Saccharomyces cerevisiae G₁ Cyclins Differ in Their Intrinsic Functional Specificities

Abstract

The three budding yeast CLN genes appear to be functionally redundant for cell cycle Start: any single CLN gene is sufficient to promote Start, while the cln1 cln2 cln3 triple mutant is Start defective and inviable. Both quantitative and apparently qualitative differences between CLN genes have been reported, but available data do not in general allow distinction between qualitative functional differences as opposed to simply quantitative differences in expression or function. To determine if there are intrinsic qualitative differences between Cln proteins, we compared CLN2, CLN3, and crippled (but still partially active) CLN2 genes in a range of assays that differentiate genetically between CLN2 and CLN3. The results suggest that different potencies of Cln2, Cln3, and Cln2 mutants in functional assays cannot be accounted for by a simple quantitative model for their action, since Cln3 is at least as active as Cln2 and much more active than the Cln2 mutants in driving Swi4/Swi6 cell cycle box (SCB)-regulated transcription and cell cycle initiation in cln1 cln2 cln3 bck2 strains, but Cln3 has little or no activity in other assays in which Cln2 and the Cln2 mutants function. Differences in Cln protein abundance are unlikely to account for these results. Cln3-associated kinase is therefore likely to have an intrinsic in vivo substrate specificity distinct from that of Cln2-associated kinase, despite their functional redundancy. Consistent with the idea that Cln3 may be the primary transcriptional activator of CLN1, CLN2, and other genes, the activation of CLN2 transcription was found to be sensitive to the gene dosage of CLN3 but not to the gene dosage of CLN2.The Start transition in the Saccharomyces cerevisiae cell cycle requires the activity of one of three cyclin homologs encoded by the CLN1, CLN2, and CLN3 gene family, complexed with the cyclin-dependent kinase encoded by the CDC28 gene. Although the CLN genes are functionally redundant for cell cycle Start (39), the CLN3 gene differs sharply from the CLN1-CLN2 gene pair in structure and regulation (see reference 7 for a review). Searches for mutations resulting in lethality in strains deficient in CLN1 and CLN2 have yielded mutations in a wide range of genes, with widely varying lethal phenotypes. The general conclusion from this result has been that Cln1 and Cln2 are potent at execution of various cell biological processes associated with Start (e.g., cell polarization [4,12] and septin ring formation [4,18]), while Cln3 is relatively weak at activating these processes. Mutations in one gene (BCK2) are lethal in the absence of CLN3 but not in the absence of CLN1 and CLN2. The latter defect is significantly but not completely rescued by placing CLN2 under the control of heterologous promoters (14, 20), as Bck2 is required for efficient transcriptional activation of CLN1 and CLN2 in the absence of CLN3. This result is consistent with other data which suggest that Cln3 is the main or only physiological activator of CLN1 and CLN2 transcription (15,43,44). These r...

Discovering gene association networks by multi-objective evolutionary quantitative association rules

“…The association CLN2-CLN1 is consistent with the knowledge that CLN2 and CLN1 have their transcription maximum in G1 [9] whereas CDC20 is transcribed in late S/G 2 phase. Finally, the associations CLN1-CLN2 and CLN2-CLB5 are not inferred by the benchmark method [9], but they are consistent with observations on the partial functional redundancy existing among CLB5, CLN1 and CLN2, which have been reported in [66] and [67].…”

High Functional Overlap Between MluI Cell-Cycle Box Binding Factor and Swi4/6 Cell-Cycle Box Binding Factor in the G1/S Transcriptional Program in Saccharomyces cerevisiae

“…To construct the normally inviable double mutant strain, we made use of the observation of Koch et al (1993) that mbp1 swi4 mutants were viable when CLN2 was placed under control of a constitutive promoter. We observed reasonably efficient rescue of viable mbp1 swi4 mutant strains by inclusion of an integrated construct containing CLN2 under the control of the CLN3 promoter, P CLN3 -CLN2 (Valdivieso et al 1993;Levine et al 1996).…”

Molecular basis of the functional distinction between Cln1 and Cln2 cyclins

“…However, intrinsic differences between these cyclins also exist, because Cln2 and Cln3 differ even when both are expressed under the control of the CLN3 promoter. 10 One important factor that determines the function of a cyclin-Cdc28 kinase is its localization, and this is highlighted for Cln2 and Cln3, because the localization pattern contributes to Cln2 and Cln3's ability to affect different processes. The Cln3-Cdc28 complex is associated with the endoplasmic reticulum membrane until late G 1 , when it enters the nucleus to activate transcription.…”