The glucanosyltransferase Gas1 functions in transcriptional silencing

Abstract

Transcriptional silencing is a crucial process that is mediated through chromatin structure. The histone deacetylase Sir2 silences genomic regions that include telomeres, ribosomal DNA (rDNA) and the cryptic mating-type loci. Here, we report an unsuspected role for the enzyme Gas1 in locus-specific transcriptional silencing. GAS1 encodes a ␤-1,3-glucanosyltransferase previously characterized for its role in cell wall biogenesis. In gas1 mutants, telomeric silencing is defective and rDNA silencing is enhanced. We show that the catalytic activity of Gas1 is required for normal silencing, and that Gas1's role in silencing is distinct from its role in cell wall biogenesis. Established hallmarks of silent chromatin, such as Sir2 and Sir3 binding, H4K16 deacetylation, and H3K56 deacetylation, appear unaffected in gas1 mutants. Thus, another event required for telomeric silencing must be influenced by GAS1. Because the catalytic activity of Gas1 is required for telomeric silencing, Gas1 localizes to the nuclear periphery, and Gas1 and Sir2 physically interact, we propose a model in which carbohydrate modification of chromatin components provides a new regulatory element that may be critical for chromatin function but which is virtually unexplored in the current landscape of chromatin analysis.acetylation ͉ beta-glucan ͉ chromatin ͉ S. cerevisiae ͉ telomeres T he formation of silent chromatin leads to the transcriptional repression of regions of the genome. In the yeast Saccharomyces cerevisiae, these regions include the telomeres, ribosomal DNA (rDNA), and the cryptic mating-type loci (the HM loci, HML, and HMR), all of which require the silent information regulator 2 protein (Sir2), an NAD ϩ -dependent protein deacetylase (reviewed in ref. 1). Sir2 is the founding member of the conserved sirtuin deacetylase family (2), with Sir2 directing its activity toward lysine 16 of histone H4 (H4K16) to promote silent chromatin formation (reviewed in ref.3). There is also evidence that deacetylation of lysine 56 of histone H3 (H3K56) by Sir2 (4), or the sirtuins Hst3 and Hst4 (5), enables silencing at the telomeres and HM loci. Sir2 functions in the SIR complex with Sir3 and Sir4 to silence at telomeres, and also at the HM loci, where an additional protein, Sir1, also participates (1). However, within the rDNA, the Sir2 containing RENT complex acts independently of the other Sir proteins (1). Distinctions among the 3 silenced regions led to the hypothesis that different mechanisms of silent chromatin formation and regulation exist for each region.The basic model for silent chromatin formation at telomeres and HM loci involves recruitment of the SIR complex by DNA binding proteins, followed by Sir2-mediated histone deacetylation and additional SIR complex spreading. Hypoacetylation of histones enables silent chromatin spreading in the absence of Sir2 deacetylase activity, but is not sufficient for full silencing (6). Instead, deacetylase activity must be targeted to the silenced loci through Sir3 or some other means (7). Although...

Unexpected Function of the Glucanosyltransferase Gas1 in the DNA Damage Response Linked to Histone H3 Acetyltransferases in Saccharomyces cerevisiae

“…As previously reported, deletion of GAS1 did not alter levels of H3K9Ac, H3K14Ac at 30° ( Koch and Pillus 2009), which are targets of both Gcn5 and Sas3 (reviewed in Lafon et al 2007). At 37°, a condition under which deletion of SAS3 suppresses gas1 temperature sensitivity, neither the gas1 strain nor gas1 sas3 had altered global levels of H3K9Ac, H3K14Ac ( Figure S2).…”

“…Whereas loss of SAS3 leads to an increase in silencing at the HM loci (Reifsnyder et al 1996), gas1 mutants have impaired silencing at telomeric loci and improved silencing within rDNA (Koch and Pillus 2009). We demonstrate that deletion of both enzymes leads to restoration of silencing to wild-type levels at all loci analyzed (Figure 2A).…”

“…Specifically, loss of Gas1 catalytic activity increases rDNA silencing and decreases telomeric silencing, yet has no observable change at the HM cryptic mating-type loci. These alterations in silencing are not remediated by the osmoregulator sorbitol (Koch and Pillus 2009), which rescues the cell wall-associated defects of gas1 and other cell wall mutants (Turchini et al 2000;Levin 2005). Combined, these data support a function for Gas1 in chromatin-mediated processes that is separable from its role at the cell wall.…”

Palmitoylation controls the dynamics of budding-yeast heterochromatin via the telomere-binding protein Rif1

“…Although several studies establish functional links between the inner nuclear membrane and yeast silencing (37)(38)(39), palmitoylation of Rif1 is unique as an example in which a known silencing protein's association with the inner nuclear membrane is controlled by a fatty acid modification. A recent study shows that carbohydrate modification of Sir2 can regulate silencing (40). Perhaps these unexpected modifications represent an emerging theme for how cells use nonhistone proteins to make chromatin structures more responsive to a cell physiology.…”

Identification of a Chloroplast Ribonucleoprotein Complex Containing Trans-splicing Factors, Intron RNA, and Novel Components

“…This was done because fusion proteins frequently cannot fold correctly in yeast and thus are incapable of interacting. Moreover, full-length proteins can be locked in a "closed" formation that masks binding domains (59,60). This effect might explain why, for example, subfragment Rab1-B interacts with several fusion proteins, whereas its full-length variant Rab1-FL fails to bind the respective proteins.…”