Regulatory GTP-binding proteins: Emerging concepts on their role in cell function

The last few years have evidenced a tremendous expansion in our appreciation of the role of regulatory GTP-binding proteins in cellular activation. The availability of cholera and pertussis toxins to detect G proteins as well as methodological advances in the study of cellular function has afforded the opportunity to examine G protein participation in many cellular events. Regulation of adenylyl cyclase and cyclic GMP phosphodiesterase by G proteins has been demonstrated. Phosphatidylinositol-4,5-bisphosphate specific phospholipase C activity appears to be subject to G protein control. G proteins regulate inward K⁺ and Ca²⁺ channels through a mechanism which may be independent of effects on the above mentioned enzymes. Certainly, the number of G proteins which have been identified from sequencing of complementary DNA affords the potential for G protein involvement in many cellular events. Only three G proteins have however been isolated and functionally characterized, G_s, G_i and transducin. Whether all the functions of these proteins have been identified remains to be seen.