Insights into the ATP / GTP selectivity of a GTPase, adenylosuccinate synthetase from Leishmania donovani.

Many GTPases have been shown to utilize ATP too as the phosphoryl donor. Both GTP and ATP are important molecules in the cellular environments and play multiple and discrete functional role within the cells. In our present study, we showed that one of the purine metabolic enzymes Adenylosuccinate synthetase from Leishmania donovani (LdAdSS) which belongs to the BioD-superfamily of GTPases can also carry out the catalysis by hydrolysing ATP instead of its cognate substrate GTP albeit with less efficiency. Biochemical and biophysical studies indicated its ability to bind to ATP too but at a higher concentration of ATP compared to that of GTP. Sequence analysis and molecular dynamic simulations suggested that residues of the switch loop and the G4-G5 (593SAXD596) connected motif of LdAdSS plays a role in determining the nucleotide specificity. Though the crucial interaction between Asp596 and the nucleotide is broken when ATP is bound, interactions between the Ala594 and the adenine ring of ATP could still hold ATP in the GTP binding site. The results of the present study suggested that though LdAdSS is GTP specific, it still shows ATP hydrolysing activity.

Structure of the GDP-bound state of the SRP GTPase FlhF.

The GTPase FlhF, a signal recognition particle (SRP)-type enzyme, is pivotal for spatial-numerical control and bacterial flagella assembly across diverse species, including pathogens. This study presents the X-ray structure of FlhF in its GDP-bound state at a resolution of 2.28 Å. The structure exhibits the classical N- and G-domain fold, consistent with related SRP GTPases such as Ffh and FtsY. Comparative analysis with GTP-loaded FlhF elucidates the conformational changes associated with GTP hydrolysis. These topological reconfigurations are similarly evident in Ffh and FtsY, and play a pivotal role in regulating the functions of these hydrolases.

Insights into the role of the conserved GTPase domain residues T62 and S277 in yeast Dnm1.

Mitochondrial division is a highly regulated process. The master regulator of this process is the multi-domain, conserved protein called Dnm1 in yeast. In this study, we systematically analyzed two residues, T62 and S277, reported to be putatively phosphorylated in the GTPase domain of the protein. These residues lie in the G2 and G5 motifs of the GTPase domain. Both residues are important for the function of the protein, as evident from in vivo and in vitro analysis of the non-phosphorylatable and phosphomimetic variants. Dnm1T62A/D and Dnm1S277A/D showed differences with respect to the protein localization and puncta dynamics in vivo, albeit both were non-functional as assessed by mitochondrial morphology and GTPase activity. Overall, the secondary structure of the protein variants was unaltered, but local conformational changes were observed. Interestingly, both Dnm1T62A/D and Dnm1S277A/D exhibited dominant-negative behavior when expressed in cells containing endogenous Dnm1. To our knowledge, we report for the first time a single residue (S277) change that does not alter the localization of Dnm1 but makes it non-functional in a dominant-negative manner. Intriguingly, the two residues analyzed in this study are present in the same domain but exhibit variable effects when mutated to alanine or aspartic acid.

The variable domain from dynamin-related protein 1 promotes liquid-liquid phase separation that enhances its interaction with cardiolipin-containing membranes.

Dynamins are an essential superfamily of mechanoenzymes that remodel membranes and often contain a "variable domain" important for regulation. For the mitochondrial fission dynamin, dynamin-related protein 1, a regulatory role for the variable domain (VD) is demonstrated by gain- and loss-of-function mutations, yet the basis for this is unclear. Here, the isolated VD is shown to be intrinsically disordered and undergo a cooperative transition in the stabilizing osmolyte trimethylamine N-oxide. However, the osmolyte-induced state is not folded and surprisingly appears as a condensed state. Other co-solutes including known molecular crowder Ficoll PM 70, also induce a condensed state. Fluorescence recovery after photobleaching experiments reveal this state to be liquid-like indicating the VD undergoes a liquid-liquid phase separation under crowding conditions. These crowding conditions also enhance binding to cardiolipin, a mitochondrial lipid, which appears to promote phase separation. Since dynamin-related protein 1 is found assembled into discrete punctate structures on the mitochondrial surface, the inference from the present work is that these structures might arise from a condensed state involving the VD that may enable rapid tuning of mechanoenzyme assembly necessary for fission.

