Janet M. Shaw

The dynamin-related GTPase Dnm1 regulates mitochondrial fission in yeast.

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.

Mitochondrial dynamics and division in budding yeast.

Mitochondria adopt a variety of different shapes in eukaryotic cells, ranging from multiple, small compartments to elaborate tubular networks. The establishment and maintenance of different mitochondrial morphologies depends, in part, on the equilibrium between opposing fission and fusion events. Recent studies in yeast, flies, worms and mammalian cells indicate that three high-molecular-weight GTPases control mitochondrial membrane dynamics. One of these is a dynamin-related GTPase that acts on the outer mitochondrial membrane to regulate fission. Recently, genetic approaches in budding yeast have identified additional components of the fission machinery. These and other new findings suggest a common mechanism for membrane fission events that has been conserved and adapted during eukaryotic evolution.

Moving Mitochondria: Establishing Distribution of an Essential Organelle

Mitochondria form a dynamic network responsible for energy production, calcium homeostasis and cell signaling. Appropriate distribution of the mitochondrial network contributes to organelle function and is essential for cell survival. Highly polarized cells, including neurons and budding yeast, are particularly sensitive to defects in mitochondrial movement and have emerged as model systems for studying mechanisms that regulate organelle distribution. Mitochondria in multicellular eukaryotes move along microtubule tracks. Actin, the primary cytoskeletal component used for transport in yeast, has more subtle functions in other organisms. Kinesin, dynein and myosin isoforms drive motor‐based movement along cytoskeletal tracks. Milton and syntabulin have recently been identified as potential organelle‐specific adaptor molecules for microtubule‐based motors. Miro, a conserved GTPase, may function with Milton to regulate transport. In yeast, Mmr1p and Ypt11p, a Rab GTPase, are implicated in myosin V‐based mitochondrial movement. These potential adaptors could regulate motor activity and therefore determine individual organelle movements. Anchoring of stationary mitochondria also contributes to organelle retention at specific sites in the cell. Together, movement and anchoring ultimately determine mitochondrial distribution throughout the cell.

RNA editing and the mitochondrial cryptogenes of kinetoplastid protozoa

Le RNA editing est une modification post-transcriptionnelle des mRNA. Il consiste en une addition de residus de base unidine a differentes positions du messager. Il est observe sur le RNA mitochondrial des trypanosomes au niveau de leur kinetoplaste. On peut le constater aussi sur 3 autres especes: L. tarentolae, T. brucei et C. fasciculata. 2 enzymes seraient les responsables: par modification de la structure primaire codante, elles sont obligatoirement impliquees dans la regulation du metabolisme

Mitochondrial GFA2 Is Required for Synergid Cell Death in Arabidopsis

Little is known about the molecular processes that govern female gametophyte (FG) development and function, and few FG-expressed genes have been identified. We report the identification and phenotypic analysis of 31 new FG mutants in Arabidopsis. These mutants have defects throughout development, indicating that FG-expressed genes govern essentially every step of FG development. To identify genes involved in cell death during FG development, we analyzed this mutant collection for lines with cell death defects. From this analysis, we identified one mutant, gfa2, with a defect in synergid cell death. Additionally, the gfa2 mutant has a defect in fusion of the polar nuclei. We isolated the GFA2 gene and show that it encodes a J-domain–containing protein. Of the J-domain–containing proteins in Saccharomyces cerevisiae (budding yeast), GFA2 is most similar to Mdj1p, which functions as a chaperone in the mitochondrial matrix. GFA2 is targeted to mitochondria in Arabidopsis and partially complements a yeast mdj1 mutant, suggesting that GFA2 is the Arabidopsis ortholog of yeast Mdj1p. These data suggest a role for mitochondria in cell death in plants.

Gem1 and ERMES do not directly affect phosphatidylserine transport from ER to mitochondria or mitochondrial inheritance.

In yeast, a protein complex termed the ER‐Mitochondria Encounter Structure (ERMES) tethers mitochondria to the endoplasmic reticulum. ERMES proteins are implicated in a variety of cellular functions including phospholipid synthesis, mitochondrial protein import, mitochondrial attachment to actin, polarized mitochondrial movement into daughter cells during division, and maintenance of mitochondrial DNA (mtDNA). The mitochondrial‐anchored Gem1 GTPase has been proposed to regulate ERMES functions. Here, we show that ERMES and Gem1 have no direct role in the transport of phosphatidylserine (PS) from the ER to mitochondria during the synthesis of phosphatidylethanolamine (PE), as PS to PE conversion is not affected in ERMES or gem1 mutants. In addition, we report that mitochondrial inheritance defects in ERMES mutants are a secondary consequence of mitochondrial morphology defects, arguing against a primary role for ERMES in mitochondrial association with actin and mitochondrial movement. Finally, we show that ERMES complexes are long‐lived, and do not depend on the presence of Gem1. Our findings suggest that the ERMES complex may have primarily a structural role in maintaining mitochondrial morphology.

