Dimeric Dnm1-G385D interacts with Mdv1 on mitochondria and can be stimulated to assemble into fission complexes containing Mdv1 and Fis1

Interactions between yeast Dnm1p, Mdv1p, and Fis1p are required to form fission complexes that catalyze division of the mitochondrial compartment. During the formation of mitochondrial fission complexes, the Dnm1p GTPase self-assembles into large multimeric complexes on the outer mitochondrial membrane that are visualized as punctate structures by fluorescent labeling. Although it is clear that Fis1p.Mdv1p complexes on mitochondria are required for the initial recruitment of Dnm1p, it is not clear whether Dnm1p puncta assemble before or after this recruitment step. Here we show that the minimum oligomeric form of cytoplasmic Dnm1p is a dimer. The middle domain mutant protein Dnm1G385Dp forms dimers in vivo but fails to assemble into punctate structures. However, this dimeric mutant stably interacts with Mdv1p on the outer mitochondrial membrane, demonstrating that assembly of stable Dnm1p multimers is not required for Dnm1p-Mdv1p association or for mitochondrial recruitment of Dnm1p. Dnm1G385Dp is reported to be a terminal dimer in vitro. We describe conditions that allow assembly of Dnm1G385Dp into functional fission complexes on mitochondria in vivo. Using these conditions, we demonstrate that multimerization of Dnm1p is required to promote reorganization of Mdv1p from a uniform mitochondrial localization into punctate fission complexes. Our studies also reveal that Fis1p is present in these assembled fission complexes. Based on our results, we propose that Dnm1p dimers are initially recruited to the membrane via interaction with Mdv1p.Fis1p complexes. These dimers then assemble into multimers that subsequently promote the reorganization of Mdv1p into punctate fission complexes.     

The WD repeat protein,Mdv1p,functions as a molecular adaptor by interacting with Dnm1p and Fis1p during mitochondrial fission

Yeast mitochondrial fission is a multistep process during which the dynamin-related GTPase, Dnm1p, assembles into punctate structures that associate with the outer mitochondrial membrane and mediate mitochondrial division. Steps in the Dnm1p-dependent process of fission are regulated by the actions of the WD repeat protein, Mdv1p, and the mitochondrial outer membrane protein, Fis1p. Our previous studies suggested a model where Mdv1p functions to regulate fission at a post-Dnm1p assembly step and Fis1p functions at two distinct steps, at an early point, to regulate Dnm1p assembly, and later, together with Mdv1p, to facilitate Dnm1p-dependent mitochondrial fission. To test this model, we have examined the physical and functional relationship between Mdv1p and Fis1p and present genetic, biochemical, and two-hybrid data indicating that a Fis1p-Mdv1p complex is required to regulate mitochondrial fission. To further define the role of Mdv1p in fission, we examined the structural features of Mdv1p required for its interactions with Dnm1p and Fis1p. Data from two-hybrid analyses and GFP-tagged domains of Mdv1p indicate that it contains two functionally distinct domains that enable it to function as a molecular adaptor to regulate sequential interactions between Dnm1p and Fis1p and catalyze a rate-limiting step in mitochondrial fission.     

The role of Fis1p-Mdv1p interactions in mitochondrial fission complex assembly

Mitochondrial division requires coordinated interactions among Fis1p, Mdv1p, and the Dnm1p GTPase, which assemble into fission complexes on the outer mitochondrial membrane. The integral outer membrane protein Fis1p contains a cytoplasmic domain consisting of a tetratricopeptide repeat (TPR)-like fold and a short NH(2)-terminal helix. Although it is known that the cytoplasmic domain is necessary for assembly of Mdv1p and Dnm1p into fission complexes, the molecular details of this assembly are not clear. In this study, we provide new evidence that the Fis1p-Mdv1p interaction is direct. Furthermore, we show that conditional mutations in the Fis1p TPR-like domain cause fission complex assembly defects that are suppressed by mutations in the Mdv1p-predicted coiled coil. We also define separable functions for the Fis1p NH(2)-terminal arm and TPR-like fold. These studies suggest that the concave binding surface of the Fis1p TPR-like fold interacts with Mdv1p during mitochondrial fission and that Mdv1p facilitates Dnm1p recruitment into functional fission complexes.     

