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Distinct isoforms of the cofactor BAG-1 differentially affect Hsc70 chaperone function
1Max Planck Institute for Biochemistry, Department of Molecular Cell Biology, D-82152 Martinsried, Germany.
Insights
The anti-apoptotic protein BAG-1 has distinct isoforms that oppositely regulate the Hsc70 chaperone. BAG-1S stimulates Hsc70 activity, while BAG-1M inhibits it, influencing protein folding outcomes.
Area of Science:
- Molecular biology
- Cellular biology
- Protein biochemistry
Background:
- Hsc70 chaperone activity is modulated by cofactors in the cytosol and nucleus.
- BAG-1 is an anti-apoptotic protein functioning as an Hsc70 cofactor.
- BAG-1 exists in multiple isoforms differing in their amino termini.
Purpose of the Study:
- To investigate differential Hsc70-regulating activities among BAG-1 isoforms.
- To compare the effects of BAG-1M and BAG-1S on Hsc70-assisted protein folding.
Main Methods:
- Comparative analysis of BAG-1M and BAG-1S isoforms in chaperone-assisted folding reactions.
- Assessment of Hsc70 ATPase activity modulation by BAG-1 isoforms.
- Investigation of Hsp40-dependent mechanisms.
Main Results:
- BAG-1M inhibited Hsc70-mediated refolding of nonnative polypeptides.
- BAG-1S isoform stimulated Hsc70 chaperone activity.
- Both isoforms accelerated ADP-ATP exchange, stimulating Hsc70 ATPase activity in an Hsp40-dependent manner.
Conclusions:
- Distinct BAG-1 isoform amino termini dictate Hsc70-mediated folding outcomes.
- Isoform-specific substrate interactions likely mediate differential effects on Hsc70 function.
- Differential cofactor isoform substrate binding influences Hsc70 chaperone function beyond ATPase cycle modulation.
Abstract:
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.