Relationships within the family of GTP-binding proteins isolated from bovine central nervous system

Four members of a family of GTP-binding proteins (G-proteins) which translate stimulation of extracellular receptors into regulation of intracellular enzymes were isolated from the bovine central nervous system. These proteins were examined for functional similarities and cross-reactivity with antibodies to the G-protein (transductin, G(t)) from the photoreceptor system. Two proteins, G(s) and G(i), can be distinguished by their respective abilities to stimulate or inhibit adenylate cyclase. The activated α subunits of G(t) and a fourth member of the family, G(o), did not affect this enzyme. G(t) was shown to be unique in its ability to stimulate cGMP-dependent phosphodiesterase. While functionally diverse, the G-proteins were shown to have some common antigenic properties. Antibodies directed against the β subunit of G(t) recognize the β₃₆ subunits of all preparations but not a putative second β₃₅ subunit. Antibodies specific for the α subunit of G(t) did not recognize other α subunits when immune blots from sodium dodecyl sulfate gels were examined. However, G(o)α, but not G(s)α or G(i)α, reacted strongly with the antibodies when the native subunit was spotted directly. This suggests that G(o)α and G(t)α have homologous structural determinants. An antiserum that recognized G(t)γ did not recognize γ subunits from other sources. These data support the proposed diversity of function and similarity of structure among the four G-proteins. The α and potentially γ subunits appear to be responsible for the specificity of function.