Rat liver GTP-binding proteins mediate changes in mitochondrial membrane potential and organelle fusion

The variety of mitochondrial morphology in healthy and diseased cells can be explained by regulated mitochondrial fusion. Previously, a mitochondrial outer membrane fraction containing fusogenic, aluminum fluoride (AlF4)sensitive GTP-binding proteins (mtg) was separated from rat liver (J. D. Cortese, Exp. Cell Res. 240: 122-133, 1998). Quantitative confocal microscopy now reveals that mtg transiently increases mitochondrial membrane potential (Delta Psi) when added to permeabilized rat hepatocytes (15%), rat fibroblasts (19%), and rabbit myocytes (10%). This large mtg-induced Delta Psi increment is blocked by fusogenic GTPase-specific modulators such as guanosine 5'-O-(3-thiotriphosphate), excess GTP (>100 mu M), and AlF4, suggesting a linkage between Delta Psi and mitochondrial fusion. Accordingly, stereometric analysis shows that decreasing Delta Psi or ATP synthesis with respiratory inhibitors limits mtg- and AlF4-induced mitochondrial fusion. Also, a specific G protein inhibitor (Bordetella pertussis toxin) hyperpolarizes mitochondria and leads to a loss of AlF4-dependent mitochondrial fusion. These results place mtg-induced Delta Psi changes upstream of AlF4-induced mitochondrial fusion, suggesting that GTPases exert Delta Psi-dependent control of the fusion process. Mammalian mitochondrial morphology thus can be modulated by cellular energetics.