Regulation of squid visual phospholipase C by activated G-protein alpha

Phospholipase C (PLC) is the key enzyme in the phototransduction cascade of invertebrate rhabdomeric photoreceptors. In addition to 130 kDa PLC, a 95 kDa protein recognized by antibody against the catalytic site of PLC was fo und in the squid retina. The PLC-like 95 kDa protein (95 kDa PLC) was produ ced from 130 kDa PLC by an intrinsic protease in the presence of calcium. T he 130 kDa PLC was stimulated by the active form of Gq-class G-protein alph a (Gq alpha), but the 95 kDa PLC was not, although their PLC activity was s imilar. A 35 kDa fragment, the counterpart of 95 kDa PLC, was not recognize d by antibodies against catalytic site or N-terminal site of the 130 kDa PL C, indicating that the cleavage site is on the C-terminal side beyond the c atalytic site. In the presence of a large excess of the 35 kDa fragment, 95 kDa PLC was stimulated by Gq alpha to a similar extent as intact 130 kDa P LC. These results indicate that the C-terminal polypeptide of PLC is necess ary for regulation of its enzyme activity by Gq alpha. The uncoupling of PL C from Gq alpha, caused by limited proteolysis, is therefore a candidate re gulatory mechanism of the phototransduction cascade in rhabdomeric photorec eptors. (C) 1999 Elsevier Science Inc. All rights reserved.