Movement of retinal along the visual transduction path

Movement of the ligand/receptor complex in rhodopsin (Rh) has been traced. Bleaching of diazoketo rhodopsin (DK-Rh) containing 11-cis-3-diazo-4-oxo-re tinal yields batho-, lumi-, meta-I-, and meta-II-Rh intermediates correspon ding to those of native kh but at lower temperatures. Photoaffinity labelin g of DK-Rh and these bleaching intermediates shows that the ionone ring cro ss-links to tryptophan-265 on helix F in DK-Rh and batho-Rh, and to alanine -169 on helix D in lumi-, meta-I-, and meta-II-Rh intermediates, It is like ly that these movements involving a flip-over of the chromophoric ring trig ger changes in cytoplasmic membrane loops resulting in heterotrimeric guani ne nucleotide-binding protein (G protein) activation.