Conservation of Heterochromatin Protein 1 Function

Abstract

Heterochromatin represents a cytologically visible state of heritable gene repression. In the yeast, Schizosaccharomyces pombe, the swi6 gene encodes a heterochromatin protein 1 (HP1)-like chromodomain protein that localizes to heterochromatin domains, including the centromeres, telomeres, and the donor mating-type loci, and is involved in silencing at these loci. We identify here the functional domains of swi6p and demonstrate that the chromodomain from a mammalian HP1-like protein, M31, can functionally replace that of swi6p, showing that chromodomain function is conserved from yeasts to humans. Site-directed mutagenesis, based on a modeled three-dimensional structure of the swi6p chromodomain, shows that the hydrophobic amino acids which lie in the core of the structure are critical for biological function. Gel filtration, gel overlay experiments, and mass spectroscopy show that HP1 proteins can self-associate, and we suggest that it is as oligomers that HP1 proteins are incorporated into heterochromatin complexes that silence gene activity.

ACKNOWLEDGMENTS

We thank K. Maundrell for the pREP1, A. M. Carr for pREP81 expression vectors, and M. Yanagida for the CN2 strain. We thank G. W. Butcher and A. Hutchings for help with monoclonal antibody production, J. Mellor for help with production and verification of the strains used in this study, M. Fricker and D. Spiller for help with confocal microscopy, J. Coadwell for initial structural analysis of swi6p, and J. P. Brown for critical reading of the manuscript.

This work was funded by BBSRC grant LRG43/AO1809.