Thermal Adaptation of the Yeast Mitochondrial Hsp70 System is Regulated by the Reversible Unfolding of its Nucleotide Exchange Factor
Thermal Adaptation of the Yeast Mitochondrial Hsp70 System is Regulated by the Reversible Unfolding of its Nucleotide Exchange Factor
Received 30 January 2006; revised 10 March 2006; accepted 14 March 2006. Edited by M. Gottesman. Available online 29 March 2006.
Fernando Moro and Arturo Muga
Journal of Molecular Biology
Volume 358, Issue 5 , 19 May 2006
ScienceDirect
Unidad de BiofÃsica (CSIC/UPV-EHU) y Departamento de BioquÃmica y BiologÃa Molecular, Facultad de Ciencia y TecnologÃa, Universidad del PaÃs Vasco, Apartado 644, 48080 Bilbao, Spain
The Hsp70 protein switches during its functional cycle from an ADP-bound state with a high affinity for substrates to a low-affinity, ATP-bound state, with concomitant release of the client protein. The rate of the chaperone cycle is regulated by co-chaperones such as nucleotide exchange factors that significantly accelerate the ADP/ATP exchange. Mge1p, a mitochondrial matrix protein with homology to bacterial GrpE, serves as the nucleotide exchange factor of mitochondrial Hsp70. Here, we analyze the influence of temperature on the structure and functional properties of Mge1p from the yeast Saccharomyces cerevisiae. Mge1p is a dimer in solution that undergoes a reversible thermal transition at heat-shock temperatures, i.e. above 37 ?C, that involves protein unfolding and dimer dissociation. The thermally denatured protein is unable to interact stably with mitochondrial Hsp70, and therefore is unable to regulate its ATPase and chaperone cycle. Crosslinking of wild-type mitochondria reveals that Mge1p undergoes the same dimer to monomer temperature-dependent shift, and that the nucleotide exchange factor does not associate with its Hsp70 partner at stress temperatures (i.e. =45 ?C). Once the stress conditions disappear, Mge1p refolds and recovers both structure and functional properties. Therefore, Mge1p can act as a thermosensor for the mitochondrial Hsp70 system, regulating the nucleotide exchange rates under heat shock, as has been described for two bacterial GrpE proteins. The thermosensor activity is conserved in the GrpE-like nucleotide exchange factors although, as discussed here, it is achieved through a different structural mechanism.
Keywords: nucleotide exchange factor; Mge1p; GrpE; Hsp70; mitochondria
Abbreviations: DSC, differential scanning calorimetry; DSS, dissuccinimidyl suberate
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