Heterochromatin protein 1 (HP1) is a conserved eukaryotic chromosomal protein that is prominently associated with pericentric heterochromatin and mediates the concomitant gene silencing. Mechanistic studies implicate HP1 family proteins as 'hub proteins', able to interact with a variety of chromosomal proteins through the chromo-shadow domain (CSD), as well as to recognize key histone modification sites through the chromodomain (CD). Consequently, HP1 has many important roles in chromatin architecture and impacts both gene expression and gene silencing, utilizing a variety of mechanisms. Several studies in different organisms have also indicated that HP1 plays a pivotal role in development and aging. HP1-dependent heterochromatin organization is indeed essential for a proper development in Drosophila. In Arabidopsis, the HP1 ortholog, LHP1, is required for the epigenetic maintenance of gene repression that is also essential for plant development. Recent works in human cells point out a role of HP1 in the context of senescence, a cellular manifestation of aging. In particular, senescence of human cells is associated with mislocalization of HP1 proteins and altered chromatin compaction. The multiple functions of the HP1 are likely to be correlated with the many post-translational modifications (PTMs) described for these proteins.
In this proposal we will undertake complementary approaches in Drosophila, Arabidopsis and human cells to unravel how ubiquitination may regulate HP1 functions in telomere maintenance, gene expression and replicative senescence. As these pathways are fundamental in ensuring genome integrity, we are confident that our results will provide new insights in the comprehension on the epigenetic mechanisms that prevent genome instability onset underlying several pathological conditions including cancer.
This proposal aims at unraveling how ubiquitination may regulate HP1 functions in telomere maintenance, gene expression and replicative senescence in different model systems. As these pathways are fundamental in ensuring genome integrity, we are confident that our results will provide novel mechanisms to prevent genome instability onset that underlie aging and several pathological conditions including cancer. The results of this project will highlight for the first time the contribution of HP1 ubiquitination in specific cellular processes such as telomere maintenance and senescence. In addition, by using Drosophila and Arabidopsis as model systems, we will provide an unanticipated evidence of the involvement of Effete/UbcD1 and atUBC28, two well conserved ubiquitin conjugating enzymes from Drosophila and arabidopsis, respectively, in the regulation of HP1 ubiquitination. Moreover, our site direct mutagenesis will allow the generation of HP1 variant proteins in which all conserved lysine residues have been replaced by arginine. These variants will enable us to finely dissect the contribution of each specific ubiquitinated lysine on HP1 function.