Glycoprotein Function as Receptor
The functions of glycoproteins are tremendously broad and include cell attachment recognition, homeostasis, transport of molecules, and enzymatic and immunology recognition domains. The following are some examples of glycoproteins that function as receptors.
Example 1: Myelin-associated Glycoprotein (MAG)
The MAG is a 100 kDa transmembrane glycoprotein that is selectively localized in periaxonal Schwann cell and oligodendroglial membranes of myelin sheaths, suggesting that it functions in glia-axon interactions in both the PNS and CNS. It appears to function both as a ligand for an axonal receptor that is needed for the maintenance of myelinated axons and as a receptor for an axonal signal that promotes the differentiation, maintenance, and survival of oligodendrocytes.
Fig.1 Structure of MAG and biochemical mechanisms by which MAG may mediate interactions and signaling between myelin-forming cells and axons. (Quarles, 2007)
Example 2: Endocytic Transmembrane Glycoprotein Endo180 as a Novel Collagen Receptor
Collagen is the most abundant group of proteins in vertebrates. It plays an important role in tissue development, morphogenesis and growth. In addition to its mechanical properties, it also acts as a substrate for cell adhesion and migration and mediates signal events by binding to cell surface receptors. Endo180, a member of the mannose receptor family, cycles constitutively between clathrin-coated pits on the cell surface and endosomes in the cell core. Endo180 is the fourth and final member of the mannose receptor family, which also comprises the mannose receptor, the M-type phospholipase A2 (PLA2R) receptor, and DEC-205. All 4 receptors contain an N-terminal cysteine-rich or ricin-type domain, a fibronectin type II (FNII) domain, 8 C-type-lectin domains, a single transmembrane domain, and a short cytoplasmic domain. The proposal that Endo180 might function as a collagen receptor comes from two sources. First, FNII domains in other proteins including fibronectin and the gelatinases MMP2 and MMP9 have been demonstrated to confer binding to denatured collagen. Second, Endo180 has been identified as part of a trimolecular cell surface complex together with prourokinase plasminogen activator (prouPA) and the uPA receptor (uPAR), hence its alternative name of uPAR associated protein, uPARAP.
Fig.2 Structure of Endo180 showing the FNII domain located between the cysteine-rich domain (cys) and eight C-type lectin-like domains (CTLD). (Wienke, 2003)
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References
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Quarles, R.H. Myelin-associated glycoprotein (MAG): past, present and beyond. Journal of neurochemistry. 2007, 100(6): 1431-1448.
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Wienke, D.; et al. Identification and characterization of the endocytic transmembrane glycoprotein Endo180 as a novel collagen receptor. Molecular biology of the cell. 2003, 14(9): 3592-3604.
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