CLONING, EXPRESSION AND SUBCELLULAR-LOCALIZATION OF THE HUMAN HOMOLOGOF P40(MO15) CATALYTIC SUBUNIT OF CDK-ACTIVATING KINASE

Transitions of the cell cycle are controlled by cyclin-dependent prote in kinases (cdks) whose phosphorylation on the Thr residue included in the conserved sequence YTHEVV dramatically increases the activity. A kinase responsible for this specific phosphorylation, called CAK for c dk-activating kinase, has been recently purified from starfish and Xen opus oocytes and shown to contain the MO15 gene product as a catalytic subunit. In the present paper, we have cloned the human homolog of Xe nopus p40(MO15) by probing a HeLa cell cDNA library with degenerate ol igonucleotides deduced from Xenopus and starfish MO15 sequences. Human and Xenopus MO15 displayed a strong homology showing 86% identity wit h regard to amino acid sequences. Northern blot analysis of RNA extrac ts from a series of human tissues as well as from cultured rodent fibr oblasts revealed a unique 1.4 kb MO15 mRNA. No variation in the amount of MO15 transcript or protein was found along the entire course of th e fibroblast cell cycle. Fluorescence in situ hybridization on human l ymphocyte metaphases showed two distinct chromosomal locations of huma n MO15 gene at 5q12-q13 and 2q22-q24. By using gene tagging and mammal ian cell transfection, we demonstrate that the KRKR motif located at t he carboxy terminal end of MO15 is required for nuclear targeting of t he protein. Mutation of KRKR to NGER retains MO15 in the cytoplasmic c ompartment, whilst the wild-type protein is detected exclusively in th e nucleus. Interestingly, we demonstrate that the nuclear targeting of MO15 is necessary to confer the protein its CAK activity. In contrast to the wild-type, the NLS-mutated MO15 expressed in Xenopus oocytes i s unable to generate CAK as long as the nuclear envelope is not broken . The nuclear localization of both the MO15 gene product and CAK activ ity may imply that cdks activation primarily occurs in the cell nucleu s.