The dynamin-related GTPase Dnm1 regulates mitochondrial fission in yeast

Abstract

The dynamin-related GTPase Dnm1 controls mitochondrial morphology in yeast. Here we show that dnm1 mutations convert the mitochondrial compartment into a planar 'net' of interconnected tubules. We propose that this net morphology results from a defect in mitochondrial fission. Immunogold labelling localizes Dnm1 to the cytoplasmic face of constricted mitochondrial tubules that appear to be dividing and to the ends of mitochondrial tubules that appear to have recently completed division. The activity of Dnm1 is epistatic to that of Fzo1, a GTPase in the outer mitochondrial membrane that regulates mitochondrial fusion. dnm1 mutations prevent mitochondrial fragmentation in fzo1 mutant strains. These findings indicate that Dnm1 regulates mitochondrial fission, assembling on the cytoplasmic face of mitochondrial tubules at sites at which division will occur.The mitochondrion is a complex organelle with a double membrane, its own genome and an independent protein-synthetic machinery. Although the role of mitochondria in metabolism and ATP production is more widely recognized, alterations in mitochondrial shape and abundance are also important for cellular function and differentiation. For example, mitochondrial morphology and copy number change in response to nutrient availability in yeast cells 1 , cell damage and apoptosis in mammalian cells 2 and developmental cues in Xenopus and Drosophila 3,4 . Cytological studies indicate that the 'steady-state' mitochondrial morphology and copy number can vary dramatically in different cell types, ranging from multiple, spherical organelles to the branched, tubular networks found in budding yeast and some mammalian cells [5][6][7] . These differences in mitochondrial morphology and copy number are largely determined by the balance between ongoing mitochondrial fission and fusion events.Little is known about molecules that regulate mitochondrial fission and fusion. The fuzzy onions (fzo) family of transmembrane GTPases was recently shown to control mitochondrial fusion in different organisms and cell types. In Drosophila, fzo is required for a developmentally regulated mitochondrial fusion event during spermatogenesis 8 . In budding yeast, mutations in fzo1 cause mitochondrial networks to fragment 9,10 and prevent mitochondrial fusion during yeast mating 9 . Molecules required for mitochondrial fission © 1999 Macmillan Magazines Ltd § Correspondence and requests for materials should be addressed to J.M.S. shaw@bioscience.utah.edu. NIH Public Access Results Mitochondrial membranes form nets in dnm1 mutant cellsWe previously reported that mitochondrial membranes collapse to one side of the cell in a dnm1Δ mutant strain 11 . Transmission electron microscopy indicated that these collapsed membranes might be organized in an unusual structure 11 . To characterize this structure further, we studied mitochondrial morphology in dnm1Δ cells under several different conditions.As reported previously, dnm1Δ mutants grown at 25 °C lack the highly branched mitochondrial network charac...

Could the low level of expression of the gene encoding skeletal muscle mitofusin-2 account for the metabolic inflexibility of obesity?

“…In yeast, selective inhibition of either processes significantly modifies mitochondrial size and interconnectivity [23,24]. Among the best-known proteins involved in mitochondrial dynamics are Fzo/mitofusin, a transmembrane GTPase involved in fusion [25], and Dnm1p/dynamin-related protein 1, a dynamin-related protein involved in fission [26].…”

Mitochondrial dynamics and division in budding yeast

“…Subsequent studies (described above) suggested that loss of Dnm1p function blocked mitochondrial fission but allowed the fusion of mitochondrial tips to continue, resulting in the formation of elaborate net-like structures [9,10]. Most importantly, the mitochondrial fragmentation induced by a fzo1 mutation no longer occurred in cells containing a dnm1 mutation [9,10].…”

“…In dnm1 mutant cells, mitochondrial fission is abolished but fusion of mitochondrial tips continues, resulting in the formation of elaborate 'net-like' structures ( Fig. 3, left) [9,10]. By contrast, mitochondrial networks rapidly fragment in fzo1 mutant cells because mitochondrial fusion is blocked but fission continues unopposed (Fig.…”

“…The fission and fusion of the inner and outer mitochondrial membranes occur in tandem, although the mechanisms by which this coordination is accomplished are not understood. Fission can occur within a tubule or at a branchpoint in the reticulum and is regulated by a GTPase called Dnm1p [8][9][10]. Fusion always occurs between two mitochondrial tips or between the tip and the side of a tubule, and is regulated by the transmembrane GTPase Fzo1p [11,12].…”

Mitochondrial factors with dual roles in death and survival

“…In addition to Drp1/Dnm1, a dynamin-like GTPase, three other proteins were identified in yeast that are required for normal mitochondrial fission, Fis1, Mdv1/ Net2 and Caf4 (Bleazard et al, 1999;Sesaki and Jensen, 1999;Jensen et al, 2000;Shaw and Nunnari, 2002;Griffin et al, 2005). A critical role for each of these factors in yeast mitochondrial fission is evident from the fact that deletion of any one of them results in fusion of normally tubular mitochondria into continuous nets.…”