Heterogeneity of the retinal G-protein transducin from frog rod photoreceptors. Biochemical identification and characterization of new subunits.

Transducin, a retinal G-protein, has been shown to exist as heterotrimers of alpha (39,000), beta (36,000), and gamma (approximately 7,000) subunits. Blue Sepharose CL-6B column chromatography of a transducin preparation extracted with a metal-free, low salt buffer containing GTP showed three distinct alpha and two distinct beta gamma activities in frog (Rana catesbeiana) rod outer segment. The binding of a hydrolysis-resistant GTP analog in these alpha fractions was proportional to the amount of the M(r) 39,000 protein. The first alpha was eluted in a complex with an inhibitory subunit of cGMP phosphodiesterase, but alpha subunits in the second and the third fractions were not complexed with any proteins. Two-dimensional gel electrophoresis and characterization with regard to the interaction with the inhibitory subunit of cGMP phosphodiesterase suggested that the first and the second alpha s were the same protein; however, the third alpha showed different characters as follows. We designated alpha in the first two fractions as alpha 1, and alpha in the third fraction as alpha 2. Nonlinear regression analysis for the binding of a hydrolysis-resistant GTP analog to both alpha subunits revealed a single class of GTP binding sites with an apparent stoichiometry of 1 mol of GTP/mol of alpha. Compared with alpha 1, alpha 2 required larger amounts of rhodopsin and beta gamma for the binding of a hydrolysis-resistant GTP analog. alpha 2 also showed less binding with the inhibitory subunit of cGMP phosphodiesterase. Both alpha 1 and alpha 2 complexed with beta gamma or beta delta (described below) were substrates for pertussis toxin-dependent ADP-ribosylation. The protein profiles of two beta gamma fractions revealed that the main fraction was composed of a beta gamma complex; however, the second active fraction was composed of beta complexed with delta (M(r) 12,000). Compared with beta gamma, beta delta stimulated GTP binding to alpha 1 at approximately 10-fold higher concentration. Two-dimensional gel electrophoresis revealed five beta and two gamma isoforms in beta gamma. Only one beta isoform was present in beta delta. The diversity of transducin subunits may reflect different signaling pathways in visual signal transduction.