title: Chemical modification of bovine transducin: probing the GTP-binding site with affinity analogues.
abstract: The structure of the GTP-binding site of transducin, a signal-transducing G-protein involved in the visual excitation process, was studied by affinity labeling. Radioactive GTP analogues with reactive groups attached to different moieties of the GTP molecule were obtained and include 8-azido-GTP, P3-(4-azidoanilino)-P1-5'-GTP (AA-GTP), 5'-[p-(fluorosulfonyl)benzoyl]guanosine (FSBG), 3'-O-(3-[N-(4-azido-2-nitrophenyl)amino]propionyl)-GTP (ANPAP-GTP), the 2',3'-dialdehyde derivative of GTP (oGTP), and a bifunctional cross-linking analogue, 8-azido-P3-(4-azidoanilino)-P1-5'-GTP (8-azido-AA-GTP). With the exception of FSBG, all of the analogues were found to bind to transducin specifically and serve as a cofactor to activate the retinal cGMP cascade or act as a competitive inhibitor for the GTPase activity of transducin. The labeling sites of these analogues were localized by tryptic peptide mapping. ANPAP-GTP and oGTP were unable to covalently modify transducin, suggesting that the 2'- and 3'-hydroxy groups on the ribose ring of GTP are not in direct contact with the protein. AA-GTP only labeled the T alpha subunit of transducin and was localized on the 21-kDa tryptic fragment of T alpha. This indicates that the phosphate moiety of the bound GTP is in direct contact with this peptide. On the other hand, 8-azido-GTP labeled both the T alpha and T beta gamma subunits of transducin. The labeling on T alpha was on the 12-kDa tryptic fragment, suggesting that the guanine ring binding site is composed of a different peptide fragment than the phosphate binding region. Treatment with the bifunctional analogue 8-azido-AA-GTP generated the cross-linked products of T alpha and T beta gamma. This observation implies that the guanine ring of the bound GTP on T alpha could be in close proximity with T beta gamma.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Biological Chemistry, University of Illinois, Chicago.
authors.forename: V N
authors.initials: VN
authors.lastname: Hingorani
authors.forename: L F
authors.initials: LF
authors.lastname: Chang
authors.forename: Y K
authors.initials: YK
authors.lastname: Ho
mesh_terms: D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D002417:Cattle; D003432:Cross-Linking Reagents; D020558:GTP Phosphohydrolases / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000031:analogs & derivatives / Q000378:metabolism*; D008954:Models, Biological; D015394:Molecular Structure; D010449:Peptide Mapping; D011487:Protein Conformation; D015293:Transducin / Q000378:metabolism*
chemical_list: D003432:Cross-Linking Reagents; D006160:Guanosine Triphosphate; D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins; D015293:Transducin