Mechanisms of mitochondrial reorganization.

The cytoplasm of eukaryotes is dynamically zoned by membrane-bound and membraneless organelles. Cytoplasmic zoning allows various biochemical reactions to take place at the right time and place. Mitochondrion is a membrane-bound organelle that provides a zone for intracellular energy production and metabolism of lipids and iron. A key feature of mitochondria is their high dynamics: mitochondria constantly undergo fusion and fission, and excess or damaged mitochondria are selectively eliminated by mitophagy. Therefore, mitochondria are appropriate model systems to understand dynamic cytoplasmic zoning by membrane organelles. In this review, we summarize the molecular mechanisms of mitochondrial fusion and fission as well as mitophagy unveiled through studies using yeast and mammalian models.

Complete set of the Atg8-E1-E2-E3 conjugation machinery forms an interaction web that mediates membrane shaping.

Atg8, a ubiquitin-like protein, is conjugated with phosphatidylethanolamine (PE) via Atg7 (E1), Atg3 (E2) and Atg12-Atg5-Atg16 (E3) enzymatic cascade and mediates autophagy. However, its molecular roles in autophagosome formation are still unclear. Here we show that Saccharomyces cerevisiae Atg8-PE and E1-E2-E3 enzymes together construct a stable, mobile membrane scaffold. The complete scaffold formation induces an in-bud in prolate-shaped giant liposomes, transforming their morphology into one reminiscent of isolation membranes before sealing. In addition to their enzymatic roles in Atg8 lipidation, all three proteins contribute nonenzymatically to membrane scaffolding and shaping. Nuclear magnetic resonance analyses revealed that Atg8, E1, E2 and E3 together form an interaction web through multivalent weak interactions, where the intrinsically disordered regions in Atg3 play a central role. These data suggest that all six Atg proteins in the Atg8 conjugation machinery control membrane shaping during autophagosome formation.

Protocol for real-time imaging of membrane fission by mitofissin.

Yeast mitofissin Atg44 is a mitochondrial intermembrane space protein that causes membrane fission required for mitophagy. Here, we present a protocol for observing Atg44-mediated membrane fission. We describe steps for recombinant Atg44 purification, lipid nanotube preparation as model membranes, and Atg44-mediated membrane fission real-time observation. We then detail procedures for tube radius estimation using confocal microscopy. This protocol can also be adapted to the study of membrane fission by other proteins. For complete details on the use and execution of this protocol, please refer to Fukuda et al. (2023).1.

Mitofissin: a novel mitochondrial fission protein that facilitates mitophagy.

ABBREVIATIONS: Atg: autophagy related; IMM: inner mitochondrial membrane; IMS: intermembrane space; PAS: phagophore assembly site; SAR: selective autophagy receptor.

The mitochondrial intermembrane space protein mitofissin drives mitochondrial fission required for mitophagy.

Mitophagy plays an important role in mitochondrial homeostasis by selective degradation of mitochondria. During mitophagy, mitochondria should be fragmented to allow engulfment within autophagosomes, whose capacity is exceeded by the typical mitochondria mass. However, the known mitochondrial fission factors, dynamin-related proteins Dnm1 in yeasts and DNM1L/Drp1 in mammals, are dispensable for mitophagy. Here, we identify Atg44 as a mitochondrial fission factor that is essential for mitophagy in yeasts, and we therefore term Atg44 and its orthologous proteins mitofissin. In mitofissin-deficient cells, a part of the mitochondria is recognized by the mitophagy machinery as cargo but cannot be enwrapped by the autophagosome precursor, the phagophore, due to a lack of mitochondrial fission. Furthermore, we show that mitofissin directly binds to lipid membranes and brings about lipid membrane fragility to facilitate membrane fission. Taken together, we propose that mitofissin acts directly on lipid membranes to drive mitochondrial fission required for mitophagy.

Delineating the lipidated Atg8 structure for unveiling its hidden activity in autophagy.

Atg8 has attracted attention as a central factor in autophagosome biogenesis for a long time. However, the molecular activities of Atg8 on the phagophore membranes as the physiologically functional lipidated form remain enigmatic. In our recent study, we unveiled the hidden physicochemical activity of lipidated Atg8 toward the membrane. Structural analysis revealed that lipidated Atg8 adopts a preferred orientation on the membrane, contacting the membrane using aromatic residues and at the same time exposing cargo binding pockets to the solvent, enabling this small protein to perturb and transform membranes while recognizing autophagic cargos. The membrane perturbation activity was shown to be essential for efficient autophagosome biogenesis, yet questions on the mechanistic roles of Atg8 remain open.

