The BAG-1 protein modulates the chaperone activity of Hsc70 and Hsp70 in the mammalian cytosol and nucleus. Remarkably, BAG-1 possesses a ubiquitin-like domain at its amino terminus, suggesting a link to the ubiquitin/proteasome system. Here we show that BAG-1 is indeed associated with the 26 S proteasome in HeLa cells. Binding of the chaperone cofactor to the proteolytic complex is regulated by ATP hydrolysis and is not mediated by Hsc70 and Hsp70. The presented findings reveal a role of BAG-1 as a physical link between the Hsc70/Hsp70 chaperone system and the proteasome. In fact, targeting of BAG-1 to the proteasome promotes an association of the chaperones with the proteolytic complex in vitro and in vivo. A regulatory function of the chaperone cofactor at the interface between protein folding and protein degradation is thus indicated.
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