Mgm1 is a member of the dynamin family of GTP-binding proteins. Mgm1 was first 
identified in yeast, where it affects mitochondrial morphology. The human 
homologue of Mgm1 is called OPA1. Mutations in the OPA1 gene are the prevailing 
cause of dominant optic atrophy, a hereditary disease in which progressive 
degeneration of the optic nerve can lead to blindness. Here we investigate the 
properties of the Mgm1/OPA1 protein in mammalian cells. We find that Mgm1/OPA1 
is localized to the mitochondrial intermembrane space, where it is tightly bound 
to the outer surface of the inner membrane. Overexpression of wild type or 
mutant forms of the Mgm1/OPA1 protein cause mitochondria to fragment and, in 
some cases, cluster near the nucleus, whereas the loss of protein caused by 
small interfering RNA (siRNA) leads to dispersal of mitochondrial fragments 
throughout the cytosol. The cristae of these fragmented mitochondria are 
disorganized. At early time points after transfection with Mgm1/OPA1 siRNA, the 
mitochondria are not yet fragmented. Instead, the mitochondria swell and 
stretch, after which they form localized constrictions similar to the 
mitochondrial abnormalities observed during the early stages of apoptosis. These 
abnormalities might be the earliest effects of losing Mgm1/OPA1 protein.