OPA1 helical structures give perspective to mitochondrial dysfunction.

Dominant optic atrophy is one of the leading causes of childhood blindness. Around 60-80% of cases1 are caused by mutations of the gene that encodes optic atrophy protein 1 (OPA1), a protein that has a key role in inner mitochondrial membrane fusion and remodelling of cristae and is crucial for the dynamic organization and regulation of mitochondria2. Mutations in OPA1 result in the dysregulation of the GTPase-mediated fusion process of the mitochondrial inner and outer membranes3. Here we used cryo-electron microscopy methods to solve helical structures of OPA1 assembled on lipid membrane tubes, in the presence and absence of nucleotide. These helical assemblies organize into densely packed protein rungs with minimal inter-rung connectivity, and exhibit nucleotide-dependent dimerization of the GTPase domains-a hallmark of the dynamin superfamily of proteins4. OPA1 also contains several unique secondary structures in the paddle domain that strengthen its membrane association, including membrane-inserting helices. The structural features identified in this study shed light on the effects of pathogenic point mutations on protein folding, inter-protein assembly and membrane interactions. Furthermore, mutations that disrupt the assembly interfaces and membrane binding of OPA1 cause mitochondrial fragmentation in cell-based assays, providing evidence of the biological relevance of these interactions.

DHA alleviates high glucose-induced mitochondrial dysfunction in Oreochromis niloticus by inhibiting DRP1-mediated mitochondrial fission.

Dynamin-related protein 1 (DRP1) is a key regulator in the maintenance of mammalian glucose homeostasis, but the relevant information remains poorly understood on aquatic animals. In the study, DRP1 is formally described for the first time in Oreochromis niloticus. DRP1 encodes a peptide of 673 amino acid residues that contained three conserved domains: a GTPase domain, a dynamin middle domain and a dynamin GTPase effector domain. DRP1 transcripts are widely distributed in all of the detected seven organs/tissues, and the highest mRNA levels in brain. High-carbohydrate (45 %) fed fish showed a significant upregulation of liver DRP1 expression than that of control (30 %) group. Glucose administration upregulated liver DRP1 expression, with peak values observed at 1 h; then its expression returned to the basal value at 12 h. In the in vitro study, DRP1 over-expression significantly decreased mitochondrial abundance in hepatocytes. DHA significantly increased mitochondrial abundance, transcriptions of mitochondrial transcription factor A (TFAM) and mitofusin 1 and 2 (MFN1 and MFN2) and complex II and III activities of high glucose-treated hepatocyte, whereas the opposite was true for DRP1, mitochondrial fission factor (MFF) and fission (FIS) expression. Together, these findings illustrated that O. niloticus DRP1 is highly conserved, and it participated in glucose control of fish. DHA could alleviate high glucose-induced mitochondrial dysfunction of fish by inhibiting DRP1-mediated mitochondrial fission.

Mitochondria-localized lncRNA HITT inhibits fusion by attenuating formation of mitofusin-2 homotypic or heterotypic complexes.

Long noncoding RNAs (lncRNAs) are emerging as essential players in multiple biological processes. Mitochondrial dynamics, comprising the continuous cycle of fission and fusion, are required for healthy mitochondria that function properly. Despite long-term recognition of its significance in cell-fate control, the mechanism underlying mitochondrial fusion is not completely understood, particularly regarding the involvement of lncRNAs. Here, we show that the lncRNA HITT (HIF-1α inhibitor at translation level) can specifically localize in mitochondria. Cells expressing higher levels of HITT contain fragmented mitochondria. Conversely, we show that HITT knockdown cells have more tubular mitochondria than is present in control cells. Mechanistically, we demonstrate HITT directly binds mitofusin-2 (MFN2), a core component that mediates mitochondrial outer membrane fusion, by the in vitro RNA pull-down and UV-cross-linking RNA-IP assays. In doing so, we found HITT disturbs MFN2 homotypic or heterotypic complex formation, attenuating mitochondrial fusion. Under stress conditions, such as ultraviolet radiation, we in addition show HITT stability increases as a consequence of MiR-205 downregulation, inhibiting MFN2-mediated fusion and leading to apoptosis. Overall, our data provide significant insights into the roles of organelle (mitochondria)-specific resident lncRNAs in regulating mitochondrial fusion and also reveal how such a mechanism controls cellular sensitivity to UV radiation-induced apoptosis.

