Autophosphorylation of rhodopsin kinase from retinal rod outer segments

Rhodopsin kinase has been identified as a 68K protein that is more readily extracted from dark-adapted rod outer segments (dark-extract) than from illuminated rod outer segments (light-extract). We observed that a 68K protein is phosphorylated by endogenous protein kinase of dark- or light-extract of bovine rod outer segments and that the amount of incorporated radioactivity (32P) was greater in the dark-than in the light-extract. Phosphorylation of the 68K protein is neither stimulated by cyclic nucleotides nor affected by the light or dark conditions of the phosphorylation reaction. Light-and dark-extracts were centrifuged simultaneously on individual sucrose density gradients revealing that the 68K phosphoprotein cosediments with endogenous rhodopsin kinase activity and that both greater 32P incorporation and higher rhodopsin kinase activity are found in dark-extract as compared to light-extract. These findings suggest strongly that the 68K phosphoprotein and rhodopsin kinase are identical and that rhodopsin kinase undergoes autophosphorylation.