ROLE OF COOH-TERMINAL PHOSPHORYLATION IN THE REGULATION OF CASEIN KINASE I-DELTA

Casein kinase I delta is a member of the casein kinase I (CKI) family, a group of second messenger independent protein kinases. We present e vidence that the COOH-terminal domain of CKI delta has regulatory prop erties. CKI delta expressed in Escherichia coli was activated by hepar in, as found previously, and by treatment with the catalytic subunit o f type-1 protein phosphatase (CS1). Concomitant with activation by CS1 , there was a reduction in the apparent molecular weight of CKI delta from 55,000 to 49,000 as judged by polyacrylamide gel electro-phoresis in the presence of sodium dodecyl sulfate. Truncation of CKI delta by removal of the COOH-terminal 110 amino acids eliminated the ability o f CS1 to activate or to increase electrophoretic mobility. Casein kina se I alpha, a 37-kDa isoform that lacks an extended COOH-terminal doma in, was not activated by CS1 or the presence of heparin. However, a ch imeric enzyme consisting of CKI alpha fused to the COOH-terminal domai n of CKI delta was activated by both heparin and CS1. Analysis of the effects of CS1 on a series of CKI delta COOH-terminal truncation mutan ts identified an inhibitory region between His(317) and Pro(342), whic h contained six potential phosphorylation sites. From analysis of the specific activities of these truncation mutants, removal of the same r egion resulted in enzyme with a specific activity nearly 10-fold great er than wild-type. Thus, CKI delta activity can be regulated by phosph orylation of its COOH terminus, which may serve to create an autoinhib itory domain. This mechanism of regulation could have important conseq uences in vivo.