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

Abstract
Phospholipase C (PLC) was purified from squid retina. Soluble Gqα, membrane Gqα and Gβγ were isolated from GTPγS-treated and light-illuminated photoreceptor membranes. The membrane Gqα stimulated phosphatidyl inositoi-phospholipase C (PI-PLC) activity in a dose-dependent manner. Soluble Gqα and membrane Gβγ showed no stimulating effects on PLC. GTPγS-binding was found exclusively in membrane fraction, with very little present in the KCl-soluble fraction which contained soluble Gqα. These results indicate that light-activated rhodopsin activates PLC through membrane-bound Gqα and suggest that the rhodopsin/Gq/PLC cascade might be the pathway of phototransduction in squid photoreceptors