title: The two mitochondrial heat shock proteins 70, Ssc1 and Ssq1, compete for the cochaperone Mge1.
abstract: Two members of the heat shock protein 70 kDa (Hsp70) family, Ssc1 and Ssq1, perform important functions in the mitochondrial matrix. The essential Ssc1 is an abundant ATP-binding protein required for both import and folding of mitochondrial proteins. The function of Ssc1 is supported by an interaction with the preprotein translocase subunit Tim44, the cochaperone Mdj1, and the nucleotide exchange factor Mge1. In contrast, only limited information is available on Ssq1. So far, a basic characterization of Ssq1 has demonstrated its involvement in the maintenance of mitochondrial DNA, the maturation of the yeast frataxin (Yfh1) after import, and assembly of the mitochondrial Fe/S cluster. Here, we analyzed the biochemical properties and the interaction partners of Ssq1 in detail. Ssq1 showed typical chaperone properties by binding to unfolded substrate proteins in an ATP-regulated manner. Ssq1 was able to form a specific complex with the nucleotide exchange factor Mge1. In particular, complex formation in organello was enhanced significantly when Ssc1 was inactivated selectively. However, even under these conditions, no interaction of Ssq1 with the two other mitochondrial Hsp70-cochaperones, Tim44 and Mdj1, was observed. The Ssq1-Mge1 interaction showed a lower overall stability but the same characteristic nucleotide-dependence as the Ssc1-Mge1 interaction. A quantitative analysis of the interaction properties indicated a competition of Ssq1 with Ssc1 for binding to Mge1. Perturbation of Mge1 function or amounts resulted in direct effects on Ssq1 activity in intact mitochondria. We conclude that mitochondria represent the unique case where two Hsp70s compete for the interaction with one nucleotide exchange factor.
authors.affiliation: Institut für Biochemie und Molekularbiologie, Universität Freiburg, Hermann-Herder-Str. 7, D-79104, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Schmidt
authors.forename: A
authors.initials: A
authors.lastname: Strub
authors.forename: K
authors.initials: K
authors.lastname: Röttgers
authors.forename: N
authors.initials: N
authors.lastname: Zufall
authors.forename: W
authors.initials: W
authors.lastname: Voos
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000595:Amino Acid Sequence; D001667:Binding, Competitive; D000252:Calcium-Transporting ATPases / Q000737:chemistry / Q000378:metabolism*; D002352:Carrier Proteins / Q000737:chemistry / Q000378:metabolism*; D017124:Conserved Sequence; D005656:Fungal Proteins / Q000737:chemistry / Q000378:metabolism*; D050956:HSP40 Heat-Shock Proteins; D018840:HSP70 Heat-Shock Proteins; D006360:Heat-Shock Proteins / Q000378:metabolism; D008565:Membrane Proteins / Q000378:metabolism; D026901:Membrane Transport Proteins; D008928:Mitochondria / Q000737:chemistry / Q000378:metabolism; D033681:Mitochondrial Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D024101:Mitochondrial Proteins / Q000378:metabolism*; D008958:Models, Molecular; D018832:Molecular Chaperones / Q000737:chemistry / Q000378:metabolism*; D008969:Molecular Sequence Data; D011233:Precipitin Tests; D011485:Protein Binding; D011487:Protein Conformation; D011498:Protein Precursors / Q000378:metabolism; D021381:Protein Transport; D012441:Saccharomyces cerevisiae / Q000166:cytology / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism*
chemical_list: D002352:Carrier Proteins; D005656:Fungal Proteins; D050956:HSP40 Heat-Shock Proteins; D018840:HSP70 Heat-Shock Proteins; D006360:Heat-Shock Proteins; C087041:MDJ1 protein, S cerevisiae; C086039:MGE1 protein, S cerevisiae; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D033681:Mitochondrial Membrane Transport Proteins; D000090622:Mitochondrial Precursor Protein Import Complex Proteins; D024101:Mitochondrial Proteins; D018832:Molecular Chaperones; D011498:Protein Precursors; C101767:SSC1 protein, S cerevisiae; C115913:SSQ1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; C077016:TIM44 protein, S cerevisiae; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases