Cells do not like simplicity. So instead of being spherical, mitochondria form tubular networks with complex inner membranes. How do cells build these complicated structures? To tackle this question, a few years ago, Rob Jensen's laboratory screened for Saccharomyces cerevisiae mutants defective in mitochondrial shape, and isolated one mutant, mmm1, in which mitochondria were large and spherical.

The Jensen group have now analysed the localization of an Mmm1–GFP chimera in living yeast cells. They found Mmm1 in spots on the surface of mitochondria, adjacent to a subset of mitochondrial DNA (mtDNA) nucleoids. In mmm1 mutants, mtDNA collapses into one large structure, and cannot be properly segregated during mitochondrial division, which leads to its loss.

But Mmm1 is an outer membrane protein, so how can it bind to mtDNA inside mitochondria? The authors suggest that Mmm1 functions in the formation of contact sites between the outer and inner membranes, and this is supported by the fact that inner membranes collapse in mmm1 mutants.

So the authors' model is that Mmm1 is part of a scaffold-like complex — the 'mitoskeleton' — that holds the outer and inner membranes together and is required for normal mitochondrial shape and mtDNA segregation.