Autoinhibition of casein kinase I ε (CKIε) is relieved by protein phosphatases and limited proteolysis

Abstract

Casein kinase I ε (CKIε) is a member of the CKI gene family, members of which are involved in the control of SV40 DNA replication, DNA repair, and cell metabolism. The mechanisms that regulate CKIε activity and substrates specificity are not will understood. We report that CKIε, which contains a highly phosphorylated 123-amino acid carboxyl-terminal extension not present in CKIα, is substantially less active than CKIα in phosphorylating a number of substrates including SV40 DNA replication. Two mechanisms for the activation of CKIε have been identified. First, limited tryptic digestion of CKIε produces a protease-resistant amino-terminal 39-kDa core kinase with several-fold enhanced activity. Second, phosphatase treatment of CKIε activates CKIε 5-20-fold toward T antigen. Similar treatment of a truncated form of CKIε produced only a 2-fold activation. Notably, this activation was transient; reautophosphorylation led to a rapid down-regulation of the kinase within 5 min. Phosphatase treatment also activated CKIε toward the novel substrate IκBα and Ets-1. These mechanisms may serve to regulate CKIε and related forms of CKI in the cell, perhaps in response to DNA damage.