CryoEM of OPA1 helical structures gives perspective to mitochondrial dysfunction.

Abstract

The leading cause of childhood blindness, Dominant Optic Atrophy (DOA), is primarily caused by mutation of the gene encoding the Optic Atrophy 1 (OPA1) protein. These OPA1 mutations putatively dysregulate the inner-membrane (IM) fusion processes of the mitochondria, leading to mitochondrial network disruption and DOA pathology. OPA1 exists in two forms in vivo: the long-form which is tethered to the inner mitochondrial membrane through a single transmembrane domain, and the short form that results from proteolytic cleavage in the inner membrane space as a response to mitochondrial membrane depolarization. Using cryo-electron microscopy, we solved helical structures of the short form of human OPA1 isoform-1 (sOPA1) with and without nucleotide. These helical assemblies formed around a lipid core, with densely packed protein rungs marked by minimal inter-rung connectivity. Notably, we also observed nucleotide-dependent dimerization of the GTPase domains, a hallmark of dynamin superfamily proteins. The sOPA1 assemblies contained several unique secondary structures to strengthen membrane association, pointing to the importance of sOPA1 membrane interaction to its function. The novel structural features and interfaces also shed light on the effects of pathogenic point mutations on protein folding, inter-protein assembly, and membrane interactions. Further, the biological relevance of the sOPA1 helical assemblies was supported by in-cell assays, where disruption of the structurally identified interfaces and membrane binding regions caused fragmentation of the mitochondrial network.