The yeast SIN1 protein is a nuclear protein that together with other proteins behaves as a transcriptional repressor of a family of genes. In addition, sin1 mutants are defective in proper mitotic chromosome segregation. In an effort to understand the basis for these phenotypes, we employed the yeast two-hybrid system to identify proteins that interact with SIN1 in vivo. Here, we demonstrate that SAP1, a novel protein belonging to the 'AAA' family of ATPases, is able to directly interact with SIN1. Furthermore, we show, using recombinant molecules in vitro, that a short 27 amino acid sequence near the N-terminal of SIN1 is sufficient to bind SAP1. Previous experiments defined different domains of SIN that interact with other proteins and with DNA. The C-terminal domain of SIN1 was shown to be responsible for interaction with a protein that binds the regulatory region of HO, a gene whose transcription is repressed by SIN1. The central 'HMG1-like region' of SIN1 binds DNA, while the N-terminal of SIN1 can bind CDC23, a protein that regulates chromosome segregation. These data, taken together with the results presented here, suggest that SIN1 is a multifunctional chromatin protein that can interact with a number of different proteins that are involved in several different cellular functions.