CLN3 encodes a G1 cyclin involved in regulation of the cell cycle. Progression through the cell cycle is a carefully regulated process that is conserved throughout eukaryotes. Periodic activation of cyclin-dependent kinases (CDKs) are required for this process; the critical CDK involved in cell cycle progression in yeast is Cdc28p.

Cyclins are the regulatory subunits that activate CDKs at the appropriate time in the cell cycle; they were named for their cyclical accumulation during particular phases of the cell cycle (8). Distinct CDK-cyclin complexes are required for progression through different stages of the cell cycle (9). CLN1, CLN2, and CLN3 encode the yeast cyclins involved in the G1 to S phase transition.

While transcription of CLN1 and CLN2 is regulated by the cell cycle, CLN3 transcription is not. Instead, Cln3p is regulated post-translationally (6, 5). Cln3p is an unstable protein that contains several PEST motifs, which are sequences rich in proline, glutamic acid, serine, and threonine and are found in many other unstable proteins (6, 5). Cln3p is phosphorylated by Cdc28p; this phosphorylation may make the Cln3 protein less stable (5). The chaperone Ydj1p is required for the phosphorylation and degradation of Cln3p (10). In addition to being carefully regulated itself, Cln3p also plays a role in regulating the transcriptional activation of the other G1 cyclins, CLN1 and CLN2 (11, 12). An excellent review by Lew et al. describes cell cycle control in S. cerevisiae in detail (9).

Last updated: 1999-12-17