Msa1 and Msa2 Modulate G1-Specific Transcription to Promote G1 Arrest and the Transition to Quiescence in Budding Yeast

Abstract

Yeast that naturally exhaust their glucose source can enter a quiescent state that is characterized by reduced cell size, and high cell density, stress tolerance and longevity. The transition to quiescence involves highly asymmetric cell divisions, dramatic reprogramming of transcription and global changes in chromatin structure and chromosome topology. Cells enter quiescence from G1 and we find that there is a positive correlation between the length of G1 and the yield of quiescent cells. The Swi4 and Swi6 transcription factors, which form the SBF transcription complex and promote the G1 to S transition in cycling cells, are also critical for the transition to quiescence. Swi6 forms a second complex with Mbp1 (MBF), which is not required for quiescence. These are the functional analogues of the E2F complexes of higher eukaryotes. Loss of the RB analogue, Whi5, and the related protein Srl3/Whi7, delays G1 arrest, but it also delays recovery from quiescence. Two MBF- and SBF-Associated proteins have been identified that have little effect on SBF or MBF activity in cycling cells. We show that these two related proteins, Msa1 and Msa2, are specifically required for the transition to quiescence. Like the E2F complexes that are quiescence-specific, Msa1 and Msa2 are required to repress the transcription of many SBF target genes, including SWI4, the CLN2 cyclin and histones, specifically after glucose is exhausted from the media. They also activate transcription of many MBF target genes. msa1msa2 cells fail to G1 arrest and rapidly lose viability upon glucose exhaustion. msa1msa2 mutants that survive this transition are very large, but they attain the same thermo-tolerance and longevity of wild type quiescent cells. This indicates that Msa1 and Msa2 are required for successful transition to quiescence, but not for the maintenance of that state.

A common strategy for initiating the transition from proliferation to quiescence

“…It has been shown to bind SBF transiently in DNA damage (Travesa, et al 2013) and mediate the nuclear localization of Cln3 (Yahya, et al 2014). The whi5srl3 mutant is slow to arrest in G1 (Miles, et al 2016), but after seven days in culture, it achieves G1 arrest just like wild type (Fig 4A). It’s most pronounced defect is that it is very delayed in budding and DNA synthesis upon re-feeding quiescent cells ((Miles, et al 2016) and Fig 4B).…”

“…The whi5srl3 mutant is slow to arrest in G1 (Miles, et al 2016), but after seven days in culture, it achieves G1 arrest just like wild type (Fig 4A). It’s most pronounced defect is that it is very delayed in budding and DNA synthesis upon re-feeding quiescent cells ((Miles, et al 2016) and Fig 4B). This role in speeding cell cycle re-entry is unexpected for negative regulators of the G1 to S transition and suggests novel functions for these proteins.…”

“…It is known that recruitment of Rpd3 to the SBF-targeted G1/S genes of budding yeast requires the presence of Whi5 and Stb1 ( S in T hree B inding protein 1) in the SBF complexes (Takahata, et al 2009). We have looked at whi5stb1 mutants and find that they produce wild type levels of quiescent cells (Miles, et al 2016), while rpd3 mutants produce very few quiescent cells (McKnight, et al 2015). This indicates that, at least in budding yeast, the global chromatin remodeling and transcriptional repression that Rpd3 mediates is critically important for the full transition to a quiescent state.…”

“…Consistent with this, Cln3, which is rate limiting for the G1 to S transition in budding yeast (Cross 1988) reduces quiescent cell yields when it is overproduced and increases quiescent cell yields when it is absent (Miles, et al 2013). In a survey of 26 mutants that affect the length of G1 during rapid growth, we found that there is a strong correlation between the length of G1 and the quiescent cell yield (Miles, et al 2016). This suggests that, as cells enter quiescence from G1, some time is required to assemble the requisite complexes.…”

“…Interestingly, Whi5, the negative regulator of SBF and functional paralog of RB, is not absolutely required for the transition to quiescence, but it does influence the efficiency of the transition in and out of quiescence (Miles, et al 2016). This phenotype is enhanced by elimination of the Whi5-related protein Srl3.…”