Dnm1p GTPase-mediated mitochondrial fission is a multi-step process requiring the novel integral membrane component Fis1p

Yeast Dnm1p is a soluble, dynamin-related GTPase that assembles on the outer mitochondrial membrane at sites where organelle division occurs. Although these Dnm1p-containing complexes are thought to trigger constriction and fission, little is known about their composition and assembly, and molecules required for their membrane recruitment have not been isolated. Using a genetic approach, we identified two new genes in the fission pathway, FIS1 and FIS2. FIS1 encodes a novel, outer mitochondrial membrane protein with its amino terminus exposed to the cytoplasm. Fis1p is the first integral membrane protein shown to participate in a eukaryotic membrane fission event. In a related study (Tieu, Q., and J. Nunnari. 2000. J. Cell Biol. 151:353-365), it was shown that the FIS2 gene product (called Mdv1p) colocalizes with Dnm1p on mitochondria. Genetic and morphological evidence indicate that Fis1p, but not Mdv1p, function is required for the proper assembly and distribution of Dnm1p-containing fission complexes on mitochondrial tubules. We propose that mitochondrial fission in yeast is a multi-step process, and that membrane-bound Fis1p is required for the proper assembly, membrane distribution, and function of Dnm1p-containing complexes during fission.     

The WD40 protein Caf4p is a component of the mitochondrial fission machinery and recruits Dnm1p to mitochondria

The mitochondrial division machinery regulates mitochondrial dynamics and consists of Fis1p, Mdv1p, and Dnm1p. Mitochondrial division relies on the recruitment of the dynamin-related protein Dnm1p to mitochondria. Dnm1p recruitment depends on the mitochondrial outer membrane protein Fis1p. Mdv1p interacts with Fis1p and Dnm1p, but is thought to act at a late step during fission because Mdv1p is dispensable for Dnm1p localization. We identify the WD40 repeat protein Caf4p as a Fis1p-associated protein that localizes to mitochondria in a Fis1p-dependent manner. Caf4p interacts with each component of the fission apparatus: with Fis1p and Mdv1p through its NH2-terminal half and with Dnm1p through its COOH-terminal WD40 domain. We demonstrate that mdv1delta yeast contain residual mitochondrial fission due to the redundant activity of Caf4p. Moreover, recruitment of Dnm1p to mitochondria is disrupted in mdv1delta caf4delta yeast, demonstrating that Mdv1p and Caf4p are molecular adaptors that recruit Dnm1p to mitochondrial fission sites. Our studies support a revised model for assembly of the mitochondrial fission apparatus.     

Mdv1 interacts with assembled dnm1 to promote mitochondrial division

The dynamin-related GTPase, Dnm1, self-assembles into punctate structures that are targeted to the outer mitochondrial membrane where they mediate mitochondrial division. Post-targeting, Dnm1-dependent division is controlled by the actions of the WD repeat protein, Mdv1, and the mitochondrial tetratricopeptide repeat-like outer membrane protein, Fis1. Our previous studies suggest a model where at this step Mdv1 functions as an adaptor linking Fis1 with Dnm1. To gain insight into the exact role of the Fis1.Mdv1.Dnm1 complex in mitochondrial division, we performed a structure-function analysis of the Mdv1 adaptor. Our analysis suggests that dynamic interactions between Mdv1 and Dnm1 play a key role in division by regulating Dnm1 self-assembly.     

