TI - Phosphorylation of ovine rhodopsin . Identification of the phosphorylated sites . AB - Light -dependent phosphorylation of sheep opsin was obtained in purified discs to which was added a partially purified preparation of rhodopsin kinase . A maximum ratio of 1.8 mol of phosphate/mol of rhodopsin bleached was obtained . Perturbing the lipid bilayer did not alter the phosphorylation ratio . Dephosphorylation in both segments and discs was only achieved when the supernatant fraction from a retina homogenate was added . Complete dephosphorylation required the presence of the detergent dodecyltrimethylammonium bromide in the incubation medium . Treatment of phosphorylated disc membranes with Staphylococcal aureus V8 proteinase generated two membrane-bound fragments , only one of which ( V8-S , Mr 12 000 ) was labelled , together with a soluble seven - residue peptide that contained [32P] phosphoserine . Peptide sequencing , together with subdigestion procedures , localized the phosphorylation sites to serine residues at positions 334 , 338 and 343 in the whole sequence and threonine residues at positions 335 and 336 .