CLONING OR P57(KIP2), A CYCLIN-DEPENDENT KINASE INHIBITOR WITH UNIQUE DOMAIN-STRUCTURE AND TISSUE DISTRIBUTION

Progression through the cell cycle is catalyzed by cyclin-dependent kinases (CDKs) and is negatively controlled by CDK inhibitors (CDIs). We have isolated a new member of the p21(CIP1)/p27(KIP1) CDI family and named it p57(KIP2) to denote its apparent molecular mass and higher similarity to p27(KIP1). Three distinct p57 cDNAs were cloned that differ at the start of their open reading frames and correspond to messages generated by the use of distinct splice acceptor sites. p57 is distinguished from p21 and p27 by its unique domain structure. Pour distinct domains follow the heterogeneous amino-terminal region and include, in order, a p21/p27-related CDK inhibitory domain, a proline-rich (28% proline) domain, an acidic (36% glutamic or aspartic acid) domain, and a carboxy-terminal nuclear targeting domain that contains a putative CDK phosphorylation site and has sequence similarity to p27 but not to p21. Most of the acidic domain consists of a novel, tandemly repeated 4-amino acid motif. p57 is a potent inhibitor of G(1)- and S-phase CDKs (cyclin E-cdk2, cyclin D2-cdk4, and cyclin A-cdk2) and, to lesser extent, of the mitotic cyclin B-Cdc2. In mammalian cells, p57 localizes to the nucleus, associates with G(1) CDK components, and its overexpression causes a complete cell cycle arrest in G(1) phase. In contrast to the widespread expression of p21 and p27 in human tissues, p57 is expressed in a tissue-specific manner, as a 1.5-kb species in placenta and at lower levels in various other tissues and a 7-kb mRNA species observed in skeletal muscle and heart. The expression pattern and unique domain structure of p57 suggest that this CDI may play a specialized role in cell cycle control.