Peroxisome fission in Hansenula polymorpha requires Mdv1 and Fis1, two proteins also involved in mitochondrial fission

We show that Mdv1 and Caf4, two components of the mitochondrial fission machinery in Saccharomyces cerevisiae , also function in peroxisome proliferation. Deletion of MDV1 , CAF4 or both, however, had only a minor effect on peroxisome numbers at peroxisome-inducing growth conditions, most likely related to the fact that Vps1 – and not Dnm1 – is the key player in peroxisome fission in this organism. In contrast, in Hansenula polymorpha , which has only a Dnm1-dependent peroxisome fission machinery, deletion of MDV1 led to a drastic reduction of peroxisome numbers. This phenotype was accompanied by a strong defect in mitochondrial fission. The MDV1 paralog CAF4 is absent in H. polymorpha . In wild-type H. polymorpha , cells Dnm1–mCherry and green fluorescent protein (GFP)–Mdv1 colocalize in spots that associate with both peroxisomes and mitochondria. Furthermore, Fis1 is essential to recruit Mdv1 to the peroxisomal and mitochondrial membrane. However, formation of GFP–Mdv1 spots – and related to this normal organelle fission – is strictly dependent on the presence of Dnm1. In dnm1 cells, GFP–Mdv1 is dispersed over the surface of peroxisomes and mitochondria. Also, in H. polymorpha mdv1 or fis1 cells, the number of Dnm1–GFP spots is strongly reduced. These spots still associate to organelles but are functionally inactive.     

A novel motif in the yeast mitochondrial dynamin Dnm1 is essential for adaptor binding and membrane recruitment

To initiate mitochondrial fission, dynamin-related proteins (DRPs) must bind specific adaptors on the outer mitochondrial membrane. The structural features underlying this interaction are poorly understood. Using yeast as a model, we show that the Insert B domain of the Dnm1 guanosine triphosphatase (a DRP) contains a novel motif required for association with the mitochondrial adaptor Mdv1. Mutation of this conserved motif specifically disrupted Dnm1–Mdv1 interactions, blocking Dnm1 recruitment and mitochondrial fission. Suppressor mutations in Mdv1 that restored Dnm1–Mdv1 interactions and fission identified potential protein-binding interfaces on the Mdv1 β-propeller domain. These results define the first known function for Insert B in DRP–adaptor interactions. Based on the variability of Insert B sequences and adaptor proteins, we propose that Insert B domains and mitochondrial adaptors have coevolved to meet the unique requirements for mitochondrial fission of different organisms.     

The Mitochondrial Fission Adaptors Caf4 and Mdv1 Are Not Functionally Equivalent

Mitochondrial fission in eukaryotes is mediated by protein complexes that encircle and divide mitochondrial tubules. In budding yeast, fission requires the membrane-anchored protein Fis1 and the dynamin-related GTPase Dnm1. Dnm1 is recruited to mitochondria via interactions with the adaptor proteins Caf4 and Mdv1, which bind directly to Fis1. Unlike Mdv1, a function for Caf4 in mitochondrial membrane scission has not been established. In this study, we demonstrate that Caf4 is a bona fide fission adaptor that assembles at sites of mitochondrial division. We also show that fission complexes may contain Caf4 alone or both Caf4 and Mdv1 without compromising fission function. Although there is a correspondence between Caf4 and Mdv1 expression levels and their contribution to fission, the two adaptor proteins are not equivalent. Rather, our functional and phylogenetic analyses indicate that Caf4 mitochondrial fission activity has diverged from that of Mdv1.     

