Receptor Classification By Ligand Size

The crystal structure of bovine rhodopsin2 is considered to be applicable to all of the GPCR superfamily, with the a-helices traversing the lipid bilayer seven times. Bacteriorhodopsin and the many other prokaryotic opsins are not GPCRs, although their sequences and structures imply relatedness. A recent genomics survey of 16 species of Archaea and 96 species of bacteria identified two widespread families of novel 7TMRs that exhibit a bewildering array of amino and carboxyl terminal acquisitions, via gene fusion, characteristic of extracellular substrate-binding molecules and intracellular adenylate cyclases, histidine kinases, phosphatases, and other effectors typical of prokaryotic signal transduction pathways.3

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