Interspecies conservation of retinal guanosine 5′-triphosphatase. Characterization by photoaffinity labelling and tryptic-peptide mapping

Abstract

Light-activated hydrolysis of cGMP is achieved through the photoexcitation of rhodopsin, a process which then triggers the replacement of GDP for GTP by a retinal guanosine 5''-triphosphatase referred to as transducin. The transducin-GTP complex then switches on the phosphodiesterase. The bovine transducin consists of an .alpha.-subunit (39,000 MW), which is a GTP-binding component, together with a .beta.-(37,000 MW) and a .gamma.-subunit (10,000 MW). Retinal transducin was purified from cow, pig, chick and frog. The enzyme specific activities and sodium dodecyl sulfate/polyacrylamide-gel-electrophoretic profiles indicate that this enzyme is similar in all species except the frog. Whereas the bovine, pig and chick transducins consist of major 37,000- and 39,000-MW components, that of the frog consists of a single 75,000-MW component. Labeling of the GTP-binding components with the photoaffinity label 8-azidoguanosine [.gamma.-32P]triphosphate demonstrated that the 37,000-MW components of the cow, pig and chick and the 75,000-MW component of the frog were major GTP-binding components. In addition, peptide maps of radioiodinated tryptic peptides indicate that the frog 75,000-MW protein is highly related to the pig transducin. These results demonstrate evolutionary conservation of retinal transducin and the presence of a higher-MW, but nonetheless highly conserved form, of transducin in the frog. The relationship of this component to the recently reported rod-outer-segment inhibitor protein is discussed.