How is CBX1/M31 related to position-effect variegation?
['First, forced expression of full-length SUV39H1 (412 amino acids) redistributes endogenous M31 (HP1beta) and induces abundant associations with inter- and metaphase chromatin', 'Together, our data reveal a dominant role(s) for the SET domain of SUV39H1 in the distribution of prominent heterochromatic proteins and suggest a possible link between a chromosomal SU(VAR) protein and histone H3', 'Using this motif, termed chromo box, we have cloned a mouse candidate modifier gene, M31, that also shows considerable sequence homology to Drosophila HP1', 'Here we report evidence of at least four independently segregating loci in the mouse homologous to the M31 cDNA. One of these loci--Cbx-rs1--maps to the X Chromosome (Chr), 1 cM proximal to Amg and outside the X-inactivation center region', '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', 'which complex with the heterochromatin component M31.', 'In addition, Suv39h1/SUV39H1 proteins associate with M31, currently the only other characterized mammalian SU(VAR) homologue', 'M31, a murine homolog of Drosophila HP1, is concentrated in the XY body during spermatogenesis', 'The HP1 class of chromobox genes are thought to encode proteins involved in the packaging of chromosomal DNA into repressive heterochromatin domains, as seen, for example, in position-effect variegation', 'Study of the distribution of a murine HP1-like chromodomain protein, M31, during spermatogenesis revealed spreading from the tip of the XY body in mid-stage pachytene spermatocytes to include the whole of the XY body in late-pachytene spermatocytes', 'We also demonstrate that the formation of the XY body during spermatogenic progression in neonatal mice coincides with the expression of a novel nuclear isoform of M31, M31(p21', 'Furthermore, by overexpressing a mammalian homologue (M31) of Drosophila melanogaster heterochromatin protein 1 (HP1; refs 7,8) in transgenic mouse lines that exhibit PEV, it is possible to modify the proportion of cells that silence the transgene in a dose-dependent manner', 'Thus, we show M31 overexpression to have two contrasting effects which are dependent on chromosomal context: (i) it enhanced PEV in those lines with centromeric or pericentromeric transgene locations; and (ii) it suppressed PEV when the transgene was non-centromeric']
['M31 is a heterochromatin component, that is concentrated in the XY body during spermatogenesis. M31 overexpression has two contrasting effects which are dependent on chromosomal context: (i) it enhanced PEV in those lines with centromeric or pericentromeric transgene locations; and (ii) it suppressed PEV when the transgene was non-centromeric.']
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