HAX1 drives assembly and activation of the mitochondrial intermembrane space chaperone CLPB

The multicellular metazoan lineage acquired a novel chaperone in the mitochondrial intermembrane space, the AAA+ disaggregase and refoldase CLPB. Although it is not known how they function together, CLPB and the intrinsically disordered IMS protein HAX1 interact and share disease and cellular phenotypes; loss of function in either gene causes severe congenital neutropenia as well as neuropathology and causes many proteins in the IMS and its bounding membranes to become insoluble. We sought to determine how HAX1 functions with CLPB. Through biochemical reconstitution, we find that HAX1 is a stimulatory cofactor of CLPB. HAX1 promotes oligomerization of CLPB into an active disaggregase and stimulates the ATPase and refoldase activities of the oligomeric complex. A short peptide within HAX1 is necessary for direct interaction with the ankyrin domain of CLPB, but stimulation of CLPB activity requires additional elements of HAX1. Characterization of CLPB and CLPB-HAX1 oligomers indicates that HAX1 shifts the predominant oligomeric state of CLPB from a dodecamer to a hexamer elaborated 1:1 with HAX1, suggesting that this smaller oligomer is important during the cycle of CLPB function with clients.