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The dynamin-related GTPase, Mgm1p, is an intermembrane space protein required for maintenance of fusion competent mitochondria.

Wong ED, Wagner JA, Gorsich SW, McCaffery JM, Shaw JM, Nunnari J - J. Cell Biol. (2000)

Bottom Line: To study the primary function of MGM1, we characterized new temperature sensitive MGM1 alleles.Although Mgm1p was recently reported to localize to the mitochondrial outer membrane, our studies indicate that Mgm1p is localized to the mitochondrial intermembrane space.Based on our localization data and Mgm1p's structural homology to dynamin, we postulate that it functions in inner membrane remodeling events.

View Article: PubMed Central - PubMed

Affiliation: Section of Molecular and Cellular Biology, University of California Davis, Davis, California 95616, USA.

ABSTRACT
Mutations in the dynamin-related GTPase, Mgm1p, have been shown to cause mitochondrial aggregation and mitochondrial DNA loss in Saccharomyces cerevisiae cells, but Mgm1p's exact role in mitochondrial maintenance is unclear. To study the primary function of MGM1, we characterized new temperature sensitive MGM1 alleles. Examination of mitochondrial morphology in mgm1 cells indicates that fragmentation of mitochondrial reticuli is the primary phenotype associated with loss of MGM1 function, with secondary aggregation of mitochondrial fragments. This mgm1 phenotype is identical to that observed in cells with a conditional mutation in FZO1, which encodes a transmembrane GTPase required for mitochondrial fusion, raising the possibility that Mgm1p is also required for fusion. Consistent with this idea, mitochondrial fusion is blocked in mgm1 cells during mating, and deletion of DNM1, which encodes a dynamin-related GTPase required for mitochondrial fission, blocks mitochondrial fragmentation in mgm1 cells. However, in contrast to fzo1 cells, deletion of DNM1 in mgm1 cells restores mitochondrial fusion during mating. This last observation indicates that despite the phenotypic similarities observed between mgm1 and fzo1 cells, MGM1 does not play a direct role in mitochondrial fusion. Although Mgm1p was recently reported to localize to the mitochondrial outer membrane, our studies indicate that Mgm1p is localized to the mitochondrial intermembrane space. Based on our localization data and Mgm1p's structural homology to dynamin, we postulate that it functions in inner membrane remodeling events. In this context, the observed mgm1 phenotypes suggest that inner and outer membrane fission is coupled and that loss of MGM1 function may stimulate Dnm1p-dependent outer membrane fission, resulting in the formation of mitochondrial fragments that are structurally incompetent for fusion.

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Mitochondrial fusion does not require MGM1 function. Mitochondrial fusion was assessed as described in Fig. 2. Mitochondria in homozygous zygotes formed at 37°C from Δdnm1 (A–D), mgm1-5 Δdnm1 (E–H), fzo1-1 Δdnm1 (I–L), and mgm1-5 fzo1-1 Δdnm1 (M–P) cells were visualized by fluorescence confocal microscopy. Bars, 2 μm.
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Figure 4: Mitochondrial fusion does not require MGM1 function. Mitochondrial fusion was assessed as described in Fig. 2. Mitochondria in homozygous zygotes formed at 37°C from Δdnm1 (A–D), mgm1-5 Δdnm1 (E–H), fzo1-1 Δdnm1 (I–L), and mgm1-5 fzo1-1 Δdnm1 (M–P) cells were visualized by fluorescence confocal microscopy. Bars, 2 μm.

Mentions: Although these data suggest that MGM1 might play a role in the fusion process, the structural similarity of Mgm1p to dynamin-like proteins suggests that it is involved in membrane remodeling and/or fission events. Thus, we considered the alternative explanation, that mitochondrial fragmentation in mgm1-5 cells results from an increase in Dnm1p-dependent mitochondrial fission. In this scenario, mitochondrial fragments might fail to fuse because of their geometry/structure and not as a primary consequence of loss of MGM1 function. To test this possibility, we assayed mitochondrial fusion during mating in mgm1-5 Δdnm1 cells, where deletion of DNM1 blocks mitochondrial fragmentation and restores mitochondrial tubular structures (Fig. 3G and Fig. H; Table ). As shown previously, deletion of DNM1 has no effect on mitochondrial fusion during mating, consistent with its role in fission (Bleazard et al. 1999; Fig. 4, A–D; Table ). Thus, as expected at the permissive temperature where MGM1 is functional, mitochondrial fusion and content mixing occurred in zygotes formed from mgm1-5 Δdnm1 haploids (Table ). In contrast to mgm1-5 cells at nonpermissive temperature, where both mitochondrial fragmentation and a block in mitochondrial fusion are observed, deletion of DNM1 in mgm1-5 cells restored mitochondrial fusion during mating (Fig. 4, E–H; Table ). Thus, the presence of mitochondrial tubular and net structures in mgm1-5 Δdnm1 cells correlates with the restoration of mitochondrial fusion (Table ). In contrast, FZO1 function is still required for mitochondrial fusion, even when tubules are restored in fzo1-1 Δdnm1 zygotes, consistent with Fzo1p's proposed direct role in the fusion process (Bleazard et al. 1999; Fig. 4, I–L; Table ).