Molecular basis for the recruitment of the Rab effector protein WDR44 by the GTPase Rab11.

The formation of complexes between Rab11 and its effectors regulates multiple aspects of membrane trafficking, including recycling and ciliogenesis. WD repeat-containing protein 44 (WDR44) is a structurally uncharacterized Rab11 effector that regulates ciliogenesis by competing with prociliogenesis factors for Rab11 binding. Here, we present a detailed biochemical and biophysical characterization of the WDR44-Rab11 complex and define specific residues mediating binding. Using AlphaFold2 modeling and hydrogen/deuterium exchange mass spectrometry, we generated a molecular model of the Rab11-WDR44 complex. The Rab11-binding domain of WDR44 interacts with switch I, switch II, and the interswitch region of Rab11. Extensive mutagenesis of evolutionarily conserved residues in WDR44 at the interface identified numerous complex-disrupting mutations. Using hydrogen/deuterium exchange mass spectrometry, we found that the dynamics of the WDR44-Rab11 interface are distinct from the Rab11 effector FIP3, with WDR44 forming a more extensive interface with the switch II helix of Rab11 compared with FIP3. The WDR44 interaction was specific to Rab11 over evolutionarily similar Rabs, with mutations defining the molecular basis of Rab11 specificity. Finally, WDR44 can be phosphorylated by Sgk3, with this leading to reorganization of the Rab11-binding surface on WDR44. Overall, our results provide molecular detail on how WDR44 interacts with Rab11 and how Rab11 can form distinct effector complexes that regulate membrane trafficking events.

Cryo-EM structure of the SEA complex.

The SEA complex (SEAC) is a growth regulator that acts as a GTPase-activating protein (GAP) towards Gtr1, a Rag GTPase that relays nutrient status to the Target of Rapamycin Complex 1 (TORC1) in yeast1. Functionally, the SEAC has been divided into two subcomplexes: SEACIT, which has GAP activity and inhibits TORC1, and SEACAT, which regulates SEACIT2. This system is conserved in mammals: the GATOR complex, consisting of GATOR1 (SEACIT) and GATOR2 (SEACAT), transmits amino acid3 and glucose4 signals to mTORC1. Despite its importance, the structure of SEAC/GATOR, and thus molecular understanding of its function, is lacking. Here, we solve the cryo-EM structure of the native eight-subunit SEAC. The SEAC has a modular structure in which a COPII-like cage corresponding to SEACAT binds two flexible wings, which correspond to SEACIT. The wings are tethered to the core via Sea3, which forms part of both modules. The GAP mechanism of GATOR1 is conserved in SEACIT, and GAP activity is unaffected by SEACAT in vitro. In vivo, the wings are essential for recruitment of the SEAC to the vacuole, primarily via the EGO complex. Our results indicate that rather than being a direct inhibitor of SEACIT, SEACAT acts as a scaffold for the binding of TORC1 regulators.

Lipid and Lipidation in Membrane Fusion.

Membrane fusion plays a lead role in the transport of vesicles, neurotransmission, mitochondrial dynamics, and viral infection. There are fusion proteins that catalyze and regulate the fusion. Interestingly, various types of fusion proteins are present in nature and they possess diverse mechanisms of action. We have highlighted the importance of the functional domains of intracellular heterotypic fusion, homotypic endoplasmic reticulum (ER), homotypic mitochondrial, and type-I viral fusion. During intracellular heterotypic fusion, the SNAREs and four-helix bundle formation are prevalent. Type-I viral fusion is controlled by the membrane destabilizing properties of fusion peptide and six-helix bundle formation. The ER/mitochondrial homotypic fusion is controlled by GTPase activity and the membrane destabilization properties of the amphipathic helix(s). Although the mechanism of action of these fusion proteins is diverse, they have some similarities. In all cases, the lipid composition of the membrane greatly affects membrane fusion. Next, examples of lipidation of the fusion proteins were discussed. We suggest that the fatty acyl hydrophobic tail not only acts as an anchor but may also modulate the energetics of membrane fusion intermediates. Lipidation is also important to design more effective peptide-based fusion inhibitors. Together, we have shown that membrane lipid composition and lipidation are important to modulate membrane fusion.

