Daisuke Arinobu

Molecular Mechanisms Characterizing Cone Photoresponses

In the vertebrate retina, rods mediate twilight vision and cones mediate daylight vision. Their photoresponse characteristics are different. The light‐sensitivity of a cone is 102–103 times lower than that of a rod. In addition, the photoresponse time course is much faster in cones. The mechanism characterizing cone photoresponses has not been known mainly because of the difficulty in isolating cones in large quantities to perform biochemistry. Recently, we developed a method to purify cones from carp retina using a density gradient, which made it possible to analyze the differences in the molecular mechanism of phototransduction between rods and cones. The results showed that signal amplification in cones is less effective, which explains the lower light‐sensitivity of cones. The results also showed that visual pigment phosphorylation, a quenching mechanism of light‐activated visual pigment, is much more rapid in cones than in rods. The rapid phosphorylation in cones is attributed to a very high total kinase activity in cones. Because of this high activity, cone pigment is readily phosphorylated even at very high bleaching levels, which probably explains why cone photoresponses recover quickly. Based on these findings, the molecular mechanisms of the differences in the photoresponse characteristics between rods and cones are outlined.

Larger inhibition of visual pigment kinase in cones than in rods

J. Neurochem. (2010) 115, 259–268.

Amino acid residues in GRK1/GRK7 responsible for interaction with S-modulin/recoverin.

GRK1 is a visual pigment kinase in rods and is essential for inactivation of light‐activated rhodopsin. The GRK1 activity is inhibited by binding of the Ca2+‐bound form of S‐modulin/recoverin. We previously identified the S‐modulin/recoverin site to interact with GRK1. In the present study, we identified its counterpart in GRK1. We synthesized 29 of GRK1 or GRK7 partial peptides that cover the entire sequence of GRK1/GRK7, and examined whether these peptides inhibit S‐modulin/recoverin activity most probably by preoccupying the binding site for GRK1. The inhibition was the greatest with the N‐terminal peptide (p1, aa 3–23 in GRK7). On mutation of each of eight amino acid residues highly conserved in the p1 region of more than 10 orthologs, the inhibition was significantly reduced in the mutation of Leu6, Asn12 and Tyr15. We further examined the binding of the peptides, including mutated ones, to S‐modulin/recoverin with a resonance mirror biosensor. The binding correlated well with the degree of the inhibition by a peptide. The inhibition, therefore, seemed to be due to a direct binding of the kinase peptide to the binding site of active S‐modulin/recoverin. A GRK1 region close to its C‐terminus also seemed to be the binding site for S‐modulin/recoverin.