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JCS ePress
online publication date 30 Nov 2004
doi: 10.1242/jcs.01565
Research Article
Mitofusin 1 and 2 play distinct roles in mitochondrial fusion reactions via GTPase activity
Naotada Ishihara,
Yuka Eura,
and
Katsuyoshi Mihara*
* Author for correspondence (e-mail: mihara{at}cell.med.kyushu-u.ac.jp)
The mammalian homologues of yeast and Drosophila Fzo, mitofusin (Mfn) 1 and 2, are both essential for mitochondrial fusion and maintenance of mitochondrial morphology. Though the GTPase domain is required for Mfn protein function, the molecular mechanisms of the GTPase-dependent reaction as well as the functional division of the two Mfn proteins are unknown. To examine the function of Mfn proteins, tethering of mitochondrial membranes was measured in vitro by fluorescence microscopy using green fluorescence protein- or red fluorescent protein-tagged and Mfn1-expressing mitochondria, or by immunoprecipitation using mitochondria harboring HA- or FLAG-tagged Mfn proteins. These experiments revealed that Mfn1-harboring mitochondria were efficiently tethered in a GTP-dependent manner, whereas Mfn2-harboring mitochondria were tethered with only low efficiency. Sucrose density gradient centrifugation followed by co-immunoprecipitation revealed that Mfn1 produced oligomerized
250 kDa and
450 kDa complexes in a GTP-dependent manner. The
450 kDa complex contained oligomerized Mfn1 from distinct apposing membranes (docking complex), whereas the
250 kDa complex was composed of Mfn1 present on the same membrane or in the membrane-solubilized state (cis complex). These results were also confirmed using blue-native PAGE. Mfn1 exhibited higher activity for this reaction than Mfn2. Purified recombinant Mfn1 exhibited
eightfold higher GTPase activity than Mfn2. These findings indicate that the two Mfn proteins have distinct activities, and suggest that Mfn1 is mainly responsible for GTP-dependent membrane tethering.

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