DISTINCT CYTOPLASMIC AND NUCLEAR FRACTIONS OF DROSOPHILA HETEROCHROMATIN PROTEIN-1 - THEIR PHOSPHORYLATION LEVELS AND ASSOCIATIONS WITH ORIGIN RECOGNITION COMPLEX PROTEINS

The distinct structural properties of heterochromatin accommodate a di verse group of vital chromosome functions, yet we have only rudimentar y molecular details of its structure. A powerful tool in the analyses of its structure in Drosophila has been a group of mutations that reve rse the repressive effect of heterochromatin on the expression of a ge ne placed next to it ectopically, Several genes from this group are kn own to encode proteins enriched in heterochromatin. The best character ized of these is the heterochromatin-associated protein, HP1. HP1 has no known DNA-binding activity, hence its incorporation into heterochro matin is likely to be dependent upon other proteins. To examine HP1 in teracting proteins, we isolated three distinct oligomeric species of H P1 from the cytoplasm of early Drosophila embryos and analyzed their c ompositions. The two larger oligomers share two properties with the fr action of HP1 that is most tightly associated with the chromatin of in terphase nuclei: an underphosphorylated HP1 isoform profile and an ass ociation with subunits of the origin recognition complex (ORC). We als o found that HP1 localization into heterochromatin is disrupted in mut ants for the ORC2 subunit, These findings support a role for the ORC-c ontaining oligomers in localizing HP1 into Drosophila heterochromatin that is strikingly similar to the role of ORC in recruiting the Sir1 p rotein to silencing nucleation sites in Saccharomyces cerevisiae.