Germ cell development in C. elegans requires that the X chromosomes be globally silenced during mitosis and early meiosis. We previously found that the nuclear proteins MES-2, MES-3, MES-4 and MES-6 regulate the different chromatin states of autosomes versus X chromosomes and are required for germline viability. Strikingly, the SET-domain protein MES-4 is concentrated on autosomes and excluded from the X chromosomes. Here, we show that MES-4 has histone H3 methyltransferase (HMT) activity in vitro, and is required for histone H3K36 dimethylation in mitotic and early meiotic germline nuclei and early embryos. MES-4 appears unlinked to transcription elongation, thus distinguishing it from other known H3K36 HMTs. Based on microarray analysis, loss of MES-4 leads to derepression of X-linked genes in the germ line. We discuss how an autosomally associated HMT may participate in silencing genes on the X chromosome, in coordination with the direct silencing effects of the other MES proteins.

The C. elegans proteins MES-2 and MES-6, orthologs of the Polycomb group (PcG) chromatin repressors E(Z) and ESC, exist in a complex with their novel partner MES-3. The MES system participates in silencing the X chromosomes in the hermaphrodite germline. Loss of maternal MES function leads to germline degeneration and sterility. We report here that the MES complex is responsible for di- and trimethylation of histone H3 Lys27 (H3-K27) in the adult germline and in early embryos and that MES-dependent H3-K27 marks are concentrated on the X's. Another H3-K27 HMT functions in adult somatic cells, oocytes, and the PGCs of embryos. In PGCs, the MES complex may specifically convert dimethyl to trimethyl H3-K27. The HMT activity of the MES complex appears to be dependent on the SET domain of MES-2. MES-2 thus joins its orthologs Drosophila E(Z) and human EZH2 among SET domain proteins known to function as HMTs (reviewed in ). Methylation of histones is important for long-term epigenetic regulation of chromatin and plays a key role in diverse processes such as X inactivation and oncogenesis. Our results contribute to understanding the composition and roles of E(Z)/MES-2 complexes across species.

Abstract. The SH3 domain-containing protein Bemlp is needed for normal bud emergence and mating projection formation, two processes that require asymmetric reorganizations of the cortical cytoskeleton in Saccharomyces cerevisiae.

Abstract. The src homology region 3 (SH3) domainbearing protein Bemlp and the Rho-type GTPase Cdc42p are important for bud emergence in Saccharomyces cerevisiae. Here, we present evidence that through its second SH3 domain, Bemlp binds to the structurally and functionally similar proteins Boilp and Boi2p, each of which contain an SH3 and a pleckstrin homology (PH) domain. Deletion of BOll and BOI2 together leads to impaired morphogenesis and poor viability. A PH domain-bearing segment of Boilp that lacks the Bemlp-binding site is necessary and sufficient for function. This segment of Boilp displays a twohybrid interaction with Cdc42p, suggesting that Boilp either binds directly to or is part of a larger complex that contains Cdc42p. Consistent with these possibilities, overexpression of Boilp inhibits bud emergence, but this inhibition is counteracted by cooverexpression of Cdc42p. Increased expression of the Rho-type GTPase Rho3p, which is implicated in bud growth, suppresses the growth defects of boil boi2 mutants, suggesting that Boilp and Boi2p may also play roles in the activation or function of Rho3p. These findings provide an example of a tight coupling in function between PH domain-bearing proteins and both Rho-type GTPases and SH3 domain-containing proteins, and they raise the possibility that Boilp and Boi2p play a role in linking the actions of Cdc42p and Rho3p.T HE src homology region 3 (SH3) 1 and pleckstrin homology (PH) domains are present in many proteins that are involved in signal transduction and the organization of the cortical cytoskeleton (Pawson, 1995). The binding sites for SH3 domains generally appear to be short, proline-rich sequences . Less is known about the binding sites for PH domains. The findings that the PH domains in some proteins overlap with sequences that can bind to the 13y subunit of some trimeric G proteins (Touhara et al., 1994) and that some PH domains can bind PIP2 (Harlan et al., 1994), however, raise the possibility that a general role of PH domains might be to target proteins to membranes.Rho-type GTPases also are involved in signal transduction and the organization of the cortical cytoskeleton

The Maternal-Effect Sterile (MES) proteins are essential for germline viability in Caenorhabditis elegans. Here, we report that MES-4, a SET-domain protein, binds to the autosomes but not to the X chromosomes. MES-2, MES-3, and MES-6 are required to exclude MES-4 and markers of active chromatin from the X chromosomes. These findings strengthen the emerging view that in the C. elegans germ line, the X chromosomes differ in chromatin state from the autosomes and are generally silenced. We propose that all four MES proteins participate in X-chromosome silencing, and that the role of MES-4 is to exclude repressors from the autosomes, thus enabling efficient repression of the Xs.