EXISTENCE OF MULTIPLE PHOSPHORYLATED FORMS OF HUMAN PLATELET ACTIN-BINDING PROTEIN

Platelet actin binding protein (ABP) as isolated from human platelets exists in at least four phosphorylated forms which we have designated ABP-0, ABP-1, ABP-2, and ABP-3 whose phosphate content ranges from 18 (ABP-0) to 40 (ABP-3) moles Pi/mole ABP. These forms differ in their r esistance to calpain cleavage and ability to cross-link F-actin with A BP-3 being the best in each of these properties. Attempts to phosphory late ABP-1, two or three with protein kinase C (PKC) were unsuccessful except if the proteins were pretreated with Escherichia coli alkaline phosphatase. All of the forms could be phosphorylated with cAMP-depen dent kinase (PKA) and subsequent resistance to calpain cleavage confer red. Phosphorylation/dephosphorylation of ABP may be an important regu latory mechanism by which the cytoskeletal architecture is stabilized or transformed.