The INs and OUTs of mitofusins.

Mitofusins are outer membrane proteins essential for mitochondrial fusion. Their accepted topology posits that both N and C termini face the cytoplasm. In this issue, Mattie et al. (2018. J. Cell Biol. https://doi.org/10.1083/jcb.201611194) demonstrate instead that their C termini reside in the intermembrane space. These findings call for a revision of the current models of mitochondrial fusion.