Human CCDC51 and yeast Mdm33 are functionally conserved mitochondrial inner membrane proteins that demarcate a subset of organelle fission events.

Mitochondria are highly dynamic double membrane-bound organelles that exist in a semi- continuous network. Mitochondrial morphology arises from the complex interplay of numerous processes, including opposing fission and fusion dynamics and the formation of highly organized cristae invaginations of the inner membrane. While extensive work has examined the mechanisms of mitochondrial fission, it remains unclear how fission is coordinated across two membrane bilayers and how mitochondrial inner membrane organization is coupled with mitochondrial fission dynamics. Previously, the yeast protein Mdm33 was implicated in facilitating fission by coordinating with inner membrane homeostasis pathways. However, Mdm33 is not conserved outside fungal species and its precise mechanistic role remains unclear. Here, we use a bioinformatic approach to identify a putative structural ortholog of Mdm33 in humans, CCDC51 (also called MITOK). We find that the mitochondrial phenotypes associated with altered CCDC51 levels implicate the protein in mitochondrial fission dynamics. Further, using timelapse microscopy, we spatially and temporally resolve Mdm33 and CCDC51 to a subset of mitochondrial fission events. Finally, we show that CCDC51 can partially rescue yeast Δ mdm33 cells, indicating the proteins are functionally analogous. Our data reveal that Mdm33/CCDC51 are conserved mediators of mitochondrial morphology and suggest the proteins play a crucial role in maintaining normal mitochondrial dynamics and organelle homeostasis.