A fuzzy mitochondrial fusion apparatus comes into focus

Membrane fusion is fundamental to eukaryotic life. Unlike the predominant intracellular fusion machineries that fuse compartments bounded by a single membrane, the mitochondrial fusion machinery must sequentially fuse the outer and inner mitochondrial membranes. These coordinated fusion events rely on a transmembrane GTPase that is known as fuzzy onions or Fzo. Recent studies have revealed that Fzo has an evolutionarily conserved role in mitochondrial fusion, and they take the first strides in determining the molecular nature of such a role.

Interchangeable adaptors regulate mitochondrial dynamin assembly for membrane scission

Significance Mitochondrial fission is critical for mammalian cell division, mitophagy, and development. Fission initiates via recruitment of dynamin-related GTPases to the mitochondrial surface. In yeast and human, the recruitment utilizes adaptors that differ in sequence and predicted structure. Key unresolved issues are whether these adaptors function independently in membrane recruitment and whether a single adaptor and GTPase are sufficient to catalyze scission. We show that three human adaptors work interchangeably with a single mitochondrial dynamin to accomplish fission. We also show that an adaptor alters the architecture of the dynamin polymer in a manner that could facilitate membrane constriction and severing. Mitochondrial fission is mediated by the dynamin-related GTPases Dnm1/Drp1 (yeast/mammals), which form spirals around constricted sites on mitochondria. Additional membrane-associated adaptor proteins (Fis1, Mdv1, Mff, and MiDs) are required to recruit these GTPases from the cytoplasm to the mitochondrial surface. Whether these adaptors participate in both GTPase recruitment and membrane scission is not known. Here we use a yeast strain lacking all fission proteins to identify the minimal combinations of GTPases and adaptors sufficient for mitochondrial fission. Although Fis1 is dispensable for fission, membrane-anchored Mdv1, Mff, or MiDs paired individually with their respective GTPases are sufficient to divide mitochondria. In addition to their role in Drp1 membrane recruitment, MiDs coassemble with Drp1 in vitro. The resulting heteropolymer adopts a dramatically different structure with a narrower diameter than Drp1 homopolymers assembled in isolation. This result demonstrates that an adaptor protein alters the architecture of a mitochondrial dynamin GTPase polymer in a manner that could facilitate membrane constriction and severing activity.

Mdm20 protein functions with Nat3 protein to acetylate Tpm1 protein and regulate tropomyosin–actin interactions in budding yeast

The evolutionarily conserved Mdm20 protein (Mdm20p) plays an important role in tropomyosin–F-actin interactions that generate actin filaments and cables in budding yeast. However, Mdm20p is not a structural component of actin filaments or cables, and its exact function in cable stability has remained a mystery. Here, we show that cells lacking Mdm20p fail to N-terminally acetylate Tpm1p, an abundant form of tropomyosin that binds and stabilizes actin filaments and cables. The F-actin-binding activity of unacetylated Tpm1p is reduced severely relative to the acetylated form. These results are complemented by the recent report that Mdm20p copurifies with one of three acetyltransferases in yeast, the NatB complex. We present genetic evidence that Mdm20p functions cooperatively with Nat3p, the catalytic subunit of the NatB acetyltransferase. These combined results strongly suggest that Mdm20p-dependent, N-terminal acetylation of Tpm1p by the NatB complex is required for Tpm1p association with, and stabilization of, actin filaments and cables.

Loss of Miro1-directed mitochondrial movement results in a novel murine model for neuron disease.

Significance This report probes the physiological roles of mammalian mitochondrial Rho 1 (Miro1), a calcium-binding, membrane-anchored GTPase necessary for mitochondrial motility on microtubules. Using two new mouse models and primary cells, the study demonstrates a specific role for Miro1 in upper motor neuron development and retrograde transport of axonal mitochondria. Unexpectedly, Miro1 is not essential for calcium-regulated mitochondrial movement, mitochondrial-mediated calcium buffering, or maintenance of mitochondrial respiratory activity. Nevertheless, a neuron-specific Miro1 KO mouse model displays physical hallmarks of neurological disease in the brainstem and spinal cord and develops rapidly progressing upper motor neuron disease symptoms culminating in death after approximately 4 wk. These studies demonstrate that defects in mitochondrial motility and distribution alone are sufficient to cause neurological disease. Defective mitochondrial distribution in neurons is proposed to cause ATP depletion and calcium-buffering deficiencies that compromise cell function. However, it is unclear whether aberrant mitochondrial motility and distribution alone are sufficient to cause neurological disease. Calcium-binding mitochondrial Rho (Miro) GTPases attach mitochondria to motor proteins for anterograde and retrograde transport in neurons. Using two new KO mouse models, we demonstrate that Miro1 is essential for development of cranial motor nuclei required for respiratory control and maintenance of upper motor neurons required for ambulation. Neuron-specific loss of Miro1 causes depletion of mitochondria from corticospinal tract axons and progressive neurological deficits mirroring human upper motor neuron disease. Although Miro1-deficient neurons exhibit defects in retrograde axonal mitochondrial transport, mitochondrial respiratory function continues. Moreover, Miro1 is not essential for calcium-mediated inhibition of mitochondrial movement or mitochondrial calcium buffering. Our findings indicate that defects in mitochondrial motility and distribution are sufficient to cause neurological disease.