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 GTP-binding protein, Gq, which in turn activates a phospholipase C (PLC) enzyme. Gqalpha is a membrane-associated protein that is progressively released from the membrane by washing with buffers containing increasing concentrations of beta-mercaptoethanol (beta-ME). Isolated, soluble Gqalpha showed a decreased ability to be activated by rhodopsin but was more active in stimulating PLC when compared with the membrane-associated form of Gqalpha. The calcium-activated protease, calpain, selectively cleaved the soluble but not the membrane-bound form of Gqalpha. Calpain cleaved a small peptide from the amino-terminus of Gqalpha reducing the ability of the G-protein to bind GTP. The uncoupling of Gqalpha from rhodopsin and subsequent calcium-dependent proteolysis to further inactivate the G-protein may therefore be a regulatory mechanism of light adaptation in rhabdomeric photoreceptors.
Bamsey
C
C
Department of Pharmacology, University of Toronto, Ont., Canada.
Mayeenuddin
L H
LH
Cheung
R
R
Mitchell
J
J
D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002154:Calpain / Q000494:pharmacology*; D002462:Cell Membrane / Q000378:metabolism*; D020355:Cholates / Q000494:pharmacology; D049832:Decapodiformes; D003902:Detergents / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004591:Electrophoresis, Polyacrylamide Gel; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D020962:Heterotrimeric GTP-Binding Proteins / Q000378:metabolism*; D008623:Mercaptoethanol / Q000494:pharmacology; D010786:Photoreceptor Cells / Q000737:chemistry; D012243:Rhodopsin / Q000378:metabolism*; D013997:Time Factors; D010738:Type C Phospholipases / Q000378:metabolism
D020355:Cholates; D003902:Detergents; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D008623:Mercaptoethanol; D006160:Guanosine Triphosphate; D012243:Rhodopsin; D010738:Type C Phospholipases; D002154:Calpain; D020962:Heterotrimeric GTP-Binding Proteins; D002118:Calcium