Expression and purification of the NG domain from human SRα, a key component of the Signal Recognition Particle (SRP) receptor.

The Signal Recognition Particle (SRP) and the SRP receptor (SR) are responsible for protein targeting to the plasma membrane and the protein secretory pathway. Eukaryotic SRα, one of the two proteins that form the SR, is composed of the NG, MoRF and X domains. The SRα-NG domain is responsible for binding to SRP proteins such as SRP54, interacting with RNA, binding and hydrolysing GTP. The ability to produce folded SRα-NG is a prerequisite for structural studies directed towards a better understanding of its molecular mechanism and function, as well as in (counter-)screening assays for potential binders in the drug development pipeline. However, previously reported SRα-NG constructs and purification methods only used a truncated version, lacking the first N-terminal helix. This helix in other NG domains (e.g., SRP54) has been shown to be important for protein:protein interactions but its importance in SRα remains unknown. Here, we present the cloning as well as optimised expression and purification protocols of the whole SRα-NG domain including the first N-terminal helix. We have also expressed and purified isotopically labelled SRα-NG to facilitate Nuclear Magnetic Resonance (NMR) studies.

Molecular dynamics simulation study on Thermotoga maritima EngA: GTP/GDP bound state of the second G-domain influences the domain-domain interface interactions.

EngA, a GTPase involved in the late steps of ribosome maturation, consists of two GTP binding domains (G-domains) [GD1, GD2] and a C-terminal domain. The combination of GTP/GDP in G-domains dictates its binding to the ribosomal subunits by altering its conformation. Studies and comparisons on the available structures of EngA enable us to understand the correlation between nucleotide bound states and its conformation. Using all-atom molecular dynamics (MD) simulations, we have explored the conformational behavior of EngA from Thermotoga maritima (TmDer) upon binding the various combinations of GTP and GDP. Analyses of Root Mean Square Deviation (RMSD), Radius of Gyration (Rg) and Root Mean Square Fluctuation (RMSF) emphasize the importance of the second G-domain nucleotide bound state. RMSD and Rg exhibit slightly lower values when GTP is embedded in GD2 compared to GDP. These lower values are due to Sw-II of GD2, which has been observed from RMSF plot. Further investigation on the effects of GD2 nucleotide bound state using Principal Component Analysis (PCA) and Free Energy Landscape (FEL) analysis manifests an allosteric connection between GD2 nucleotide bound state and the GD1-KH interface. This is further validated by extracting electrostatic interactions and H-bonds at the GD1-KH interface. In silico mutations at the GD1 interface of KH domain affect the Sw-II mobility of GD2 by showing inverted behavior. This suggests using the second G-domain as an antibacterial target and further simulation studies on different species of EngA are to be explored.Communicated by Ramaswamy H. Sarma.

Short-form OPA1 is a molecular chaperone in mitochondrial intermembrane space.

Mitochondria, double-membrane organelles, are known to participate in a variety of metabolic and signal transduction pathways. The intermembrane space (IMS) of mitochondria is proposed to subject to multiple damages emanating from the respiratory chain. The optic atrophy 1 (OPA1), an important protein for mitochondrial fusion, is cleaved into soluble short-form (S-OPA1) under stresses. Here we report that S-OPA1 could function as a molecular chaperone in IMS. We purified the S-OPA1 (amino acid sequence after OPA1 isoform 5 S1 site) protein and showed it protected substrate proteins from thermally and chemically induced aggregation and strengthened the thermotolerance of Escherichia coli (E. coli). We also showed that S-OPA1 conferred thermotolerance on IMS proteins, e.g., neurolysin. The chaperone activity of S-OPA1 may be required for maintaining IMS homeostasis in mitochondria.