The WD-repeats of Net2p interact with Dnm1p and Fis1p to regulate division of mitochondria

The Net2, Fis1, and Dnm1 proteins are required for the division of mitochondria in the yeast Saccharomyces cerevisiae. Net2p has an amino-terminal region that contains predicted coiled-coil motifs and a carboxyl-terminal domain composed of WD-40 repeats. We found that the amino-terminal part of Net2p interacts with Fis1p, whereas the carboxyl-terminal region interacts with both Dnm1p and Fis1p. Overproduction of either domain of Net2p in yeast cells poisons mitochondrial fission, and the dominant-negative effect caused by the WD-repeats of Net2p is suppressed by increased levels of Dnm1p. Point mutations in the WD-region of Net2p or in the GTPase region of Dnm1p disrupt the normal Net2p-Dnm1p interaction, causing Net2p to lose its normal punctate distribution. Our results suggest that Dnm1p interacts with the WD-repeats of Net2p and in a GTP-dependent manner recruits Net2p to sites of mitochondrial division. Furthermore, our results indicate that Net2p is required for proper assembly of the mitochondrial fission components to regulate organelle division.     

Yeast mitochondrial division and distribution require the cortical num1 protein

Yeast mitochondrial division requires the dynamin-related Dnm1 protein. By isolating high-copy suppressors of a dominant-negative Dnm1p mutant, we uncovered an unexpected role in mitochondrial division and inheritance for Num1p, a protein previously shown to facilitate nuclear migration. num1 mutants contain an interconnected network of mitochondrial tubules, remarkably similar to cells lacking Dnm1p, and time-lapse microscopy confirms that mitochondrial fission is greatly reduced in num1Delta cells. We also find that Num1p assembles into punctate structures, which often colocalize with mitochondrial-bound Dnm1p particles. Suggesting a role for both Num1p and Dnm1p in mitochondrial inheritance, we find that num1 dnm1 double mutants accumulate mitochondria in daughter buds and that mother cells are frequently devoid of all mitochondria. Thus, our studies have revealed an additional role for Dnm1p in mitochondrial transmission through its interaction with Num1p, thereby providing a link between mitochondrial division and inheritance.     

Crystal structure of mitochondrial fission complex reveals scaffolding function for mitochondrial division 1 (Mdv1) coiled coil

The mitochondrial fission machinery is best understood in the yeast Saccharomyces cerevisiae, where Fis1, Mdv1, and Dnm1 are essential components. Fis1 is a mitochondrial outer membrane protein that recruits the dynamin-related GTPase Dnm1 during the fission process. This recruitment occurs via Mdv1, which binds both Fis1 and Dnm1 and therefore functions as a molecular adaptor linking the two molecules. Mdv1 has a modular structure, consisting of an N-terminal extension that binds Fis1, a central coiled coil for dimerization, and a C-terminal WD40 repeat region that binds Dnm1. We have solved the crystal structure of a dimeric Mdv1-Fis1 complex that contains both the N-terminal extension and coiled-coil regions of Mdv1. Consistent with previous studies, Mdv1 binds Fis1 through a U-shaped helix-loop-helix motif, and dimerization of the Mdv1-Fis1 complex is mediated by the antiparallel coiled coil of Mdv1. However, the complex is surprisingly compact and rigid due to two additional contacts mediated by the surface of the Mdv1 coiled coil. The coiled coil packs against both Fis1 and the second helix of the Mdv1 helix-loop-helix motif. Mutational analyses showed that these contacts are important for mitochondrial fission activity. These results indicate that, in addition to dimerization, the unusually long Mdv1 coiled coil serves a scaffolding function to stabilize the Mdv1-Fis1 complex.     

Constriction and Dnm1p recruitment are distinct processes in mitochondrial fission

Mitochondria undergo cycles of fusion and fission crucial for organelle homeostasis. Fission is regulated partially by recruitment of the large GTPase Dnm1p to the outer mitochondrial membrane. Using three-dimensional time-lapse fluorescence imaging of Saccharomyces cerevisiae cells, we found that Dnm1p-EGFP appears and disappears at "hot spots" along mitochondrial tubes. It forms patches that convert rapidly into different shapes regardless of whether mitochondrial fission ensues or not. Moreover, the thickness of the mitochondrial matrix displays frequent temporal fluctuations apparently unrelated to fission or to recruitment of Dnm1p-EGFP. These results suggest that mitochondrial fission requires coordination of at least two distinct processes.     

