BAG-1 modulates the chaperone activity of Hsp70/Hsc70

The 70 kDa heat shock family of molecular chaperones is essential to a variety of cellular processes, yet it is unclear how these proteins are regulated in vivo, We present evidence that the protein BAG-1 is a potential modulator of the molecular chaperones, Hsp70 and Hsc70, BAG-1 binds to the ATPase domain of Hsp70 and Hsc70, without requirement for their carboxyterminal peptide-binding domain, and can be coimmunoprecipitated with Hsp/Hsc70 from cell lysates, Purified BAG-1 and Hsp/Hsc70 efficiently form heteromeric complexes in vitro, BAG-1 inhibits Hsp/Hsc70-mediated in vitro refolding of an unfolded protein substrate, whereas BAG-1 mutants that fail to bind Hsp/Hsc70 do not affect chaperone activity, The binding of BAG-1 to one of its known cellular targets, Bcl-2, in cell lysates was found to be dependent on ATP, consistent with the possible involvement of Hsp/Hsc70 in complex formation, Overexpression of BAG-1 also protected certain cell lines from heat shock-induced cell death, The identification of Hsp/Hsc70 as a partner protein for BAG-1 may explain the diverse interactions observed between BAG-1 and several other proteins, including Raf-l, steroid hormone receptors and certain tyrosine kinase growth factor receptors, The inhibitory effects of BAG-1 on Hsp/Hsc70 chaperone activity suggest that BAG-1 represents a novel type of chaperone regulatory proteins and thus suggest a link between cell signaling, cell death and the stress response.