Mapping of the carboxyl terminus within the tertiary structure of transducin's alpha subunit using the heterobifunctional cross-linking reagent, 125I-N-(3-iodo-4-azidophenylpropionamido-S-(2-thiopyridyl) cysteine

Abstract

A heterobifunctional cross-linking reagent, 125I-N-(3-iodo-4-azidophenylpropionamido-S-(2-thiopyridyl) cysteine (125-ACTP), has been synthesized. 125I-ACTP has been used to derivative reduced sulfhydryls of the retinal G protein, transducin (Gt), to form a mixed disulfide bond under mild, nondenaturing conditions (pH 7.4, 4 degrees C). The resulting disulfide was easily cleaved using reducing reagents. A 200-fold molar excess of 125I-ACTP relative to Gt resulted in the incorporation of 1-1.3 mol of the 125I-N-(3-iodo-4-azidophenylpropionamido)cysteine moiety of ACTP into Gt alpha. In contrast to 125I-ACTP, dithionitrobenzoate and dithiopyridone derivatized six sulfhydryls in native Gt. Incubation of a 10-fold molar excess of 125I-ACTP relative to Gt resulted in the derivatization of 0.75-0.9 and 0.1 mol of reduced sulfhydryls/mol Gt alpha and beta, respectively. Gt gamma was not derivatized by 125I-ACTP. Thus, Gt alpha was preferentially derivatized by 125I-ACTP. Tryptic digestion and amino acid sequencing of Gt alpha indicated that both Cys-347 near the carboxyl terminus and Cys-210 between the second and third consensus sequences forming the GTP-binding site were derivatized by 125I-ACTP in a ratio of approximately 70 and 30%, respectively. Thus, both Cys-210 and Cys-347 are labeled, even though derivatization by 125I-ACTP does not exceed 1 mol of SH/mol Gt alpha. It appears that derivatization of one sulfhydryl, either Cys-210 or Cys-347, excludes labeling of the second cysteine either by steric hindrance or induced conformational change making the second cysteine inaccessible to 125I-ACTP. Consistent with this finding was the observation that pertussis toxin-catalyzed ADP-ribosylation of Cys-347 inhibited 125I-ACTP derivatization of Cys-210. Derivatization of Gt alpha at either Cys-210 or Cys-347 by 125I-ACTP inhibited rhodopsin-catalyzed guanosine 5'-3-O-(thio)triphosphate binding to Gt, mimicking the effect of ADP-ribosylation of Cys-347 by pertussis toxin. ACTP contains a radioiodinated phenylazide moiety which, upon activation, can cross-link the derivatized cysteine to an adjacent polypeptide domain. Following reduction of the disulfide, the [125I] iodophenyl moiety will be transferred to the azide-inserted polypeptide. When photoactivation of the phenylazide moiety of 125I-ACTP after sulfhydryl derivatization was performed, insertion of the Cys-347 which contains Cys-210, was found.(ABSTRACT TRUNCATED AT 400 WORDS)