A Comparative Study of the Degradation of Yeast Cyclins Cln1 and Cln2

The yeast cyclins Cln1 and Cln2 are very similar in both sequence and function, but some differences in their functionality and localization have been recently described. The control of Cln1 and Cln2 cellular levels is crucial for proper cell cycle initiation. In this work, we analyzed the degradation patterns of Cln1 and Cln2 in order to further investigate the possible differences between them. Both cyclins show the same half-life but, while Cln2 degradation depends on ubiquitin ligases SCF and SCF, Cln1 is affected only by SCF. Degradation analysis of chimeric cyclins, constructed by combining fragments from Cln1 and Cln2, identifies the N-terminal sequence of the proteins as responsible of the cyclin degradation pattern. In particular, the N-terminal region of Cln2 is required to mediate degradation by SCF. This region is involved in nuclear import of Cln1 and Cln2, which suggests that differences in degradation may be due to differences in localization. Moreover, a comparison of the cyclins that differ only in the presence of the Cln2 nuclear export signal indicates a greater instability of exported cyclins, thus reinforcing the idea that cyclin stability is influenced by their localization.