting that it is responsible for processing proteins containing N-terminal Met-Val and Met-Thr sequences in vivo. This protein functions both by protecting the alpha subunit of eukaryotic initiation factor 2 from inhibitory phosphorylation and by removing the amino-terminal methionine residue from nascent protein. MAP2 protects eukaryotic initiation factor EIF2S1 from translation-inhibiting phosphorylation by inhibitory kinases such as EIF2AK2/PKR and EIF2AK1/HCR. It also plays a critical role in the regulation of protein synthesis.
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