Lysine methylation of histones in vivo occurs in three states: mono-, di- and tri-methyl. Histone H3 has been found to be di-methylated at lysine 4 (K4) in active euchromatic regions but not in silent heterochromatic sites. Here we show that the Saccharomyces cerevisiae Set1 protein can catalyse di- and tri-methylation of K4 and stimulate the activity of many genes. Using antibodies that discriminate between the di- and tri-methylated state of K4 we show that di-methylation occurs at both inactive and active euchromatic genes, whereas tri-methylation is present exclusively at active genes. It is therefore the presence of a tri-methylated K4 that defines an active state of gene expression. These findings establish the concept of methyl status as a determinant for gene activity and thus extend considerably the complexity of histone modifications.

Remodelling of the nuclear membrane is essential for the dynamic changes of nuclear architecture at different stages of the cell cycle and during cell differentiation. The molecular mechanism underlying the regulation of nuclear membrane biogenesis is not known. Here we show that Smp2, the yeast homologue of mammalian lipin, is a key regulator of nuclear membrane growth during the cell cycle. Smp2 is phosphorylated by Cdc28/ Cdk1 and dephosphorylated by a nuclear/endoplasmic reticulum (ER) membrane-localized CPD phosphatase complex consisting of Nem1 and Spo7. Loss of either SMP2 or its dephosphorylated form causes transcriptional upregulation of key enzymes involved in lipid biosynthesis concurrent with a massive expansion of the nucleus. Conversely, constitutive dephosphorylation of Smp2 inhibits cell division. We show that Smp2 associates with the promoters of phospholipid biosynthetic enzymes in a Nem1-Spo7-dependent manner. Our data suggest that Smp2 is a critical factor in coordinating phospholipid biosynthesis at the nuclear/ER membrane with nuclear growth during the cell cycle.

contributed equally to this work Two membrane proteins were identified through their genetic interaction with the nucleoporin Nup84p and shown to participate in nuclear envelope morphogenesis in yeast. One component is a known sporulation factor Spo7p, and the other, Nem1p, a novel protein whose C-terminal domain is conserved during eukaryotic evolution. Spo7p and Nem1p localize to the nuclear/ER membrane and behave biochemically as integral membrane proteins. Nem1p binds to Spo7p via its conserved C-terminal domain. Although cells without Spo7p or Nem1p are viable, they exhibit a drastically altered nuclear morphology with long, porecontaining double nuclear membrane extensions. These protrusions emanate from a core nucleus which contains the DNA, and penetrate deeply into the cytoplasm. Interestingly, not only Spo7 -and Nem1 -, but also several nucleoporin mutants are defective in sporulation. Thus, Spo7p and Nem1p, which exhibit a strong genetic link to nucleoporins of the Nup84p complex, fulfil an essential role in formation of a spherical nucleus and meiotic division.