Fusion of red and green mitochondria (left) requires Mgm1p (right).

Yeast mitochondria undergo both fission and fusion events, leading to the formation of branched, reticular structures within cells. Although mitochondrial fission has been characterized relatively well, studies on fusion have been considerably more difficult. In an elegant genetic and biochemical analysis, Wong et al. (page 303) now address some previously conflicting data and establish a new model for the coordination of this complex process.

Previous studies established that the integral mitochondrial outer membrane proteins Fzo1p and Ugo1p are essential for fusion and that the dynamin-related GTPase Mgm1p might be indirectly involved in the process. The new work demonstrates that Mgm1p is in fact an essential component of a fusion complex that also includes Fzo1p and Ugo1p and that Mgm1p functions as a self-assembling GTPase in fusion. Using a novel protease-based technique, the authors also resolve a controversy about the localization of Mgm1p, showing that the protein localizes to the mitochondrial intermembrane space.

The data support a model in which Mgm1p in the inner membrane and Fzo1p and Ugo1p in the outer membrane coordinate the reorganization of both membranes to lay the groundwork for fusion. The authors are now identifying additional regulatory molecules that appear to target Mgm1p. ▪