Light regulated biochemical events in invertebrate photo receptors 1. light activated gtpase guanine nucleotide binding and cholera toxin catalyzed labeling of squid loligo opalescens photo receptor membranes/

Vandenberg, C.A.; Montal, M.

Biochemistry 23(11): 2339-2347


ISSN/ISBN: 0006-2960
Accession: 005810961

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The occurrence of a guanine nucleotide binding protein activated by squid rhodopsin was established by examination of GTPase activity, guanine nucleotide binding and cholera toxin catalyzed labeling of squid photoreceptor membranes. Purified squid (L. opalescens) photoreceptors exhibited GTPase activity that increased 3-4-fold by illumination. Half-maximal GTPase activity was observed when 2% of the rhodopsin was photoconverted to metarhodopsin. The Km of the light-regulated activity was 1 .mu.M GTP. Binding of the hydrolysis-resistant GTP analogue guanosine 5'-(.beta.,.gamma.-imidotriphosphate) [Gpp(NH)p] was enhanced > 10 times by illumination. A protein, MW 44,000, was identified as a component of the light-activated guanine nucleotide binding protein/GTPase through its specific labeling with [32P]NAD catalyzed by cholera toxin: light increased the extent of 32P incorporation 7-fold. The addition of ATP to the membrane suspension enhanced labeling; guanine nucleotides inhibited labeling with the relative potency GTP.gamma.S .mchgt. GDP > GTP > Gpp(NH)p. The 44,000-dalton protein was membrane bound irrespective of variations in ionic strength and divalent ion concentration over a wide range. A G protein, which incorporates GTP binding and hydrolysis functions, probably is intimately involved in the visual process of invertebrate photoreceptors.