J. Biochem, 2003, Vol. 134, No. 3 333-344
© 2003 Japanese Biochemical Society
CELL |
Two Mitofusin Proteins, Mammalian Homologues of FZO, with Distinct Functions Are Both Required for Mitochondrial Fusion
,11 Department of Molecular Biology, Graduate School of Medical Science, Kyushu University, Fukuoka 812-8582; and 2 Biological Program, Yamanashi Medical University, Yamanashi, 409-3898
Mitochondria are dynamic organelles that undergo frequent fission and fusion or branching. Although these morphologic changes are considered crucial for cellular functions, the underlying mechanisms remain elusive, especially in mammalian cells. We characterized two rat mitochondrial outer membrane proteins, Mfn1 and Mfn2, with distinct tissue expressions, that are homologous to Drosophila Fzo, a GTPase involved in mitochondrial fusion. Expression of the GTPase-domain mutant of Mfn2 (Mfn2K109T) in HeLa cells induced mitochondrial fragmentation in which Mfn2K109T localized at the restricted domains. Immuno-electronmicroscopy revealed that Mfn2K109T was concentrated at the contact domains between adjacent mitochondria, suggesting that fusion of the outer membrane was arrested at some intermediate step. Mfn1 expression induced highly connected tubular network structures depending on the functional GTPase domain. The Mfn1-induced tubular networks were suppressed by co-expression with Mfn2. In vivo depletion of either isoform by RNA interference revealed that both are required to maintain normal mitochondrial morphology. The fusion of differentially-labeled mitochondria in HeLa cells subjected to depletion of either Mfn isoform and subsequent cell fusion by hemagglutinating virus of Japan revealed that both proteins have distinct functions in mitochondrial fusion. We conclude that the two Mfn isoforms cooperate in mitochondrial fusion in mammalian cells.
* These authors contributed equally to this work.
To whom correspondence should be addressed. Tel: +81-92-642-6176, Fax: +81-92-642-6183, E-mail: mihara{at}cell.med.kyushu-u.ac.jp
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