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J Biol Chem, Vol. 275, Issue 20, 14817-14823, May 19, 2000
From the Max Planck Institute for Biochemistry, Department of
Molecular Cell Biology, D-82152 Martinsried, Germany
In the mammalian cytosol and nucleus the activity
of the molecular chaperone Hsc70 is regulated by chaperone cofactors
that modulate ATP binding and hydrolysis by Hsc70. Among such cofactors is the anti-apoptotic protein BAG-1. Remarkably, BAG-1 is expressed as
multiple isoforms, which are distinguished by their amino termini. We
investigated whether distinct isoforms differ with respect to their
Hsc70-regulating activity. By comparing the mainly cytosolic isoforms
BAG-1M and BAG-1S, opposite effects of the two isoforms were observed
in chaperone-assisted folding reactions. Whereas BAG-1M was found to
inhibit the Hsc70-mediated refolding of nonnative polypeptide
substrates, the BAG-1S isoform stimulated Hsc70 chaperone activity. The
opposite effects are not due to differences in the regulation of the
ATPase activity of Hsc70 by the two isoforms. Both isoforms stimulated
ATP hydrolysis by Hsc70 in an Hsp40-dependent manner
through an acceleration of ADP-ATP exchange. Our results reveal that
the different amino termini of the distinct BAG-1 isoforms determine
the outcome of an Hsc70-mediated folding event, most likely by
transiently interacting with the polypeptide substrate. Employing
isoforms of a cofactor with different substrate binding properties
appears to provide the means to influence the chaperone function of
Hsc70 in addition to modulating its ATPase cycle.
Distinct Isoforms of the Cofactor BAG-1 Differentially Affect
Hsc70 Chaperone Function*
*
This work was supported by Deutsche Forschungsgemeinschaft
Grants HO 1518/2-1, 2-2, and 3-1.The costs of publication of this article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
To whom correspondence should be addressed: Max Planck Institute
for Biochemistry, Dept. of Molecular Cell Biology, Am Klopferspitz 18a,
D-82152 Martinsried, Germany. Tel.: 49 89 8578 3027; Fax: 49 89 8578 3022; E-mail: hoehfeld@biochem.mpg.de.
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