The Sin3a gene codes for a 1,219-amino acid protein containing 4 paired amphipathic helix (PAH) domains. It has been also identified an alternatively spliced isoform with a 9-amino acid insert near the C terminus. Sin3a shares significant homology with yeast Sin3 and mouse Sin3b. Sin3a presents a nuclear localization sequence between PAH2 and PAH3, as well as a PEST motif, which mediates rapid protein degradation. The domain PAH2 of Sin3a can bind to the N-terminal 25 amino acids of Mad, which contain a putative amphipathic alpha helix. Sin3a can also form a ternary complexes in solution with Mad and Max and this complex can recognize the Mad-Max E box-binding site repressing transcription of a reporter gene. It has been hypothesized that Mad-Max represses transcription by tethering the Sin3 proteins to DNA as corepressors. Sin3a in fact can form a multiprotein complex including Sin3b, HDAC1, HDAC2 and other proteins, which can be recruited on some gene promoters to silence their transcription.
The Sin3b gene codes for a 954-amino acid protein containing 4 paired amphipathic helix (PAH) domains and shares significant homology with yeast Sin3 and mouse Sin3a. Sin3b can form a ternary complexes in solution with Mad and Max and this complex can recognize the Mad-Max E box-binding site repressing transcription of a reporter gene. It has been hypothesized that Mad-Max represses transcription by tethering the Sin3 proteins to DNA as corepressors. Sin3b in fact can form a multiprotein complex including Sin3a, HDAC1, HDAC2 and other proteins, which can be recruited on some gene promoters to silence their transcription.