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1 Physiology, McGill University, Montreal, Quebec, Canada
* To whom correspondence should be addressed. E-mail: ursula.stochaj{at}mcgill.ca.
Heat shock proteins of the hsp/hsc70 family are essential chaperones, implicated in the stress response, aging, and a growing number of human diseases. At the molecular level, hsc70s are required for the proper folding and intracellular targeting of polypeptides as well as the regulation of apoptosis. Cytoplasmic members of the hsp/hsc70 family are believed to shuttle between nuclei and cytoplasm; they are found in both compartments of unstressed cells. Our experiments demonstrate that actin filament-destabilizing drugs trigger the nuclear accumulation of hsc70s in unstressed and heat-shocked cells recovering from stress. Using human/mouse heterokaryons we show that stress inhibits shuttling and sequesters the chaperone in nuclei. The inhibition of hsc70 shuttling upon heat shock is transient only and transport is re-established when cells recover from stress. Hsc70 shuttling is controlled by hsc70 retention in the nucleus, a process that is mediated by two distinct mechanisms, ATP-sensitive binding of hsc70s to chaperone substrates and furthermore the association with nucleoli. The nucleolar protein fibrillarin and ribosomal protein rpS6 were identified as components that show an increased association with hsc70s in the nucleus upon stress exposure. Taken together, our data suggest that stress abolishes the exit of hsc70s from the nucleus to the cytoplasm, thereby limiting their function to the nuclear compartment. We propose that during recovery from stress hsc70s are released from nuclear and nucleolar anchors, which is a prerequisite to restore shuttling.
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