PURIFICATION, CHARACTERIZATION, AND PARTIAL AMINO-ACID-SEQUENCE OF A G-PROTEIN-ACTIVATED PHOSPHOLIPASE-C FROM SQUID PHOTORECEPTORS

Invertebrate visual transduction is thought to be initiated by photoactivation of rhodopsin and its subsequent interaction with a guanyl nucleotide-binding protein (G protein), The identities of the G protein and its target effector have remained elusive, although evidence suggests the involvement of a phospholipase C (PLC). We have identified a phosphatidylinositol-specific PLC from the cytosol of squid retina, The enzyme was purified to near-homogeneity by a combination of carboxymethyl-Sepharose and heparin-Sepharose chromatography. The purified PLC, identified as an approximately 140-kDa protein by sodium dodecyl sulfate-polyacrylamide gels, hydrolyzed phosphatidylinositol 4,5-bisphosphate (PIP2) at a rate of 10-15 mu mol/min/mg of protein with 1 mu M Ca2+. The partial amino acid sequence of the protein showed homology with a PLC cloned from a Drosophila head library (PLC21) and lesser homology with Drosophila norpA protein and mammalian PLC beta isozymes. Reconstitution of purified squid PLC with an AIF(-)-activated 44-kDa G protein a subunit extracted from squid photoreceptor membranes resulted in a significant increase in PIP2 hydrolysis over a range of Ca2+ concentrations while reconstitution with mammalian G(t) alpha or G(i)1 alpha was without effect, These results suggest that cephalopod phototransduction is mediated by G alpha-44 activation of a 140-kDa cytosolic PLC.