HETEROCHROMATIN PROTEIN-1 DISTRIBUTION DURING DEVELOPMENT AND DURING THE CELL-CYCLE IN DROSOPHILA EMBRYOS

Heterochromatin protein 1 (HP1) was initially discovered as a protein that is associated with the heterochromatin at the chromocenter of pol ytene chromosomes in Drosophila larval salivary glands. In this paper we investigate the localization of heterochromatin protein 1 in the di ploid nuclei of Drosophila embryos, We focus on its association with t he interphase heterochromatin in fixed embryos before and during cycle 14, the developmental time at which heterochromatin becomes most cons picuous, and also follow its localization during mitosis, The GAGA tra nscription factor was recently shown to be localized at sequences with in alpha-heterochromatin in pre-cycle 14 embryos, and an antibody agai nst this protein serves as a convenient marker for these sequences. We find an enrichment of heterochromatin protein 1 in the intensely DAPI -staining regions near the apical surface of nuclear cycle 10 embryos, At this stage GAGA factor is localized into punctate structures in th is same region, This enrichment for HP1 is markedly increased during n uclear cycle 14. Surprisingly, whereas GAGA factor retains its associa tion with the heterochromatin throughout the cell cycle, a significant fraction of HP1 is dispersed throughout the spindle around the segreg ating chromosomes during mitosis, This dispersed pool of heterochromat in protein 1 was observed during mitosis in both early and late Drosop hila embryos and in an analysis of a bacterially produced 6x histidine -heterochromatin protein 1 fusion protein injected into living Drosoph ila embryos, When Drosophila tissue culture cells were prepared by a m ethod which removes soluble protein and avoids fixation of the mitotic chromosomes, an enrichment for heterochromatin protein 1 in the heter ochromatin of the chromosomes was discovered also.