A large protein complex containing the yeast Sin3p and Rpd3p transcriptional regulators.

The SIN3 gene is required for the transcriptional repression of diverse genes in Saccharomyces cerevisiae. Sin3p does not bind directly to DNA but is thought to be targeted to promoters by interacting with sequence-specific DNA-binding proteins. We show here that Sin3p is present in a large multiprotein complex with an apparent molecular mass, estimated by gel filtration chromatography, of greater than 2 million Da. Genetic studies have shown that the yeast RPD3 gene has a function similar to that of SIN3 in transcriptional regulation, as SIN3 and RPD3 negatively regulate the same set of genes. The SIN3 and RPD3 genes are conserved from yeasts to mammals, and recent work suggests that RPD3 may encode a histone deacetylase. We show that Rpd3p is present in the Sin3p complex and that an rpd3 mutation eliminates SIN3-dependent repression. Thus, Sin3p may function as a bridge to recruit the Rpd3p histone deacetylase to specific promoters.

SIN3-dependent transcriptional repression by interaction with the Mad1 DNA-binding protein.

The SIN3 gene in Saccharomyces cerevisiae encodes a negative regulator of transcription of a large number of genes. Mouse homologs of SIN3 have been identified through screens for proteins interacting with the mammalian Mad1 protein, a transcriptional repressor. We find that yeast Sin3 (ySin3) interacts with Madl and that, as for mouse Sin3, the N terminus of Mad1 interacts with the PAH2 domain of ySin3. Although Mad1 (a basic helix-loop-helix leucine zipper [bHLH-Zip) protein) forms a heterodimer with the Max bHLH-Zip protein, LexA-Mad1 and VP16-Max do not activate transcription of a reporter gene in a two-hybrid assay. This failure in activation is due to direct repression by ySin3, as LexA-Mad1 and VP16-Max are able to activate the two-hybrid reporter in a sin3 mutant. This inhibition of activation by LexA-Mad1 and VP16-Max requires the PAH2 domain of ySin3 and the N-terminal interaction region of Mad1. These data demonstrate that ySin3 functions as a transcriptional repressor by being brought to promoters by interacting with proteins bound to DNA.

pRb and the cdks in apoptosis and the cell cycle.

Apoptosis is a fundamental biological process present in metazoan cells. Linking apoptosis to the cell cycle machinery provides a mechanism to maintain proper control of cell proliferation in a multicellular organism. pRb and the cyclin-dependent kinases may have dual roles as integral components of the cell cycle and regulators of apoptosis. In many instances manipulation of the cell cycle through these molecules can induce or inhibit apoptosis. Recent studies also identify pRb as a substrate for an apoptotic protease; however, other cell cycle components are not known substrates. While it is clear that many common molecules can affect cell proliferation and cell death, the universality of any one cell cycle molecule in apoptosis has yet to be determined.

Identification of the Saccharomyces cerevisiae genes STB1-STB5 encoding Sin3p binding proteins.

The yeast SIN3 gene functions as a transcriptional repressor, despite the fact that Sin3p does not bind DNA directly. We have conducted a two-hybrid screen to look for proteins that interact with Sin3p, using the PAH2 domain of Sin3p as bait. Five new genes, STB1-STB5 were identified, as well as the STB6 gene, which is similar to STB2. STB1, STB2, STB3, and STB6 are novel genes, and STB4 and STB5 encode C6 zinc cluster DNA-binding proteins. None of these genes is essential for viability, and several of these genes may encode transcriptional activators. Several special problems were encountered in using a transcriptional repressor in a two-hybrid screen. For example, the STB genes will interact with a LexA-Sin3(PAH2) fusion protein containing a region of Sin3p, but a LexA-Sin3p fusion protein containing full-length Sin3p, along with a STB clone, does not produce two-hybrid activation of a transcriptional reporter. In addition, a sin3 mutation reduces the transcriptional activation by two-hybrid partners, suggesting that a sin3 mutation reduces the transcriptional efficiency of the Gal4p and VP16 activation domains. We have shown previously that Sin3p is part of a large multiprotein complex, and we show here that Stb1p and Stb2p are present in this complex.