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The intramitochondrial dynamin-related GTPase, Mgm1p, is a component of a protein complex that mediates mitochondrial fusion.

Wong ED, Wagner JA, Scott SV, Okreglak V, Holewinske TJ, Cassidy-Stone A, Nunnari J - J. Cell Biol. (2003)

Bottom Line: In yeast, mitochondrial fission is regulated by the outer membrane-associated dynamin-related GTPase, Dnm1p.Using protease protection and immuno-EM, we have shown previously that Mgm1p localizes to the intermembrane space, associated with the inner membrane.To further test our conclusions, we have used a novel method using the tobacco etch virus protease and confirm that Mgm1p is present in the intermembrane space compartment in vivo.

View Article: PubMed Central - PubMed

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

ABSTRACT
A balance between fission and fusion events determines the morphology of mitochondria. In yeast, mitochondrial fission is regulated by the outer membrane-associated dynamin-related GTPase, Dnm1p. Mitochondrial fusion requires two integral outer membrane components, Fzo1p and Ugo1p. Interestingly, mutations in a second mitochondrial-associated dynamin-related GTPase, Mgm1p, produce similar phenotypes to fzo1 and ugo cells. Specifically, mutations in MGM1 cause mitochondrial fragmentation and a loss of mitochondrial DNA that are suppressed by abolishing DNM1-dependent fission. In contrast to fzo1ts mutants, blocking DNM1-dependent fission restores mitochondrial fusion in mgm1ts cells during mating. Here we show that blocking DNM1-dependent fission in Deltamgm1 cells fails to restore mitochondrial fusion during mating. To examine the role of Mgm1p in mitochondrial fusion, we looked for molecular interactions with known fusion components. Immunoprecipitation experiments revealed that Mgm1p is associated with both Ugo1p and Fzo1p in mitochondria, and that Ugo1p and Fzo1p also are associated with each other. In addition, genetic analysis of specific mgm1 alleles indicates that Mgm1p's GTPase and GTPase effector domains are required for its ability to promote mitochondrial fusion and that Mgm1p self-interacts, suggesting that it functions in fusion as a self-assembling GTPase. Mgm1p's localization within mitochondria has been controversial. Using protease protection and immuno-EM, we have shown previously that Mgm1p localizes to the intermembrane space, associated with the inner membrane. To further test our conclusions, we have used a novel method using the tobacco etch virus protease and confirm that Mgm1p is present in the intermembrane space compartment in vivo. Taken together, these data suggest a model where Mgm1p functions in fusion to remodel the inner membrane and to connect the inner membrane to the outer membrane via its interactions with Ugo1p and Fzo1p, thereby helping to coordinate the behavior of the four mitochondrial membranes during fusion.

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MGM1 is required for mitochondrial fusion during mating. Cells of the opposite mating type expressing either mito–GFP or mito–RFP were grown to log phase at 25°C, mated at 37°C, and imaged as previously described (Wong et al., 2000). Mitochondrial fusion was assessed by examining merged mito–GFP and mito–RFP images of large-budded homozygous zygotes formed from wild type (A–D), mgm1–5Δdnm1 (E–H), Δmgm1Δdnm1 (I–L), and Δfzo1Δdnm1 (M–P). Bars, 2 μm.
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fig1: MGM1 is required for mitochondrial fusion during mating. Cells of the opposite mating type expressing either mito–GFP or mito–RFP were grown to log phase at 25°C, mated at 37°C, and imaged as previously described (Wong et al., 2000). Mitochondrial fusion was assessed by examining merged mito–GFP and mito–RFP images of large-budded homozygous zygotes formed from wild type (A–D), mgm1–5Δdnm1 (E–H), Δmgm1Δdnm1 (I–L), and Δfzo1Δdnm1 (M–P). Bars, 2 μm.

Mentions: In the present study, we assayed for mitochondrial fusion in Δmgm1Δdnm1 cells during mating. As shown in Fig. 1, we observed that, as expected, mitochondrial fragmentation due to loss of MGM1 function was suppressed by blocking DNM1-dependent fission, and tubules were restored (E–H). However, in contrast to wild-type (Fig. 1, A–D) and mgm1tsΔdnm1 cells (Fig. 1, E–H; Wong et al., 2000), mitochondrial fusion was blocked in Δmgm1Δdnm1 cells (Fig. 1, E–H, 8% unfused, n = 50; I–L, 98% unfused, n = 50), a phenotype identical to that observed for Δfzo1Δdnm1 cells (Fig. 2, N–P). One explanation for this observed difference between Δmgm1 and mgm1ts strains is that the mgm1ts alleles tested are hypomorphs and, when shifted to the nonpermissive temperature, retain enough function to support mitochondrial fusion. In this case, Mgm1p may be essential for mitochondrial fusion, and our observations indicate that the requirement for Mgm1p function in fusion is diminished in the context of mitochondrial tubules, as opposed to mitochondrial fragments. Alternatively, it is possible that mitochondrial fusion is abolished in Δmgm1Δdnm1 cells as a secondary, indirect consequence of loss of MGM1 function.


