Activation of the GTP-binding protein Gq by rhodopsin in squid photoreceptors

Abstract

Photoaffinity labelling by a GTP analogue has been used to identify a 42 kDa band as the major G alpha subunit in squid photoreceptor membranes, recently identified by partial sequence analysis to be a member of the Gq sub-group of GTP-binding proteins [Pottinger, Ryba, Keen & Findlay (1991) Biochem. J. 279, 323-326]. Guanine-nucleotide-binding displacement analysis gave a stoichiometry of 1 G-protein per 12.5 rhodopsin molecules, the same as in vertebrate rod photoreceptors. Binding was not detected above background in the dark, but was rapidly activated by light. Unlike vertebrate transducin, this G-protein is very temperature-sensitive. GTP binding is maximal at temperatures less than 10 degrees C and is much decreased after several minutes above 18 degrees C. The light-stimulated GTPase rate is maximal around 10 degrees C, above which the loss of binding sites counteracts the increase in hydrolytic rate per site. Earlier studies described light-sensitive G alpha components of 40 and 45 kDa, by ADP-ribosylation in the presence of cholera and pertussis toxins. These are now shown to be very minor components, as the prolonged treatment at elevated temperature required for ADP-ribosylation is sufficient to inactivate the major G alpha totally. Unlike the minor G alpha components, the 42 kDa G alpha is not inhibited by Ca2+.

Immunological demonstration of Gq-protein inLimulusphotoreceptors

“…It is not clear at which step the cascade branches. However, in the same species (Loligo and Alloteuthis), a G-protein of the G q type has been reported by others to be the only or at least the major GTPbinding protein (Pottinger et al,1991;Nobes et al, 1992). In line with this argument is the finding that, in the electrophysiological study with intracellularly injected antibody against G q (Contzen & Nagy, 1995, see above), only one of the three components in the receptor current was suppressed.…”

Squid photoreceptor phospholipase C is stimulated by membrane Gqα but not by soluble Gqα

“…3a and also [25]). The next reaction, the stimulation of PLC by GTP-Gqc~, also was quite rapid, with little apparent time-lag (see Fig.…”

Interaction of GTP‐binding protein Gq with photoactivated rhodopsin in the photoreceptor membranes of crayfish

“…has been calculated to be about 10% of the rhodopsin [13,14], indicating that Gq is probably an important component of the photoreceptor cell membranes.…”

“…In mammalian systems, subclass q G-protein (Gq) activates PLC-p [2,8,9] and the occurrence of a Gq-type of G-protein in the photoreceptor cells of several invertebrates has been reported (Drosophila [lo, 111, crayfish, shrimp and octopus [12], and squid [12,13]). There is evidence for the activation of guanine-nucleotide-binding and GTPase activities of Gq by irradiation of squid photoreceptor membranes [14] and the relative amount of squid Gq Gt, rod transducin; HPLC, high-performance liquid chromatography; PDE, phosphodiesterase; PLC, phospholipase C; SDS-PAGE, sodium dodecyl sulphate-polyacrylamide gel electrophoresis.…”