Chemical cross-linking of bovine retinal transducin and cGMP phosphodiesterase

The bifunctional reagents para-phenyldimaleimide and maleimidobenzoyl-N-hydroxysuccinimide ester were used to chemically cross-link the subunits of the transducin and cGMP phosphodiesterase (PDE) complexes of bovine rod photoreceptor cells. The cross-linked products were identified by Western immunoblotting using antisera against purified subunits of transducin (T alpha and T beta gamma) and PDE. Oligomeric cross-linked products of transducin subunits as large as (T alpha beta gamma)3 were observed in the latent form of transducin with bound GDP. In addition to the expected T alpha beta and T beta gamma cross-linked products, a (T alpha gamma)2 structure was detected. The close proximity of T alpha and T gamma suggests that T gamma may play a role in conferring the specificity of the interaction between T alpha and rhodopsin. Most of the oligomeric cross-linked structures between T alpha and T beta gamma were diminished in the activated form of transducin, with guanosine 5'-(beta, gamma-imidotriphosphate) (Gpp(NH)p) bound. However, cross-linking between T beta and T gamma was not altered. These results suggest that transducin exists as an oligomer in solution which dissociates upon the binding of Gpp(NH)p. To identify the possible interacting domains between the T alpha, T beta, and T gamma subunits, the cross-linked products were subjected to limited tryptic proteolysis. Several cross-linked tryptic peptides of transducin subunits were found and include the cross-linked products of the N terminus 15-kDa fragment of T beta and the C terminus 5-kDa fragment of T alpha, T gamma and the 12-kDa fragment of T alpha, T gamma and the 15-kDa as well as the 23-kDa fragments of T beta, and an intra-T alpha cross-linked product of the 2- and 21-kDa fragments. These results have allowed the construction of a topographical model for the transducin subunits. The organization of the subunits of PDE (P alpha, P beta, and P gamma) was also studied. The formation of the high molecular size cross-linked products of PDE resulted in the concurrent loss of the P beta and P gamma subunits, suggesting that they are in close proximity. Finally, the interaction between transducin and PDE was examined by chemical cross-linking of transducin-Gpp(NH)p and PDE. Two additional cross-linked products of 180 and 210 kDa were obtained which could be due to the cross-linking of T alpha or T beta with P alpha beta subunits.(ABSTRACT TRUNCATED AT 250 WORDS)