Dissociation of G-protein alpha from rhabdomeric membranes decreases its interaction with rhodopsin and increases its degradation by calpain

Photoactivation of invertebrate rhodopsin activates a CTP-binding protein, G(q). which in turn activates a phospholipase C (PLC) enzyme. G(q)alpha is a membrane-associated protein that is progressively released from the membr ane by washing with buffers containing increasing concentrations of beta-me rcaptoethanol (beta-ME). Isolated, soluble G(q)alpha showed a decreased abi lity to be activated by rhodopsin but was more active in stimulating PLC wh en compared with the membrane-associated form of G(q)alpha. The calcium-act ivated protease, calpain, selectively cleaved the soluble but not the membr ane-bound form of G(q)alpha. Calpain cleaved a small peptide from the amino -terminus of G(q)alpha reducing the ability of the G-protein to bind GTP. T he uncoupling of G(q)alpha from rhodopsin and subsequent calcium-dependent proteolysis to further inactivate the G-protein may therefore be a regulato ry mechanism of light adaptation in rhabdomeric photoreceptors. (C) 2000 El scvicr Science Inc. All rights reserved.