The dynamin-related GTPase, Mgm1p, is an intermembrane space protein required for maintenance of fusion competent mitochondria.

Wong ED, Wagner JA, Gorsich SW, McCaffery JM, Shaw JM, Nunnari J - J. Cell Biol. (2000)

Mitochondrial fusion does not require MGM1 function. Mitochondrial fusion was assessed as described in Fig. 2. Mitochondria in homozygous zygotes formed at 37°C from Δdnm1 (A–D), mgm1-5 Δdnm1 (E–H), fzo1-1 Δdnm1 (I–L), and mgm1-5 fzo1-1 Δdnm1 (M–P) cells were visualized by fluorescence confocal microscopy. Bars, 2 μm.
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Related In: Results  -  Collection

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Figure 4: Mitochondrial fusion does not require MGM1 function. Mitochondrial fusion was assessed as described in Fig. 2. Mitochondria in homozygous zygotes formed at 37°C from Δdnm1 (A–D), mgm1-5 Δdnm1 (E–H), fzo1-1 Δdnm1 (I–L), and mgm1-5 fzo1-1 Δdnm1 (M–P) cells were visualized by fluorescence confocal microscopy. Bars, 2 μm.
Mentions: Although these data suggest that MGM1 might play a role in the fusion process, the structural similarity of Mgm1p to dynamin-like proteins suggests that it is involved in membrane remodeling and/or fission events. Thus, we considered the alternative explanation, that mitochondrial fragmentation in mgm1-5 cells results from an increase in Dnm1p-dependent mitochondrial fission. In this scenario, mitochondrial fragments might fail to fuse because of their geometry/structure and not as a primary consequence of loss of MGM1 function. To test this possibility, we assayed mitochondrial fusion during mating in mgm1-5 Δdnm1 cells, where deletion of DNM1 blocks mitochondrial fragmentation and restores mitochondrial tubular structures (Fig. 3G and Fig. H; Table ). As shown previously, deletion of DNM1 has no effect on mitochondrial fusion during mating, consistent with its role in fission (Bleazard et al. 1999; Fig. 4, A–D; Table ). Thus, as expected at the permissive temperature where MGM1 is functional, mitochondrial fusion and content mixing occurred in zygotes formed from mgm1-5 Δdnm1 haploids (Table ). In contrast to mgm1-5 cells at nonpermissive temperature, where both mitochondrial fragmentation and a block in mitochondrial fusion are observed, deletion of DNM1 in mgm1-5 cells restored mitochondrial fusion during mating (Fig. 4, E–H; Table ). Thus, the presence of mitochondrial tubular and net structures in mgm1-5 Δdnm1 cells correlates with the restoration of mitochondrial fusion (Table ). In contrast, FZO1 function is still required for mitochondrial fusion, even when tubules are restored in fzo1-1 Δdnm1 zygotes, consistent with Fzo1p's proposed direct role in the fusion process (Bleazard et al. 1999; Fig. 4, I–L; Table ).

Bottom Line: To study the primary function of MGM1, we characterized new temperature sensitive MGM1 alleles.Although Mgm1p was recently reported to localize to the mitochondrial outer membrane, our studies indicate that Mgm1p is localized to the mitochondrial intermembrane space.Based on our localization data and Mgm1p's structural homology to dynamin, we postulate that it functions in inner membrane remodeling events.

View Article: PubMed Central - PubMed

Affiliation: Section of Molecular and Cellular Biology, University of California Davis, Davis, California 95616, USA.

ABSTRACT
Mutations in the dynamin-related GTPase, Mgm1p, have been shown to cause mitochondrial aggregation and mitochondrial DNA loss in Saccharomyces cerevisiae cells, but Mgm1p's exact role in mitochondrial maintenance is unclear. To study the primary function of MGM1, we characterized new temperature sensitive MGM1 alleles. Examination of mitochondrial morphology in mgm1 cells indicates that fragmentation of mitochondrial reticuli is the primary phenotype associated with loss of MGM1 function, with secondary aggregation of mitochondrial fragments. This mgm1 phenotype is identical to that observed in cells with a conditional mutation in FZO1, which encodes a transmembrane GTPase required for mitochondrial fusion, raising the possibility that Mgm1p is also required for fusion. Consistent with this idea, mitochondrial fusion is blocked in mgm1 cells during mating, and deletion of DNM1, which encodes a dynamin-related GTPase required for mitochondrial fission, blocks mitochondrial fragmentation in mgm1 cells. However, in contrast to fzo1 cells, deletion of DNM1 in mgm1 cells restores mitochondrial fusion during mating. This last observation indicates that despite the phenotypic similarities observed between mgm1 and fzo1 cells, MGM1 does not play a direct role in mitochondrial fusion. Although Mgm1p was recently reported to localize to the mitochondrial outer membrane, our studies indicate that Mgm1p is localized to the mitochondrial intermembrane space. Based on our localization data and Mgm1p's structural homology to dynamin, we postulate that it functions in inner membrane remodeling events. In this context, the observed mgm1 phenotypes suggest that inner and outer membrane fission is coupled and that loss of MGM1 function may stimulate Dnm1p-dependent outer membrane fission, resulting in the formation of mitochondrial fragments that are structurally incompetent for fusion.

Show MeSH
Related in: MedlinePlus