NUCLEAR-LOCALIZATION OF VERTEBRATE CYCLIN-A CORRELATES WITH ITS ABILITY TO FORM COMPLEXES WITH CDK CATALYTIC SUBUNITS

Cyclins control the activities of cyclin-dependent protein kinases (cd ks) and hence play a key role in cell cycle regulation. While B-type- cyclins associate with p34cdc2 to trigger entry into mitosis, progress ion through S phase requires cyclin A, presumably in association with p33cdk2. Vertebrate A- and B-type cyclins display strikingly distinct subcellular localizations, but the mechanisms underlying these differe ntial distributions are unknown. Here, we have begun to study the requ irements for nuclear localization of cyclin A. We have isolated a cDNA coding for chicken cyclin A and constructed a series of deletion muta nts. These were then transfected into HeLa cells, and the subcellular distribution of the mutant cyclin A proteins was determined by indirec t immunofluorescence microscopy. In parallel, the cyclin A mutants wer e assayed for their ability to form complexes with cdk subunits. We fo und that deletion of more than 100 residues from the N terminus of cyc lin A did not impair nuclear localization or cdk subunit binding and k inase activation. In contrast, removal of as few as 15 residues from t he C terminus, or deletion of part of the internal cyclin box domain, abolished nuclear localization of cyclin A as well as its ability to b ind to and activate cdk subunits. These results suggest that nuclear t ransport of cyclin A may depend on the formation of multiprotein compl exes comprising cdk catalytic subunits.