Structure-function analysis of SUV39H1 reveals a dominant role in heterochromatin organization, chromosome segregation, and mitotic progression

SUV39H1, a human homologue of the Drosophila position effect variegationn m odifier Su(var)3-9 and of the Schizosaccharomyces pombe silencing factor cl r4, encodes a novel heterochromatic protein that transiently accumulates at centromeric positions during mitosis. Using a detailed structure-function analysis of SUV39H1 mutant proteins in transfected cells, we now show that deregulated SUV39H1 interferes at multiple levels with mammalian higher-ord er chromatin organization. First, forced expression of full-length SUV39H1 (412 amino acids) redistributes endogenous M31 (HP1 beta) and induces abund ant associations with inter- and metaphase chromatin. These properties depe nd on the C-terminal SET domain, although the major portion of the SUV39H1 protein (amino acids 89 to 412) does not display affinity for nuclear chrom atin, By contrast! the M31 interaction surface, which was mapped to the fir st 44 N-terminal amino acids, together with the immediately adjacent chrome domain, directs specific accumulation at heterochromatin. Second, cells ov erexpressing full-length SUV39H1 display severe defects in mitotic progress ion and chromosome segregation. Surprisingly, whereas localization of centr omere proteins is unaltered, the focal, G(2)-specific distribution of phosp horylated histone H3 at serine 10 (phosH3) is dispersed in these cells. Thi s phosH3 shift is not observed with C-terminally truncated mutant SUV39H1 p roteins or with deregulated M31. Together, our data reveal a dominant role( s) for the SET domain of SUV39H1 in the distribution of prominent heterochr omatic proteins and suggest a possible link between a chromosomal SU(VAR) p rotein and histone H3.