Species-specific differences in expression of G-protein-coupled receptor kinase (GRK) 7 and GRK1 in mammalian cone photoreceptor cells: Implications for cone cell phototransduction

Desensitization plays an important role in the rapid termination of G-prote in signaling pathways. This process, which involves phosphorylation by a G- protein-coupled receptor kinase (GRK) followed by arrestin binding, has bee n studied extensively in the rod photoreceptor cell of the mammalian retina . In contrast, less is known regarding desensitization in cone photorecepto r cells, which occurs more rapidly than in rod cells. Recently, our laborat ory has cloned a novel GRK family member, GRK7, from the retina of a cone-d ominant mammal, the 13-lined ground squirrel. Here we report the cloning of GRK7 from rod-dominant pig and human retinas, suggesting that this kinase plays a role in human visual signaling. Because GRK1 (rhodopsin kinase), th e GRK that mediates rhodopsin desensitization in the rod cell, is reportedl y expressed in both rods and cones, a detailed comparison of the localizati on of the two kinases is a necessary step toward determining their potentia l roles in cone visual signaling. Immunocytochemical analysis using antibod ies selective for these two GRKs unexpectedly demonstrated species-specific differences in GRK7 and GRK1 expression in cones. In pigs and dogs, cones express only GRK7, whereas in mice and rats, we detected only GRK1 in cones . These results suggest that either GRK7 or GRK1 may participate in cone op sin desensitization, depending on the expression pattern of the kinases in different species. In contrast, GRK7 and GRK1 are coexpressed in monkey and human cones, suggesting that coordinate regulation of desensitization by b oth kinases may occur in primates.