Membrane perturbation by lipidated Atg8 underlies autophagosome biogenesis.

Autophagosome biogenesis is an essential feature of autophagy. Lipidation of Atg8 plays a critical role in this process. Previous in vitro studies identified membrane tethering and hemi-fusion/fusion activities of Atg8, yet definitive roles in autophagosome biogenesis remained controversial. Here, we studied the effect of Atg8 lipidation on membrane structure. Lipidation of Saccharomyces cerevisiae Atg8 on nonspherical giant vesicles induced dramatic vesicle deformation into a sphere with an out-bud. Solution NMR spectroscopy of Atg8 lipidated on nanodiscs identified two aromatic membrane-facing residues that mediate membrane-area expansion and fragmentation of giant vesicles in vitro. These residues also contribute to the in vivo maintenance of fragmented vacuolar morphology under stress in fission yeast, a moonlighting function of Atg8. Furthermore, these aromatic residues are crucial for the formation of a sufficient number of autophagosomes and regulate autophagosome size. Together, these data demonstrate that Atg8 can cause membrane perturbations that underlie efficient autophagosome biogenesis.

A C4N4 Diaminopyrimidine Fluorophore.

A new scaffold for producing efficient organic fluorescent materials was identified: 2,5-diamino-4,6-diarylpyrimidine featuring a C4N4 elemental composition. Single-step installation of two aryl groups at the 4,6-positions of the pyrimidine core delivered fluorescent organic materials in a modular fashion. A range of fluorescent compounds with distinct absorption/emission properties was readily accessed by changing the aromatic attachments. A generally high absorption coefficient and quantum yield were observed, including C4N4 derivatives that could fluoresce even in the solid state. The two amino groups at the 2,5-positions of the pyrimidine were essential for intense fluorescence with a large Stokes shift, which was corroborated by structural relaxation to a p-iminoquinone-like structure in the excited state. Besides live-cell imaging capabilities, fluorescent labeling of a protein involved in autophagy elucidated a new protein-protein interaction, supporting potential utility in bioimaging applications.

Functional roles of Mg2+ binding sites in ion-dependent gating of a Mg2+ channel, MgtE, revealed by solution NMR.

Magnesium ions (Mg2+) are divalent cations essential for various cellular functions. Mg2+ homeostasis is maintained through Mg2+ channels such as MgtE, a prokaryotic Mg2+ channel whose gating is regulated by intracellular Mg2+ levels. Our previous crystal structure of MgtE in the Mg2+-bound, closed state revealed the existence of seven crystallographically-independent Mg2+-binding sites, Mg1-Mg7. The role of Mg2+-binding to each site in channel closure remains unknown. Here, we investigated Mg2+-dependent changes in the structure and dynamics of MgtE using nuclear magnetic resonance spectroscopy. Mg2+-titration experiments, using wild-type and mutant forms of MgtE, revealed that the Mg2+ binding sites Mg1, Mg2, Mg3, and Mg6, exhibited cooperativity and a higher affinity for Mg2+, enabling the remaining Mg2+ binding sites, Mg4, Mg5, and Mg7, to play important roles in channel closure. This study revealed the role of each Mg2+-binding site in MgtE gating, underlying the mechanism of cellular Mg2+ homeostasis.

Autophagy-regulating protease Atg4: structure, function, regulation and inhibition.

Autophagy is an intracellular degradation system that contributes to cellular homeostasis through degradation of various targets such as proteins, organelles and microbes. Since autophagy is related to various diseases such as infection, neurodegenerative diseases and cancer, it is attracting attention as a new therapeutic target. Autophagy is mediated by dozens of autophagy-related (Atg) proteins, among which Atg4 is the sole protease that regulates autophagy through the processing and deconjugating of Atg8. As the Atg4 activity is essential and highly specific to autophagy, Atg4 is a prospective target for developing autophagy-specific inhibitors. In this review article, we summarize our current knowledge of the structure, function and regulation of Atg4 including efforts to develop Atg4-specific inhibitors.The Journal of Antibiotics advance online publication, 13 September 2017; doi:10.1038/ja.2017.104.

