Our understanding of the post-translational processes involved in regulating the interferon regulatory factor-1 (IRF-1) tumor suppressor protein is limited. The introduction of mutations within the C-terminal Mf1 domain (amino acids 301-325) impacts on IRF-1-mediated gene repression and growth suppression as well as the rate of IRF-1 degradation. However, nothing is known about the proteins that interact with this region to modulate IRF-1 function. A biochemical screen for Mf1-interacting proteins has identified an LXXLL motif that is required for binding of Hsp70 family members and cooperation with Hsp90 to regulate IRF-1 turnover and activity. These conclusions are supported by the finding that Hsp90 inhibitors suppress IRF-1-dependent transcription shortly after treatment, although at later time points inhibition of Hsp90 leads to an Hsp70-dependent depletion of nuclear IRF-1. Conversely, the half-life of IRF-1 is increased by Hsp90 in an ATPase-dependent manner leading to the accumulation of nuclear but not cytoplasmic IRF-1. This study begins to elucidate the role of the Mf1 domain of IRF-1 in orchestrating the recruitment of regulatory factors that can impact on both its turnover and transcriptional activity.