The GTPase effector domain sequence of the Dnm1p GTPase regulates self-assembly and controls a rate-limiting step in mitochondrial fission

Dnm1p belongs to a family of dynamin-related GTPases required to remodel different cellular membranes. In budding yeast, Dnm1p-containing complexes assemble on the cytoplasmic surface of the outer mitochondrial membrane at sites where mitochondrial tubules divide. Our previous genetic studies suggested that Dnm1p's GTPase activity was required for mitochondrial fission and that Dnm1p interacted with itself. In this study, we show that bacterially expressed Dnm1p can bind and hydrolyze GTP in vitro. Coimmunoprecipitation studies and yeast two-hybrid analysis suggest that Dnm1p oligomerizes in vivo. With the use of the yeast two-hybrid system, we show that this Dnm1p oligomerization is mediated, in part, by a C-terminal sequence related to the GTPase effector domain (GED) in dynamin. The Dnm1p interactions characterized here are similar to those reported for dynamin and dynamin-related proteins that form higher order structures in vivo, suggesting that Dnm1p assembles to form rings or collars that surround mitochondrial tubules. Based on previous findings, a K705A mutation in the Dnm1p GED is predicted to interfere with GTP hydrolysis, stabilize active Dnm1p-GTP, and stimulate a rate-limiting step in fission. Here we show that expression of the Dnm1 K705A protein in yeast enhances mitochondrial fission. Our results provide evidence that the GED region of a dynamin-related protein modulates a rate-limiting step in membrane fission.     

Molecular architecture of a dynamin adaptor: implications for assembly of mitochondrial fission complexes

Recruitment and assembly of some dynamin-related guanosine triphosphatases depends on adaptor proteins restricted to distinct cellular membranes. The yeast Mdv1 adaptor localizes to mitochondria by binding to the membrane protein Fis1. Subsequent Mdv1 binding to the mitochondrial dynamin Dnm1 stimulates Dnm1 assembly into spirals, which encircle and divide the mitochondrial compartment. In this study, we report that dimeric Mdv1 is joined at its center by a 92-Å antiparallel coiled coil (CC). Modeling of the Fis1–Mdv1 complex using available crystal structures suggests that the Mdv1 CC lies parallel to the bilayer with N termini at opposite ends bound to Fis1 and C-terminal β-propeller domains (Dnm1-binding sites) extending into the cytoplasm. A CC length of appropriate length and sequence is necessary for optimal Mdv1 interaction with Fis1 and Dnm1 and is important for proper Dnm1 assembly before membrane scission. Our results provide a framework for understanding how adaptors act as scaffolds to orient and stabilize the assembly of dynamins on membranes.     

Gag3p, an outer membrane protein required for fission of mitochondrial tubules

Mitochondrial morphology and function depend on MGM1, a Saccharomyces cerevisiae gene encoding a dynamin-like protein of the mitochondrial outer membrane. Here, we show that mitochondrial fragmentation and mitochondrial genome loss caused by lesions in MGM1 are suppressed by three novel mutations, gag1, gag2, and gag3 (for glycerol-adapted growth). Cells with any of the gag mutations displayed aberrant mitochondrial morphology characterized by elongated, unbranched tubes and highly fenestrated structures. Additionally, each of the gag mutations prevented mitochondrial fragmentation caused by loss of the mitochondrial fusion factor, Fzo1p, or by treatment of cells with sodium azide. The gag1 mutation mapped to DNM1 that encodes a dynamin-related protein required for mitochondrial fission. GAG3 encodes a novel WD40-repeat protein previously found to interact with Dnm1p in a two-hybrid assay. Gag3p was localized to mitochondria where it was found to associate as a peripheral protein on the cytosolic face of the outer membrane. This association requires neither the DNM1 nor GAG2 gene products. However, the localization of Dnm1p to the mitochondrial outer membrane is substantially reduced by the gag2 mutation, but unaffected by loss of Gag3p. These results indicate that Gag3p plays a distinct role on the mitochondrial surface to mediate the fission of mitochondrial tubules.     