The intramitochondrial dynamin-related GTPase, Mgm1p, is a component of a protein complex that mediates mitochondrial fusion.

Wong ED, Wagner JA, Scott SV, Okreglak V, Holewinske TJ, Cassidy-Stone A, Nunnari J - J. Cell Biol. (2003)

MGM1 is required for mitochondrial fusion during mating. Cells of the opposite mating type expressing either mito–GFP or mito–RFP were grown to log phase at 25°C, mated at 37°C, and imaged as previously described (Wong et al., 2000). Mitochondrial fusion was assessed by examining merged mito–GFP and mito–RFP images of large-budded homozygous zygotes formed from wild type (A–D), mgm1–5Δdnm1 (E–H), Δmgm1Δdnm1 (I–L), and Δfzo1Δdnm1 (M–P). Bars, 2 μm.
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Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2172654&req=5

fig1: MGM1 is required for mitochondrial fusion during mating. Cells of the opposite mating type expressing either mito–GFP or mito–RFP were grown to log phase at 25°C, mated at 37°C, and imaged as previously described (Wong et al., 2000). Mitochondrial fusion was assessed by examining merged mito–GFP and mito–RFP images of large-budded homozygous zygotes formed from wild type (A–D), mgm1–5Δdnm1 (E–H), Δmgm1Δdnm1 (I–L), and Δfzo1Δdnm1 (M–P). Bars, 2 μm.
Mentions: In the present study, we assayed for mitochondrial fusion in Δmgm1Δdnm1 cells during mating. As shown in Fig. 1, we observed that, as expected, mitochondrial fragmentation due to loss of MGM1 function was suppressed by blocking DNM1-dependent fission, and tubules were restored (E–H). However, in contrast to wild-type (Fig. 1, A–D) and mgm1tsΔdnm1 cells (Fig. 1, E–H; Wong et al., 2000), mitochondrial fusion was blocked in Δmgm1Δdnm1 cells (Fig. 1, E–H, 8% unfused, n = 50; I–L, 98% unfused, n = 50), a phenotype identical to that observed for Δfzo1Δdnm1 cells (Fig. 2, N–P). One explanation for this observed difference between Δmgm1 and mgm1ts strains is that the mgm1ts alleles tested are hypomorphs and, when shifted to the nonpermissive temperature, retain enough function to support mitochondrial fusion. In this case, Mgm1p may be essential for mitochondrial fusion, and our observations indicate that the requirement for Mgm1p function in fusion is diminished in the context of mitochondrial tubules, as opposed to mitochondrial fragments. Alternatively, it is possible that mitochondrial fusion is abolished in Δmgm1Δdnm1 cells as a secondary, indirect consequence of loss of MGM1 function.

Bottom Line: In yeast, mitochondrial fission is regulated by the outer membrane-associated dynamin-related GTPase, Dnm1p.Using protease protection and immuno-EM, we have shown previously that Mgm1p localizes to the intermembrane space, associated with the inner membrane.To further test our conclusions, we have used a novel method using the tobacco etch virus protease and confirm that Mgm1p is present in the intermembrane space compartment in vivo.

View Article: PubMed Central - PubMed

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

ABSTRACT
A balance between fission and fusion events determines the morphology of mitochondria. In yeast, mitochondrial fission is regulated by the outer membrane-associated dynamin-related GTPase, Dnm1p. Mitochondrial fusion requires two integral outer membrane components, Fzo1p and Ugo1p. Interestingly, mutations in a second mitochondrial-associated dynamin-related GTPase, Mgm1p, produce similar phenotypes to fzo1 and ugo cells. Specifically, mutations in MGM1 cause mitochondrial fragmentation and a loss of mitochondrial DNA that are suppressed by abolishing DNM1-dependent fission. In contrast to fzo1ts mutants, blocking DNM1-dependent fission restores mitochondrial fusion in mgm1ts cells during mating. Here we show that blocking DNM1-dependent fission in Deltamgm1 cells fails to restore mitochondrial fusion during mating. To examine the role of Mgm1p in mitochondrial fusion, we looked for molecular interactions with known fusion components. Immunoprecipitation experiments revealed that Mgm1p is associated with both Ugo1p and Fzo1p in mitochondria, and that Ugo1p and Fzo1p also are associated with each other. In addition, genetic analysis of specific mgm1 alleles indicates that Mgm1p's GTPase and GTPase effector domains are required for its ability to promote mitochondrial fusion and that Mgm1p self-interacts, suggesting that it functions in fusion as a self-assembling GTPase. Mgm1p's localization within mitochondria has been controversial. Using protease protection and immuno-EM, we have shown previously that Mgm1p localizes to the intermembrane space, associated with the inner membrane. To further test our conclusions, we have used a novel method using the tobacco etch virus protease and confirm that Mgm1p is present in the intermembrane space compartment in vivo. Taken together, these data suggest a model where Mgm1p functions in fusion to remodel the inner membrane and to connect the inner membrane to the outer membrane via its interactions with Ugo1p and Fzo1p, thereby helping to coordinate the behavior of the four mitochondrial membranes during fusion.

Show MeSH
Related in: MedlinePlus