The mitochondrial chaperone HSPD1 folds MTHFD2 independently of its co-chaperone HSPE1

Acquiring new cellular states entails metabolic reprogramming driven by changes in the expression of cytosolic and mitochondrial metabolic enzymes. Most mitochondrial proteins are synthesized in the cytosol and imported into the mitochondria in a linear form, after which they are folded by a network of mitochondrial chaperones and co-chaperones. Which mitochondrial protein is dependent upon which chaperone for its folding is largely unknown. HSPD1/HSPE1 (HSP60/HSP10) are evolutionarily conserved mammalian homologues of the bacterial proteins GroEL/GroES, forming a chamber-and-lid chaperonin to facilitate the folding of client proteins. The endogenous clients of HSP60 in mammalian cells are not fully known, nor are the HSP60 clients that require HSP10 for folding. We used gene knockdown and stable isotope labelling of amino acids in cell culture (SILAC)-based proteomics to identify HSPD1 client proteins. We found that HSPD1 supports the expression of methylenetetrahydrofolate dehydrogenase 2 (MTHFD2), a key mitochondrial one-carbon (1C) pathway enzyme, in cells and tumours. In addition, HSPD1 directly folds MTHFD2 independently of its co-chaperone HSPE1. HSPD1 interacts with MTHFD2 in mitochondria, and MTHFD2 is degraded by LONP1 in HSPD1 knockdown cells. Our data show that HSPD1 is an MTHFD2 chaperone and can fold an endogenous client protein independently of HSPE1, providing a link between mitochondrial protein folding and the 1C pathway. This article is part of the Theo Murphy meeting issue 'ProteostaSys: a systems view of proteostasis'.