Co-chaperone required for the assembly of iron-sulfur (Fe/S) clusters in mitochondria. Stimulates the ATPase activity of its specialized Hsp70 chaperone partner SSQ1, to mediate the transfer of iron-sulfur clusters from ISU1 to GRX5. Binds to the substrate protein ISU1 and targets it to SSQ1. Interacts with ISU1 and SSQ1. Increases association between GRX5 and SSQ1 (PubMed:23615440). Inviable vegetative cell population growth (PubMed:26545917). Present with 2010 molecules/cell in log phase SD medium. Belongs to the HscB family.