Opsin exhibits cGMP-activated single-channel activity.

Low concentrations of cGMP evoked reversible single-channel currents in patches excised from liposomes that contained purified bovine opsin. Two elementary conductances, of 32 and 17 pS, were observed in the presence of 10-200 microM cGMP. Both individual channel openings (mean open times, approximately 1.6 ms for the 32-pS conductance and approximately 1.0 ms for the 17-pS conductance) and bursts of openings (mean burst duration, approximately 2-3 ms for the large events) were observed. The cGMP-activated channel activity could be observed in the presence or absence of Ca2+. These results raise the possibility that opsin or rhodopsin, an opsin/rhodopsin isoform, or an opsin/isoform multimer serves as a cGMP-modulated pore in the rod outer segment.