Mitotic phosphorylation of SUV39H1, a novel component of active centromeres, coincides with transient accumulation at mammalian centromeres

Centromeres of eukaryotes are frequently associated with constitutive heterochromatin and their activity appears to be coregulated by epigenetic modification of higher order chromatin. Recently, we isolated murine (Suv39h1) and human (SUV39H1) homologues of the dominant Drosophila suppressor of position effect variegation Su(var)3-9, which is also related to the S. pombe silencing factor Clr4. We have shown that mammalian Su(var)3-9 homologues encode novel centromeric proteins on metaphase-arrested chromosomes. Here, we describe a detailed analysis of the chromatin distribution of human SUV39H1 during the cell cycle. Although there is significant heterochromatic overlap between SUV39H1 and M31 (HP1(beta)) during interphase, mitotic SUV39H1 displays a more restricted spatial and temporal association pattern with metaphase chromosomes than M31 (HP1(beta)), or the related HP1(&agr;) gene product. SUV39H1 specifically accumulates at the centromere during prometaphase but dissociates from centromeric positions at the meta- to anaphase transition. In addition, SUV39H1 selectively associates with the active centromere of a dicentric chromosome and also with a neocentromere. Interestingly, SUV39H1 is shown to be a phosphoprotein with modifications at serine and, to a lesser degree, also at threonine residues. Whereas SUV39H1 steady-state protein levels appear constant during the cell cycle, two additional phosphorylated isoforms are detected in mitotic extracts. This intriguing localisation and modification pattern would be consistent with a regulatory role(s) for SUV39H1 in participating in higher order chromatin organisation at mammalian centromeres.