The human AAA+ ATPase CLPB (SKD3) is a protein disaggregase in the mitochondrial intermembrane space (IMS) and functions to promote the solubilization of various mitochondrial proteins. Loss-of-function CLPB mutations are associated with a few human diseases with neutropenia and neurological disorders. Unlike canonical AAA+ proteins, CLPB contains a unique ankyrin repeat domain (ANK) at its N-terminus. How CLPB functions as a disaggregase and the role of its ANK domain are currently unclear. Herein, we report a comprehensive structural characterization of human CLPB in both the apo- and substrate-bound states. CLPB assembles into homo-tetradecamers in apo-state and is remodeled into homo-dodecamers upon substrate binding. Conserved pore-loops (PLs) on the ATPase domains form a spiral staircase to grip and translocate the substrate in a step-size of 2 amino acid residues. The ANK domain is not only responsible for maintaining the higher-order assembly but also essential for the disaggregase activity. Interactome analysis suggests that the ANK domain may directly interact with a variety of mitochondrial substrates. These results reveal unique properties of CLPB as a general disaggregase in mitochondria and highlight its potential as a target for the treatment of various mitochondria-related diseases.

Expanding mitochondrial base editing tools with broad sequence compatibility is of high need for both research and therapeutic purposes. In this study, we identify a DddA homolog from Simiaoa sunii (Ddd_Ss) which can efficiently deaminate cytosine in DC context in double-stranded DNA (dsDNA). We successfully develop Ddd_Ss-derived cytosine base editors (DdCBE_Ss) and introduce mutations at multiple mitochondrial DNA (mtDNA) loci including disease-associated mtDNA mutations in previously inaccessible GC context. Finally, by introducing a single amino acid substitution from Ddd_Ss, we successfully improve the activity and sequence compatibility of DdCBE derived from DddA of Burkholderia cenocepacia (DdCBE_Bc). Our study expands mtDNA editing tool boxes and provides resources for further screening and engineering dsDNA base editors for biological and therapeutic applications.

The human AAA+ ATPase CLPB (aka, HSP78 and Skd3) is a protein disaggregase and functions to promote the solubilization of proteins in the mitochondrial intermembrane space. Unlike other AAA+ protein unfoldases or disaggregases, CLPB contains an ankyrin repeat containing domain (ANK) at its N-terminus. Mutations of CLPB are closely associated with a few human diseases, such as 3-methylglutaconic aciduria (3-MGA) and severe congenital neutropenia (SCN). The mechanism of CLPB functions as a disaggregase and the role of its unique ANK domain are currently unclear. Herein, we report a comprehensive structural characterization of human CLPB in both the apo- and substrate-bound states. Our data show that CLPB assembles into homo-tetradecamers in apo-state and dodecamers in the presence of a substrate, mediated by ANK domains via a "head-to-head" organization. Conserved pore-loops (PLs) on the ATPase domains form a spiral staircase to grasp and translocate the substrate in a step-size of two amino acid residues. We also show that the ANK domain is functionally essential, and more important, responsible for direct binding to various mitocondiral proteins in the inner membrane or intermembrane space. These results suggest that CLPB functions as a general disaggregase in mitochondria and highlight its potential as a target for the treatment of various mitochondria-related diseases.