Histone deacetylase 8 interacts with the GTPase SmRho1 in Schistosoma mansoni.

BACKGROUND: Schistosoma mansoni histone deacetylase 8 (SmHDAC8) has elicited considerable interest as a target for drug discovery. Invalidation of its transcripts by RNAi leads to impaired survival of the worms in infected mice and its inhibition causes cell apoptosis and death. To determine why it is a promising therapeutic target the study of the currently unknown cellular signaling pathways involving this enzyme is essential. Protein partners of SmHDAC8 were previously identified by yeast two-hybrid (Y2H) cDNA library screening and by mass spectrometry (MS) analysis. Among these partners we characterized SmRho1, the schistosome orthologue of human RhoA GTPase, which is involved in the regulation of the cytoskeleton. In this work, we validated the interaction between SmHDAC8 and SmRho1 and explored the role of the lysine deacetylase in cytoskeletal regulation. METHODOLOGY/PRINCIPAL FINDINGS: We characterized two isoforms of SmRho1, SmRho1.1 and SmRho1.2. Co- immunoprecipitation (Co-IP)/Mass Spectrometry (MS) analysis identified SmRho1 partner proteins and we used two heterologous expression systems (Y2H assay and Xenopus laevis oocytes) to study interactions between SmHDAC8 and SmRho1 isoforms. To confirm SmHDAC8 and SmRho1 interaction in adult worms and schistosomula, we performed Co-IP experiments and additionally demonstrated SmRho1 acetylation using a Nano LC-MS/MS approach. A major impact of SmHDAC8 in cytoskeleton organization was documented by treating adult worms and schistosomula with a selective SmHDAC8 inhibitor or using RNAi followed by confocal microscopy. CONCLUSIONS/SIGNIFICANCE: Our results suggest that SmHDAC8 is involved in cytoskeleton organization via its interaction with the SmRho1.1 isoform. The SmRho1.2 isoform failed to interact with SmHDAC8, but did specifically interact with SmDia suggesting the existence of two distinct signaling pathways regulating S. mansoni cytoskeleton organization via the two SmRho1 isoforms. A specific interaction between SmHDAC8 and the C-terminal moiety of SmRho1.1 was demonstrated, and we showed that SmRho1 is acetylated on K136. SmHDAC8 inhibition or knockdown using RNAi caused extensive disruption of schistosomula actin cytoskeleton.

The Stringent Response Inhibits 70S Ribosome Formation in Staphylococcus aureus by Impeding GTPase-Ribosome Interactions.

During nutrient limitation, bacteria produce the alarmones (p)ppGpp as effectors of a stress signaling network termed the stringent response. RsgA, RbgA, Era, and HflX are four ribosome-associated GTPases (RA-GTPases) that bind to (p)ppGpp in Staphylococcus aureus. These enzymes are cofactors in ribosome assembly, where they cycle between the ON (GTP-bound) and OFF (GDP-bound) ribosome-associated states. Entry into the OFF state occurs upon hydrolysis of GTP, with GTPase activity increasing substantially upon ribosome association. When bound to (p)ppGpp, GTPase activity is inhibited, reducing 70S ribosome assembly and growth. Here, we determine how (p)ppGpp impacts RA-GTPase-ribosome interactions. We show that RA-GTPases preferentially bind to 5'-diphosphate-containing nucleotides GDP and ppGpp over GTP, which is likely exploited as a regulatory mechanism within the cell to shut down ribosome biogenesis during stress. Stopped-flow fluorescence and association assays reveal that when bound to (p)ppGpp, the association of RA-GTPases to ribosomal subunits is destabilized, both in vitro and within bacterial cells. Consistently, structural analysis of the ppGpp-bound RA-GTPase RsgA reveals an OFF-state conformation similar to the GDP-bound state, with the G2/switch I loop adopting a conformation incompatible with ribosome association. Altogether, we highlight (p)ppGpp-mediated inhibition of RA-GTPases as a major mechanism of stringent response-mediated ribosome assembly and growth control. IMPORTANCE The stringent response is a bacterial signaling network that utilizes the nucleotides pppGpp and ppGpp to reprogram cells in order to survive nutritional stresses. However, much about how these important nucleotides control cellular reprogramming is unknown. Our previous work revealed that (p)ppGpp can bind to and inhibit the enzymatic activity of four ribosome-associated GTPases (RA-GTPases), enzymes that facilitate maturation of the 50S and 30S ribosomal subunits. Here, we examine how this occurs mechanistically and demonstrate that this interaction prevents the accommodation of RA-GTPases on ribosomal subunits both in vitro and within bacterial cells, with the ppGpp-bound state structurally mimicking the inactive GDP-bound conformation of the enzyme. We additionally reveal that these GTPase enzymes have a greater affinity for OFF-state-inducing nucleotides, which is a mechanism likely to control ribosome assembly during growth. With this, we further our understanding of how ribosome function is controlled by (p)ppGpp, enabling bacterial survival during stress.

