THE CDC2-RELATED KINASE PITALRE IS THE CATALYTIC SUBUNIT OF ACTIVE MULTIMERIC PROTEIN COMPLEXES

PITALRE is a human protein kinase identified by means of its partial s equence identity to the cell division cycle regulatory kinase CDC2. Im munopurified PITALRE protein complexes exhibit an in vitro kinase acti vity that phosphorylates the retinoblastoma protein, suggesting that P ITALRE catalyses this phosphorylation reaction. However, the presence of other kinases in the immunopurified complex could not be ruled out. In the present work, an inactive mutant of the PITALRE kinase has bee n used to demonstrate that PITALRE is the catalytic subunit responsibl e for the PITALRE-complex-associated kinase activity. Ectopic overexpr ession of PITALRE did not increase the total PITALRE kinase activity i n the cell, suggesting that PITALRE is regulated by limiting cellular factor(s). Characterization of the PITALRE-containing protein complexe s indicated that most of the cellular PITALRE protein exists as a subu nit in at least two different active multimeric complexes. Although mo nomeric PITALRE is also active in vitro, PITALRE present in multimeric complexes exhibits several-fold higher activity than monomeric PITALR E. In addition, overexpression of PITALRE demonstrated the existence o f two new associated proteins of approx. 48 and 98 kDa. Altogether the se results suggest that, in contrast to the situation with cyclin-depe ndent kinases, monomeric PITALRE is active, and that association with other proteins modulates its activity and/or its ability to recognize substrates in vivo.