SILENT INFORMATION REGULATOR PROTEIN COMPLEXES IN SACCHAROMYCES-CEREVISIAE - A SIR2 SIR4 COMPLEX AND EVIDENCE FOR A REGULATORY DOMAIN IN SIR4 THAT INHIBITS ITS INTERACTION WITH SIR3/

The SIR2, SIR3, and SIR4 silent information regulator proteins are inv olved in the assembly of silent chromatin domains in the budding yeast Saccharomyces cerevisiae. Using a series of biochemical experiments, we have studied protein-protein interactions involving these proteins. We found that yeast extracts contained a SIR2/SIR4 complex that was a ssociated with little or no SIR3, However, truncations of the N-termin al two-thirds of the SIR4 protein allowed it to efficiently associate with SIR3, suggesting that the N-terminal domain of SIR4 inhibited its interaction with SIRS, We propose that the SIR3 and SIR4 proteins int eract only during the assembly of the SIR protein complex at the silen cer and that an early step in assembly unmasks the SIR4 protein to all ow its association with SIR3, To test whether the interactions observe d in yeast extracts were direct, we tested these SIR-SIR interactions using bacterially expressed STR proteins, We observed direct interacti ons between SIR4 and SIR2, SIR4 and SIR3, SIR2 and SIR3, SIR2 and SIR2 , and SIR4 and SIR4, indicating that the associations observed in yeas t extracts were direct.