Molecular mechanism of S-modulin action: binding target and effect of ATP.
S-Modulin is suggested to increase the light sensitivity of rods by inhibiting phosphorylation of light-activated rhodopsin (Rh*) at high Ca2+ concentrations. The inhibition of rhodopsin phosphorylation was almost constant over a wide range of the Rh*/S-modulin ratio (10(-4)-approximately 10[1]). A 125I-labeled cross-linker that had been conjugated with S-modulin interacted with a protein of 60 kDa, a molecular mass close to that of frog rhodopsin kinase. These results suggested that the target molecule of S-modulin is rhodopsin kinase. To investigate the mechanism of the S-modulin action, we measured rhodopsin phosphorylation in the presence and absence of inhibition by S-modulin at various timings of ATP addition. The results suggested the following in situ mechanism of S-modulin action. After light-activation of rhodopsin kinase, the S-modulin/Ca2+ complex binds to the activated kinase and inhibits the phosphorylation of rhodopsin. The complex, however, does not affect the overall kinetics of the phosphorylation. The inhibition of the kinase by S-modulin is reversible in terms of the Ca2+ concentration. On the other hand, the kinase activity decreases as a function of time, probably via autophosphorylation.
Sato
N
N
Department of Physiology, Keio University School of Medicine, Tokyo.
Kawamura
S
S
D000255:Adenosine Triphosphate / Q000378:metabolism / Q000494:pharmacology / Q000502:physiology*; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D002135:Calcium-Binding Proteins / Q000378:metabolism / Q000494:pharmacology / Q000502:physiology*; D005136:Eye Proteins; D051606:G-Protein-Coupled Receptor Kinase 1; D051597:Hippocalcin; D007700:Kinetics; D008074:Lipoproteins; D009419:Nerve Tissue Proteins; D010766:Phosphorylation / Q000187:drug effects; D011494:Protein Kinases / Q000378:metabolism; D011892:Rana catesbeiana; D051603:Recoverin; D012243:Rhodopsin / Q000378:metabolism; D012374:Rod Cell Outer Segment / Q000187:drug effects / Q000378:metabolism / Q000502:physiology
D002135:Calcium-Binding Proteins; D005136:Eye Proteins; D008074:Lipoproteins; D009419:Nerve Tissue Proteins; D051603:Recoverin; D051597:Hippocalcin; D000255:Adenosine Triphosphate; D012243:Rhodopsin; D011494:Protein Kinases; D051606:G-Protein-Coupled Receptor Kinase 1; D002118:Calcium