Investigating the role SUMO plays in heterochromatin formation in the fission yeast Schizosaccharomyces pombe

Abstract

In eukaryotic cells, DNA is organized by histones and associated proteins into a complex that is called chromatin. The fundamental subunit of chromatin is the nucleosome that is composed of eight histone proteins and a segment of DNA. The chromatin is divided into euchromatin and heterochromatin based on the appearance of the chromosomes. Euchromatin is an open configuration reflecting regions that are allowed to be replicated and transcribed. In contrast, heterochromatin corresponds to genetically silent chromosome regions, which have an altered chromatin structure and decreased recombination frequencies. Heterochromatin domains are normally found close to the nuclear periphery, and contain transposable elements and repetitive sequences. It has also shown that heterochromatin is essential to chromosome structures such as centromeres and telomeres, associated with some chromatin proteins and different states of histone modifications. Moreover, heterochromatin regulates gene expression via epigenetic silencing mechanisms during the process of cell development and differentiation. Thus, it is crucial to study heterochromatin as it is important to maintain genome stability.

My PhD project is to investigate the role SUMO plays in heterochromatin formation in fission yeast. SUMO is a small protein modifier that is crucial for numerous cellular processes in eukaryotic cells. SUMO is attached to the substrates via the action of three enzymes, including SUMO E1 activating enzyme, E2 conjugating enzyme, and E3 ligase enzyme, that are analogous to ubiquitin pathway. SUMO has also been defined as a key mediator of genome stability, cell proliferation etc. Once SUMO targets the substrates, it can exert effects by altering the interaction with partner proteins and DNA, changing cellular localization, or disturbing protein stability by competing with ubiquitination. In S. pombe, the pmt3+ gene that encodes SUMO causes defects in heterochromatin silencing at the endogenous loci, however, the mechanism of the molecular action remains to be determined.

In this study I have first confirmed the previously published silencing defect associated with the deletion of SUMO in fission yeast cells. Moreover, I found a specific defect in in tri-methylated H3K9. Further exploration indicates that SUMO deletion is affecting heterochromatin silencing through Clr4 chromodomain (CD), probably due to Clr4 binding too tightly to the chromatin. This is the first time it has been found that SUMO is affecting the H3K9 methyltransferase Clr4 binding to the chromatin, I hope this will advance other studies in dissecting the role of SUMO in regulation of heterochromatin formation.