Expression of the Molecular Chaperone Hsp70 in Detergent-resistant Microdomains Correlates with Its Membrane Delivery and Release*
- INSERM U538, Centre Hospitalo-Universitaire Saint Antoine. 27 rue de Chaligny, 75571 Paris Cedex 12, France
- ‡To whom correspondence should be addressed. Tel.: 33-1-40-01-13-34; Fax: 33-1-40-01-13-90; E-mail: marie.bachelet{at}chusa.jussieu.fr.
Abstract
Accumulating evidence suggests that some heat shock proteins (Hsps), in particular the 72-kDa inducible Hsp70, associate to the cell membrane and might be secreted through an unknown mechanism to exert important functions in the immune response and signal transduction. We speculated that specialized structures named lipid rafts, known as important platforms for the delivery of proteins to the cell membrane, might be involved in the unknown mechanism ensuring membrane association and secretion of Hsp70. Lipid rafts are sphingolipid-cholesterol-rich structures that have been mainly characterized in polarized epithelial cells and can be isolated as detergent-resistant microdomains (DRMs). Analysis of soluble and DRM fractions prepared from unstressed Caco-2 epithelial cells revealed that Hsp70, and to a lesser extent calnexin, were present in DRM fractions. Increased expression of Hsps, through heat shock or by using drugs acting on protein trafficking or intracellular calcium level, induced an efficient translocation to DRM. We also found that Hsp70 was released by epithelial Caco-2 cells, and this release dramatically increased after heat shock. Drugs known to block the classical secretory pathway were unable to reduce Hsp70 release. By contrast, release of the protein was affected by the raft-disrupting drug methyl-β-cyclodextrin. Our data suggest that lipid rafts are part of a mechanism ensuring the correct functions of Hsps and provide a rational explanation for the observed membrane association and release of Hsp70.
Footnotes
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↵1 The abbreviations used are: Hsp, heat shock protein; DRM, detergent-resistant microdomain; ER, endoplasmic reticulum; PBS, phosphate-buffered saline; ELISA, enzyme-linked immunosorbent assay; LDH, lactase dehydrogenase.
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↵* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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- Received March 6, 2003.
- Revision received April 7, 2003.
- The American Society for Biochemistry and Molecular Biology, Inc.