Division of mitochondria requires a novel DMN1-interacting protein, Net2p

Mitochondria are dynamic organelles that undergo frequent division and fusion, but the molecular mechanisms of these two events are not well understood. Dnm1p, a mitochondria-associated, dynamin-related GTPase was previously shown to mediate mitochondrial fission. Recently, a genome-wide yeast two-hybrid screen identified an uncharacterized protein that interacts with Dnm1p. Cells disrupted in this new gene, which we call NET2, contain a single mitochondrion that consists of a network formed by interconnected tubules, similar to the phenotype of dnm1 Delta cells. NET2 encodes a mitochondria-associated protein with a predicted coiled-coil region and six WD-40 repeats. Immunofluorescence microscopy indicates that Net2p is located in distinct, dot-like structures along the mitochondrial surface, many of which colocalize with the Dnm1 protein. Fluorescence and immunoelectron microscopy shows that Dnm1p and Net2p preferentially colocalize at constriction sites along mitochondrial tubules. Our results suggest that Net2p is a new component of the mitochondrial division machinery.     

Analysis of functional domains of rat mitochondrial Fis1, the mitochondrial fission-stimulating protein

In yeast, mitochondrial-fission is regulated by the cytosolic dynamin-like GTPase (Dnm1p) in conjunction with a peripheral protein, Mdv1p, and a C-tail-anchored outer membrane protein, Fis1p. In mammals, a dynamin-related protein (Drp1) and Fis1 are involved in the mitochondrial-fission reaction as Dnm1 and Fis1 orthologues, respectively. The involvement of other component(s), such as the Mdv1 homologue, and the mechanisms regulating mitochondrial-fission remain unclear. Here, we identified rat Fis1 (rFis1) and analyzed its structure-function relationship. Blue-native-polyacrylamide gel electrophoresis revealed that rFis1 formed a approximately 200-kDa complex in the outer mitochondrial membrane. Its expression in HeLa cells promoted extensive mitochondrial fragmentation, and gene knock-down by RNAi induced extension of the mitochondrial networks. Taking advantage of these properties, we analyzed functional domains of rFis1. These experiments revealed that the N-terminal and C-terminal segments are both essential for oligomeric rFis1 interaction, and the middle TPR-like domains regulate proper oligomer assembly. Any mutations that disturb the proper oligomeric assembly compromise mitochondrial division-stimulating activity of rFis1.     

The Dynamin-related GTPase,Dnm1p,Controls Mitochondrial Morphology in Yeast

The Saccharomyces cerevisiae Dnm1 protein is structurally related to dynamin, a GTPase required for membrane scission during endocytosis. Here we show that Dnm1p is essential for the maintenance of mitochondrial morphology. Disruption of the DNM1 gene causes the wild-type network of tubular mitochondrial membranes to collapse to one side of the cell but does not affect the morphology or distribution of other cytoplasmic organelles. Dnm1 proteins containing point mutations in the predicted GTP-binding domain or completely lacking the GTP-binding domain fail to rescue mitochondrial morphology defects in a dnm1 mutant and induce dominant mitochondrial morphology defects in wild-type cells. Indirect immunofluorescence reveals that Dnm1p is distributed in punctate structures at the cell cortex that colocalize with the mitochondrial compartment. These Dnm1p-containing structures remain associated with the spherical mitochondria found in an mdm10 mutant strain. In addition, a portion of Dnm1p cofractionates with mitochondrial membranes during differential sedimentation and sucrose gradient fractionation of wild-type cells. Our results demonstrate that Dnm1p is required for the cortical distribution of the mitochondrial network in yeast, a novel function for a dynamin-related protein.     

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

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.