Abstract
Previous studies [Yu, H., Kono, M., and Oprian, D. D. (1999) Biochemistry38, xxxx−xxxx] using split receptors and disulfide cross-linking have shown that native cysteines 140 and 222 on the cytoplasmic side of transmembrane segments (TM) 3 and 5 of rhodopsin, respectively, can cross-link to each other upon treatment with the oxidant Cu(phen)32+. In this paper we show that although the 140−222 cross-link does not affect the spectral properties of rhodopsin, it completely and reversibly inactivates the ability of the receptor to activate transducin. Following on this lead we further investigate the cytoplasmic region of TM3 and TM5 and identify three additional pairs of residues that when changed to Cys are capable of forming disulfide cross-links in the protein: 140/225, 136/222, and 136/225. These disulfides are able to form without addition of the Cu(phen)32+ oxidant. Similar to the 140−222 cross-link, none of the additional disulfides affect the spectral properties of rhodopsin. Also like the 140−222 bond, the 136−222 disulfide completely and reversibly inactivates the light-dependent activation of transducin by the receptor. In contrast, the 140−225 and 136−225 disulfides have no effect on the ability of rhodopsin to activate transducin. The pattern of cross-linking observed in Cys and disulfide scans of the protein is consistent with helical secondary structure in TM3 from 130 to 142 and in TM5 from 218 to 225.