The Saccharomyces cerevisiae cyclin Clb2p is targeted to multiple subcellular locations by cis- and trans-acting determinants.

The cyclin-dependent kinase Cdc28p associates with the cyclin Clb2p to induce mitosis in the yeast Saccharomyces cerevisiae. Several cell cycle regulatory proteins have been shown to require specific nuclear transport events to exert their regulatory functions. Therefore, we investigated the subcellular localization of wild-type Clb2p and several mutant versions of the protein using green fluorescent protein (GFP) fusion constructs. Wild-type Clb2p is primarily nuclear at all points of the cell. A point mutation in a potential leucine-rich nuclear export signal (NES) enhances the nuclear localization of the protein, and delta-yrb2 cells exhibit an apparent Clb2p nuclear export defect. Clb2p contains a bipartite nuclear localization signal (NLS), and its nuclear localization requires the alpha and beta importins (Srp1p and Kap95p), as well as the yeast Ran GTPase and its regulators. Deletion of the Clb2p NLS causes increased cytoplasmic localization of the protein, as well as accumulation at the bud neck. These data indicate that Clb2p exists in multiple places in the yeast cell, possibly allowing Cdc28p to locally phosphorylate substrates at distinct subcellular sites.