Membrane-bound receptor superfamilies

Receptors can be broadly sorted into superfamilies based on their structure and mode of action. The membrane-bound receptor superfamilies are G-protein-coupled, kinase-linked, and ion channel-linked receptors. Superfamilies are divided into families named after the natural, or endogenous, ligand that binds all the members of a given family. Unnatural, or exogenous, ligands can show selectivity for specific receptors within a family and allow further subdivision.2... 

SNAREs is an acronym for soluble NSF acceptor protein receptors. They are a superfamily of small and mostly membrane-bound proteins that are distinguished by the presence of a conserved stretch of 60 amino acids referred to as a SNARE motif. With few exceptions, a single transmembrane domain is located adjacent to the SNARE motif at the C-terminal end. Many SNAREs possess in addition an independently folded N-terminal domain whose structures are more diverse. 

MAPK kinase (MAPKK). MAPK kinase itself is activated by phosphorylation by still another protein kinase, termed MAPK kinase kinase (MAPKKK). MAPK kinase kinase is activated upon interaction with a member of the Ras superfamily of small G proteins, which are bound to the plasma membrane (see Ch. 19). The exact mechanism of activation remains unknown, but it is believed that Ras and related proteins, in the activated GTP-bound form, can bind MAPK kinase kinase and thereby draw the kinase to the plasmalemma, where it is activated by as yet unknown factors, perhaps even an additional kinase, MAPK kinase kinase kinase (MAPKKKK). The mechanism governing the activation of Ras and related proteins by extracellular signals is quite complex and involves numerous Tinker proteins, for example She, Grb and Sos, that couple Ras to a variety of plasmalemma-associated growth factor-protein tyrosine kinase receptors (see Chs 20,24 and 27). 

The receptor SR-BI differs In two Important respects from the LDL receptor. First, SR-BI clusters on microvilli and in cell-surface lipid rafts (Chapter 5), not In coated pits as does the LDL receptor. Second, SR-BI mediates the transfer of lipids across the membrane, not endoc3rt osIs of entire LDL particles as mediated by the LDL receptor. A multifunctional receptor, SR-BI can mediate the selective uptake from lipoproteins of diverse lipids (e.g., cholesteryl esters, vitamin E) it also functions in the reverse direction to facilitate the export of unesterified cholesterol from cells to bound lipoproteins. SR-BI has a structure similar to that of the fatty acid transporter CD36, and they both belong to the superfamily of scavenger receptors as discussed later, some of these receptors apparently play a role In the onset of atherosclerosis. 

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