The carboxyl terminus of bovine rhodopsin is not required for G protein activation

Rhodopsin, the photoreceptor of mammalian rod cells, shares regions of structural homology with many G protein-coupled receptors. One of these domains is the "fourth cytoplasmic loop" formed by palmitoylation of two cysteines (Cys-322 and Cys-323) in the carboxyl terminus. Evidence from several laboratories suggests that this domain is important for the activation of the G protein for rhodopsin, Gt, and that it undergoes conformational changes upon exposure to light. Previously we reported that a truncation mutant with only six amino acids remaining at the proximal end of the carboxyl terminus was able to activate Gt, whereas a mutant lacking an additional five amino acids was misfolded and unable to bind retinal. In the present report, these six amino acids were mutated, to define their roles in the formation of a functional photoreceptor and in the activation of Gt. All of the point mutants displayed normal expression, post-translational processing, and Gt activation, suggesting that the fourth cytoplasmic loop in the carboxyl terminus does not play a major role in the activation of G proteins and that the specific amino acid sequence in this domain is not required for the production of a properly folded, functional photoreceptor.