A Retinal Calmodulin‐Dependent Kinase: Calcium/ Calmodulin‐Stimulated and ‐Inhibited States

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Abstract
A calcium/calmodulin‐dependent protein kinase was isolated from retina. The retinal enzyme is composed exclusively of 50‐kilodalton (kD) subunits and has a molecular mass of approximately 275 kD, in contrast to forebrain calmodulin kinase II, which is composed of 50‐kD and 60‐kD subunits in a 3:1 ratio and has a molecular mass of approximately 520 kD. Similar substrate specificities, kinetic properties, capacity to bind calmodulin, and immunoreactivity suggest that the retinal kinase is an isoenzyme of forebrain calmodulin kinase II. Both kinases autophosphorylate in an intramolecular manner; however, auto‐phosphorylation has different effects on the activities of the two enzymes. Autophosphorylation of retinal calmodulin kinase converts the enzyme from a calcium/calmodulin‐dependent to a calcium/calmodulin‐inhibited kinase, with high activity in the absence of calcium, whereas autophosphorylation of the forebrain kinase results in a less active, calcium/calmodulin‐independent enzyme. These properties of calmodulin kinase may play an important role in retinal function.