INCREASED PHOSPHORYLATION OF HP1, A HETEROCHROMATIN-ASSOCIATED PROTEIN OF DROSOPHILA, IS CORRELATED WITH HETEROCHROMATIN ASSEMBLY

The heterochromatin-associated nonhistone chromosomal protein HP1 exer ts dosage-dependent effects on the silencing of genes juxtaposed to pe ricentric heterochromatin in Drosophila melanogaster. Here, we report that HP1 is multiply phosphorylated in Drosophila tissue, predominantl y at serine and threonine residues. Pulse-labeling studies of explante d Drosophila tissues suggest that phosphorylation is relatively rapid and that phosphate is incorporated into existing protein. Maternally s ynthesized HP1 is underphosphorylated. The appearance of more highly p hosphorylated HP1 isoforms at 1.5-2 h of development coincides with th e embryonic stage at which cytologically visible heterochromatin appea rs and HP1 concentrates in heterochromatin. The extent of HP1 phosphor ylation is lower in polytene tissue, where heterochromatin is underrep resented. These results are consistent with a role for phosphorylation of HP1 in the assembly and maintenance of heterochromatin in Drosophi la.