Chaperone protein which plays an important role in mitochondrial iron-sulfur cluster (ISC) biogenesis. Interacts with and stabilizes ISC cluster assembly proteins FXN, NFU1, NFS1 and ISCU. Regulates erythropoiesis via stabilization of ISC assembly. May play a role in the control of cell proliferation and cellular aging. Interacts strongly with the intermediate form of FXN and weakly with its mature form. Associates with the mitochondrial contact site and cristae organizing system (MICOS) complex (also known as MINOS or MitOS complex). Belongs to the heat shock protein 70 family.