Identification of a new intermediate state that binds but not activates transducin in the bleaching process of bovine rhodopsin

Abstract
Using time‐resolved low‐temperature spectroscopy, we have examined whether or not bovine rhodopsin has a unique transducin‐binding state, meta Ib, previously detected from chicken rhodopsin. Unlike chicken meta Ib, bovine meta Ibwas detected only by detailed kinetics analysis of the bleaching process, but it was stabilized by transducin and visualized in the observed spectral changes. From the effect of GTPγS, it was revealed that meta Ibinduced no GDP‐GTP exchange reaction in transducin. Thus meta Ibis a common intermediate of vertebrate rhodopsin and transducin is activated in two steps by meta Iband meta II.