Mutagenesis Studies of Rhodopsin Phototransduction

Site-directed mutagenesis was employed in structure-function studies of bovine rhodopsin. Charged amino acids in the third transmembrane helix (Glut 13 G1u122, Glu'34, and Arg135) were systematically replaced We conclude that Glut13 serves as the counterion to the retinylidene Schiff base. The second and third cytoplasmic loops linking transmernbrane helices were studied by analysis of amino acid deletion and replacement mutations. Certain alterations of these loops produced rhodopsin mutants that bound but failed to activate transducin