Distinct subcellular localization patterns contribute to functional specificity of the Cln2 and Cln3 cyclins of Saccharomyces cerevisiae

The G(1) cyclins of budding yeast drive cell cycle initiation by different mechanisms, but the molecular basis of their specificity is unknown. Here w e test the hypothesis that the functional specificity of G(1) cyclins is du e to differential subcellular localization. As shown by indirect immunofluo rescence and biochemical fractionation, Cln3p localization appears to be pr imarily nuclear, with the most obvious accumulation of Cln3p to the nuclei of large budded cells. In contrast, Cln2p localizes to the cytoplasm, We we re able to shift localization patterns of truncated Cln3p by the addition o f nuclear localization and nuclear export signals, and we found that nuclea r localization drives a Cln3p-like functional profile, while cytoplasmic lo calization leads to a partial shift to a Cln2p-like functional profile. The refore, forcing Cln3p into a Cln2p-like cytoplasmic localization pattern pa rtially alters the functional specificity of Cln3p toward that of Cln2p. Th ese results suggest that there are CLN-dependent cytoplasmic and nuclear ev ents important for cell cycle initiation. This is the first indication of a cytoplasmic function for a cyclin-dependent kinase. The data presented her e support the idea that cyclin function is regulated at the level of subcel lular localization and that subcellular localization contributes to the fun ctional specificity of Cln2p and Cln3p.