Shaping the composition of the mitochondrial outer membrane.

Mitochondria are critical double-membraned organelles that act as biosynthetic and bioenergetic cellular factories, with the outer membrane providing an interface with the rest of the cell. Mitochondrial outer membrane proteins regulate a variety of processes, including metabolism, innate immunity and apoptosis. Although the biophysical and functional diversity of these proteins is highly documented, the mechanisms of their biogenesis and the integration of that into cellular homeostasis are just starting to take shape. Here, focusing on α-helical outer membrane proteins, we review recent insights into the mechanisms of synthesis and cytosolic chaperoning, insertion and assembly in the lipid bilayer, and quality control of unassembled or mislocalized transmembrane domains. We further discuss the role convergent evolution played in this process, comparing key biogenesis players from lower eukaryotes, including yeast and trypanosomes, with multicellular metazoan systems, and draw comparisons with the endoplasmic reticulum biogenesis system, in which membrane proteins face similar challenges.