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

Abstract

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

Eye evolution: common use and independent recruitment of genetic components

“…The different morphology of rhabdomeric and ciliary photoreceptors is further reflected in the level of phototransduction cascades operating in these cells, although some common elements, such as opsins or arrestins, participate in both. The rhabdomeric phototransduction cascade, which is mediated by Ga q and phospholipase C (Suzuki et al 1995), seems to be evolutionarily conserved from protostomes to vertebrate retinal ganglion cells (Koyanagi et al 2005;Panda et al 2005;Contin et al 2006;Graham et al 2008). In contrast, the ciliary photoreceptors may employ both G o -opsin (Kojima et al 1997) and c-opsins, whose Ga protein partners may be rather variable.…”

“…The rhabdomeric phototransduction cascade, which is mediated by Ga q and phospholipase C (Suzuki et al 1995), seems to be evolutionarily conserved from protostomes to vertebrate retinal ganglion cells (Koyanagi et al 2005;Panda et al 2005;Contin et al 2006;Graham et al 2008). In contrast, the ciliary photoreceptors may employ both G o -opsin (Kojima et al 1997) and c-opsins, whose Ga protein partners may be rather variable.…”