Increased phosphorylation of HP1, a heterochromatin-associated protein of Drosophila, is correlated with heterochromatin assembly

The heterochromatin-associated nonhistone chromosomal protein HP1 exerts dosage-dependent effects on the silencing of genes juxtaposed to pericentric heterochromatin in Drosophila melanogaster. Here, we report that HP1 is multiply phosphorylated in Drosophila tissue, predominantly at serine and threonine residues. Pulse-labeling studies of explanted Drosophila tissues suggest that phosphorylation is relatively rapid and that phosphate is incorporated into existing protein. Maternally synthesized HP1 is underphosphorylated. The appearance of more highly phosphorylated HP1 isoforms at 1.5-2 h of development coincides with the embryonic stage at which cytologically visible heterochromatin appears and HP1 concentrates in heterochromatin. The extent of HP1 phosphorylation is lower in polytene tissue, where heterochromatin is underrepresented. These results are consistent with a role for phosphorylation of HP1 in the assembly and maintenance of heterochromatin in Drosophila.