Interactions of HP1a, HP1b, and HP1c. Nicole C. Riddle^1,2, Tingting Gu², Sarah C. R. Elgin². 1) Biology, The University of Alabama at Birmingham, Birmingham, AL; 2) Biology, Washington University in St. Louis, St. Louis, MO.

   The heterochromatin Protein 1 (HP1) family of chromosomal proteins are involved in the formation of silent chromatin in organisms ranging from yeast to human. Their characterizing features are two conserved domains, the chromo domain and the chromo-shadow domain, which are connected by a more variable hinge domain. In Drosophila melanogaster, five HP1 family proteins exist, HP1a, HP1b, HP1c, RHINO (HP1d), and HP1e, two of which - RHINO and HP1e - are germline specific. While Su(var)205 (encoding HP1a), HP1b, and HP1c are all expressed ubiquitously, the proteins show significant differences in their localization patterns, HP1a associating mainly with heterochromatin, HP1c with euchromatin, and HP1b localizing to both on polytene chromsomes. Interestingly, chromatin immunoprecipitation experiments show that HP1a, HP1b and HP1c colocalize to a significant number of loci. These loci correspond to transcription start sites, both in euchromatin and heterochromatin. Focusing on HP1b, which is less well studied, we find that in third instar larvae, HP1b localizes to 3360 genes, 3127 in euchromatin, 233 in heterochromatin. Of these, 3231 genes are also bound by HP1c, and 472 genes are associated with HP1a, HP1b and HP1c. To determine the binding relationships between the three proteins, we analyzed mutant third instar larvae lacking either HP1a, HP1b, or HP1c. Lack of either HP1b or HP1c does not influence the binding patterns of the other family members. However, chromatin immunoprecipitation experiments suggest that lack of HP1a - while not affecting the distribution of HP1c - can disrupt proper association of HP1b with chromatin. On-going experiments explore how the presence of HP1 family members at transcription start sites influences gene expression in different genomic contexts, given that HP1b and HP1c act as transcriptional activators and HP1a is generally considered a repressor.