Phosphorylation of CD45 by Casein Kinase 2

CD45 is a receptor-type protein-tyrosine phosphatase (PTP) that is required for antigen-specific stimulation and proliferation in lymphocytes. This study was designed to determine the nature of specific kinases in lymphocytes that phosphorylate CD45 and to determine the effect of phosphorylation on CD45 PTP activity. A major cytoplasmic lymphocyte kinase that phosphorylated CD45 was identified as casein kinase 2 (CK2) by use of an in-gel kinase assay in combination with immunoprecipitation, immunodepletion, and specific inhibition. Mutational analysis of CK2 consensus sites showed that the target for CK2 was in an acidic insert of 19 amino acids in the D2 domain, and Ser to Ala mutations at amino acids 965, 968, 969, and 973 abrogated CK2 phosphorylation of CD45. CK2 phosphorylation increased CD45 activity 3-fold toward phosphorylated myelin basic protein, and this increase was reversible by PP2A treatment. Mutation of Ser to Glu at the CK2 sites had the same effect as phosphorylation and also tripled the Vmax of CD45. CD45 isolatedin vivo was highly phosphorylated and could not be phosphorylated by CK2 without prior dephosphorylation with phosphatase PP2A. We conclude that CK2 is a major lymphocyte kinase that is responsible for in vivo phosphorylation of CD45, and phosphorylation at specific CK2 sites regulates CD45 PTP activity.