Comparative Genomic Analysis of the DUF34 Protein Family Suggests Role as a Metal Ion Chaperone or Insertase.

Members of the DUF34 (domain of unknown function 34) family, also known as the NIF3 protein superfamily, are ubiquitous across superkingdoms. Proteins of this family have been widely annotated as "GTP cyclohydrolase I type 2" through electronic propagation based on one study. Here, the annotation status of this protein family was examined through a comprehensive literature review and integrative bioinformatic analyses that revealed varied pleiotropic associations and phenotypes. This analysis combined with functional complementation studies strongly challenges the current annotation and suggests that DUF34 family members may serve as metal ion insertases, chaperones, or metallocofactor maturases. This general molecular function could explain how DUF34 subgroups participate in highly diversified pathways such as cell differentiation, metal ion homeostasis, pathogen virulence, redox, and universal stress responses.

A fast protein binding site comparison algorithm for proteome-wide protein function prediction and drug repurposing.

The expansion of three-dimensional protein structures and enhanced computing power have significantly facilitated our understanding of protein sequence/structure/function relationships. A challenge in structural genomics is to predict the function of uncharacterized proteins. Protein function deconvolution based on global sequence or structural homology is impracticable when a protein relates to no other proteins with known function, and in such cases, functional relationships can be established by detecting their local ligand binding site similarity. Here, we introduce a sequence order-independent comparison algorithm, PocketShape, for structural proteome-wide exploration of protein functional site by fully considering the geometry of the backbones, orientation of the sidechains, and physiochemical properties of the pocket-lining residues. PocketShape is efficient in distinguishing similar from dissimilar ligand binding site pairs by retrieving 99.3% of the similar pairs while rejecting 100% of the dissimilar pairs on a dataset containing 1538 binding site pairs. This method successfully classifies 83 enzyme structures with diverse functions into 12 clusters, which is highly in accordance with the actual structural classification of proteins classification. PocketShape also achieves superior performances than other methods in protein profiling based on experimental data. Potential new applications for representative SARS-CoV-2 drugs Remdesivir and 11a are predicted. The high accuracy and time-efficient characteristics of PocketShape will undoubtedly make it a promising complementary tool for proteome-wide protein function inference and drug repurposing study.

Mutation spectrum of the OPA1 gene in a large cohort of patients with suspected dominant optic atrophy: Identification and classification of 48 novel variants.

