Functional differences in the βγ complexes of transducin and the inhibitory guanine nucleotide regulatory protein

The authors have examined the mechanism of inhibition of adenylate cyclase using the purified α and βγ subunits of bovine brain inhibitory guanine nucleotide regulatory protein (N/sub i/) (i.e., α/sub i/ and βγ/sub N/) and bovine retinal transducin (α/sub T/ and βγ/sub T/) in reconstituted phospholipid vesicle systems. The addition of βγ/sub N/ or βγ/sub T/ to lipid vesicles containing the pure stimulatory guanine nucleotide regulatory protein (N/sub s/) from human erythrocytes as well as a resolved preparation of the catalytic moiety (C) of bovine caudate adenylate cyclase results in significant inhibition of guanine nucleotide stimulated cyclase activity (80-90%). The inhibition by these βγ subunit complexes appears to fully account for the inhibitory effects observed with holo-N/sub i/ or holotransducin. A variety of structure-function comparison of the βγ/sub N/ and βγ/sub T/ complexes were performed in order to further probe the molecular mechanisms involved in the inhibitory pathway. Whereas the β subunits of βγ/sub N/ and βγ/sub T/ appear to be very similar, if not identical, on the basis of comparisons of their gel electrophoretic mobility and immunological cross-reactivity, clear differences exist in the apparent structures of γ/sub N/ and γ/sub T/. The functional differences between the βγ subunits of brain N/sub i/ and retinal transducin may reflect the clear structural differences in their γ subunits, thus suggesting a possible role for γ in the inhibition of adenylate cyclase by N/sub i/. Overall, these results indicate that the βγ complexes, as well as the α subunits, may have an important role in imparting the functional specificity necessary for nucleotide regulatory protein mediated signal transduction