In brief

Mitochondria exist in many cells as more of a continuous reticulum than the isolated boats-in-the-ocean depicted in textbooks. They get that way through fusion. This is no mean feat, given that mitochondria have two membranes, which adhere but remain physically distinct. (Nuclei also have two membranes, but the membranes are continuous at the nuclear pores.) Drosophila that lack the Fuzzy onions protein have defective sperm because the sperm mitochondria fail to fuse. Hermann et al. investigate the budding yeast homologue of Fuzzy onions, Fzo1p, and find that its loss causes the yeast mitochondrial reticulum to fragment (page 359). Cells lacking Fzo1p are viable, but their mitochondria cannot fuse after mating. Fzo1p is an integral membrane protein with its GTPase domain in the cytoplasm. Fractionation suggests that it spans both mitochondrial membranes, perhaps at the contact sites where inner and outer mitochondrial membranes are directly apposed. Heptad repeats in the cytoplasmic domain may mediate self association in docking, fusion, or both, and two hydrophobic peptides are candidate fusion peptides.