Serine/threonine protein phosphatases type 1, 2A and 2C in vertebrate retinae

A number of retinal proteins are phosphorylated by a variety of kinases, re sult ing in well-known regulatory effects. The identity and role of corresp onding phosphatases is less clear. We simultaneously measured the activity of serine/threonine protein phosphatases type 1, 2A and 2C in bovine retina e. The enzymes were classified according to substrate specificity, divalent cation require ment and the effect of phosphatase subtype-specific inhibit ors. The total- and specific activity of phosphatase type 2A was prevalent. Type 2C was 10-fold less abundant. 80% of type 1 and 50% of type 2A and ty pe 2C, respectively, were soluble. An economic purification scheme was deve loped. We demonstrated the presence of phosphatase isozymes 2C alpha and 2C beta in bovine rod outer segments by enzymatic analysis as well as immunol ogical techniques. The results suggest a yet unknown role of phosphatase ty pe 2C in phototransduction. On the other hand, the immense amount of protei n phosphatases found to be soluble - therefore not associated with rod oute r segment membranes - points towards participation of these enzymes in the process of visual transduction not considered thus far.