Co-chaperone required for the assembly of iron-sulfur (Fe/S) clusters in mitochondria (PubMed:26545917). Stimulates the ATPase activity of the mitochondrial Hsp70 chaperone ssc1, to mediate the transfer of iron-sulfur clusters from isu1 to grx5 (PubMed:26545917). Interacts with ssc1. Inviable. Belongs to the HscB family.