Sin3p

Sin3p is a nuclear transcriptional regulator in the yeast Saccharomyces cerevisiae. It functions as a core scaffold component of the Rpd3 histone deacetylase (HDAC) complexes and is essential for targeted histone deacetylation and transcriptional repression. The Sin3 protein coordinates the assembly of HDAC machinery by providing interaction surfaces for Rpd3 and other subunits, enabling the enzymatic activity to be directed to chromatin.

The protein contains multiple paired amphipathic helix (PAH) domains at the N-terminus (PAH1–PAH4) that mediate contacts with transcription factors and other subunits, facilitating recruitment of the HDAC complex to specific promoters or coding regions. The C-terminal region participates in complex stability and regulatory interactions. Sin3p is predominantly localized to the nucleus and is found in various HDAC-containing assemblies that repress gene expression.

Functionally, Sin3p itself does not possess catalytic activity; rather, it serves as a scaffold that anchors the Rpd3 histone deacetylase to chromatin. HDAC activity leads to removal of acetyl groups from histone H3 and H4 tails, promoting chromatin condensation and reduced transcription. Through its interactions, Sin3p participates in multiple regulatory pathways, including general transcriptional repression and the maintenance of transcriptional programs in response to environmental cues.

Evolutionarily, Sin3p is conserved across eukaryotes. The yeast protein is the homolog of the metazoan SIN3 family, whose members SIN3A and SIN3B assemble near-identical HDAC complexes with histone deacetylases and act as transcriptional corepressors in higher organisms.