Investigation of the Sin3a-HDAC1-SDS3 transcriptional co-repressor complex

The transcriptional co-repressor Sin3a is a ubiquitous eukaryotic protein complex that has a multitude of critical functions, including the regulation of embryonic development, cell division and maintenance of genomic integrity. It incorporates the highly related HDAC1 and HDAC2 enzymes as its catalytic subunits, which interact with the complex through the HID domain of the Sin3a co-repressor. Sin3a is responsible for deacetylating lysines of Histone tails, condensing chromatin and consequently repressing the transcription of genes. The enzymatic activities of HDAC1 and 2 within Sin3a depend on the association of the Sin3aspecific SDS3 protein, which also interacts with the complex via the HID domain. The mechanism by which Sin3a recruits HDAC1, HDAC2 and SDS3 remains unknown, and elucidating it would represent a big step forward in understanding the epigenetic regulation of genes through the deacetylation of chromatin. The aim of this thesis is to understand how Sin3a recruits its catalytic subunits in to the complex as well as to get a deeper insight into the role of SDS3 in the HID domain by using both structural (X-ray crystallography) and biochemical approaches. Our data suggests that HDAC1 may interact with Sin3a through an extended surface of the co-repressor and that SDS3 stabilizes this interaction by simultaneously binding to HDAC1 and Sin3a. Enzymatic and kinetic assays indicate that Sin3a may be the only Class I HDAC containing complex that is not regulated by IP4. IP4 is a co-factor that regulates the activity of the Class I HDACdependent complexes NuRD and SMRT-NCoR. Thus, our results suggest that Sin3a may have followed a separate evolutionary pattern and its activity may be regulated in a different way.