ATP-dependent modulation of MgtE in Mg2+ homeostasis.

Magnesium is an essential ion for numerous physiological processes. MgtE is a Mg2+ selective channel involved in the maintenance of intracellular Mg2+ homeostasis, whose gating is regulated by intracellular Mg2+ levels. Here, we report that ATP binds to MgtE, regulating its Mg2+-dependent gating. Crystal structures of MgtE-ATP complex show that ATP binds to the intracellular CBS domain of MgtE. Functional studies support that ATP binding to MgtE enhances the intracellular domain affinity for Mg2+ within physiological concentrations of this divalent cation, enabling MgtE to function as an in vivo Mg2+ sensor. ATP dissociation from MgtE upregulates Mg2+ influx at both high and low intracellular Mg2+ concentrations. Using site-directed mutagenesis and structure based-electrophysiological and biochemical analyses, we identify key residues and main structural changes involved in the process. This work provides the molecular basis of ATP-dependent modulation of MgtE in Mg2+ homeostasis.MgtE is an Mg2+ transporter involved in Mg2+ homeostasis. Here, the authors report that ATP regulates the Mg+2-dependent gating of MgtE and use X-ray crystallography combined with functional studies to propose the molecular mechanisms involved in this process.

Backbone resonance assignments for the cytoplasmic region of the Mg(2+) transporter MgtE in the Mg (2+)-unbound state.

Magnesium ion (Mg(2+)) is an essential metal element for life, and has many cellular functions, including ATP utilization, activation of enzymes, and maintenance of genomic stability. The intracellular Mg(2+) concentration is regulated by a class of transmembrane proteins, called Mg(2+) transporters. One of the prokaryotic Mg(2+) transporters, MgtE, is a 450-residue protein, and functions as a dimer. We previously reported that MgtE exhibits the channel-like electrophysiological property, i.e., it permeates Mg(2+) according to the electrochemical potential of Mg(2+). The Mg(2+)-permeation pathway opens in response to the decrease of the intracellular Mg(2+) concentration, while it is completely closed at the intracellular Mg(2+) concentration of 10 mM. The crystal structures of the MgtE dimer revealed that the Mg(2+)-sensing cytoplasmic region consists of the N and CBS domains. The Mg(2+)-bound state of MgtE adopts a compact, globular conformation, which is stabilized by the coordination of a number of Mg(2+) ions between these domains. On the other hand, in the Mg(2+)-unbound state, these domains are far apart, and fixed by the crystal packing. Therefore, structural analyses in solution were awaited, in order to characterize the Mg(2+)-dependent alteration of the MgtE structure and dynamics relevant to its gating. In this paper, we report the backbone resonance assignments of the dimer of the cytoplasmic region of the MgtE from Thermus thermophilus with a molecular weight of 60 KDa, in the Mg(2+)-unbound state.

Spatial distribution of cytoplasmic domains of the Mg(2+)-transporter MgtE, in a solution lacking Mg(2+), revealed by paramagnetic relaxation enhancement.

MgtE is a prokaryotic Mg(2+) transporter that controls cellular Mg(2+) concentrations. We previously reported crystal structures of the cytoplasmic region of MgtE, consisting of 2 domains, that is, N and CBS, in the Mg(2+)-free and Mg(2+)-bound forms. The Mg(2+)-binding sites lay at the interface of the 2 domains, making the Mg(2+)-bound form compact and globular. In the Mg(2+)-free structure, however, the domains are far apart, and the Mg(2+)-binding sites are destroyed. Therefore, it is unclear how Mg(2+)-free MgtE changes its conformation to accommodate Mg(2+) ions. Here, we used paramagnetic relaxation enhancement (PRE) to characterize the relative orientation of the N and CBS domains in the absence of Mg(2+) in solution. When the residues on the surface of the CBS domain were labeled with nitroxide tags, significant PRE effects were observed for the residues in the N domain. No single structure satisfied the PRE profiles, suggesting that the N and CBS domains are not fixed in a particular orientation in solution. We then conducted ensemble simulated annealing calculations in order to obtain the atomic probability density and visualize the spatial distribution of the N domain in solution. The results indicate that the N domain tends to occupy the space near its position in the Mg(2+)-bound crystal structure, facilitating efficient capture of Mg(2+) with increased intracellular Mg(2+) concentration, which is necessary to close the gate.