Autosomal dominant optic atrophy is one of the most common inherited optic neuropathies. This disease is genetically heterogeneous, but most cases are due to pathogenic variants in the OPA1 gene: depending on the population studied, 32-90% of cases harbor pathogenic variants in this gene. The aim of this study was to provide a comprehensive overview of the entire spectrum of likely pathogenic variants in the OPA1 gene in a large cohort of patients. Over a period of 20 years, 755 unrelated probands with a diagnosis of bilateral optic atrophy were referred to our laboratory for molecular genetic investigation. Genetic testing of the OPA1 gene was initially performed by a combined analysis using either single-strand conformation polymorphism or denaturing high performance liquid chromatography followed by Sanger sequencing to validate aberrant bands or melting profiles. The presence of copy number variations was assessed using multiplex ligation-dependent probe amplification. Since 2012, genetic testing was based on next-generation sequencing platforms. Genetic screening of the OPA1 gene revealed putatively pathogenic variants in 278 unrelated probands which represent 36.8% of the entire cohort. A total of 156 unique variants were identified, 78% of which can be considered null alleles. Variant c.2708_2711del/p.(V903Gfs*3) was found to constitute 14% of all disease-causing alleles. Special emphasis was placed on the validation of splice variants either by analyzing cDNA derived from patients´ blood samples or by heterologous splice assays using minigenes. Splicing analysis revealed different aberrant splicing events, including exon skipping, activation of exonic or intronic cryptic splice sites, and the inclusion of pseudoexons. Forty-eight variants that we identified were novel. Nine of them were classified as pathogenic, 34 as likely pathogenic and five as variant of uncertain significance. Our study adds a significant number of novel variants to the mutation spectrum of the OPA1 gene and will thereby facilitate genetic diagnostics of patients with suspected dominant optic atrophy.

Mimicking human Drp1 disease-causing mutations in yeast Dnm1 reveals altered mitochondrial dynamics.

The dynamin-related protein 1 (Drp1) and its homologs in various eukaryotes are essential to maintain mitochondrial morphology and regulate mitochondrial division. Several mutations in different domains of Drp1 have been reported, which result in debilitating conditions. Four such disease-causing mutations of the middle domain of Drp1 were mimicked in the yeast dynamin-related GTPase (Dnm1) and were characterized in this study. Mitochondrial morphology and protein function were observed to be altered to a variable extent in cells expressing the mutated variants of Dnm1. Several aspects related to the protein such as punctate formation, localization to mitochondria, dynamic behavior and structure were analyzed by microscopy, biochemical studies and molecular dynamics simulations. Significant effects on the protein structure and function were observed in cells expressing A430D and G397D mutations. Overall, our data provide insight into the molecular and cellular alterations resulting from middle domain mutations in Dnm1.

[New structures of mTORC1: Focus on Rag GTPases]. / Une moisson de nouvelles structures de mTORC1 - Coup de projecteur sur les GTPases Rag.

mTORC1 is a central player in cell growth, a process that is tightly regulated by the availability of nutrients and that controls various aspects of metabolism in the normal cell and in severe diseases such as cancers. mTORC1 is a large multiprotein complex, composed of the kinase subunit mTOR, of Ragulator, which attaches mTOR to the lysosome membrane, of the atypical Rag GTPases and the small GTPase RheB, whose nucleotide states directly dictate its localization to the lysosome and its kinase activity, and of RAPTOR, an adaptor that assembles the complex. The activity of the Rag GTPases is further controlled by the GATOR1 and folliculin complexes, which regulate their GTP/GDP conversion. Here, we review recent structures of important components of the mTORC1 machinery, determined by cryo-electron microscopy for the most part, which allow to reconstitute the architecture of active mTORC1 at near atomic resolution. Notably, we discuss how these structures shed new light on the roles of Rag GTPases and their regulators in mTORC1 regulation, and the perspectives that they open towards understanding the inner workings of mTORC1 on the lysosomal membrane.

TITLE: Une moisson de nouvelles structures de mTORC1 - Coup de projecteur sur les GTPases Rag. ABSTRACT: mTORC1 est un acteur central de la croissance cellulaire, un processus étroitement régulé par la disponibilité de nutriments et qui contrôle diverses étapes du métabolisme dans la cellule normale et au cours de maladies, comme les cancers. mTORC1 est un complexe multiprotéique de grande taille constitué de nombreuses sous-unités, parmi lesquelles deux types de GTPases, Rag et RheB, contrôlent directement sa localisation membranaire et son activité kinase. Dans cette revue, nous faisons le point sur une moisson de structures récentes, déterminées pour la plupart par cryo-microscopie électronique, qui sont en passe de reconstituer le puzzle de l'architecture de mTORC1. Nous discutons ce que ces structures révèlent sur le rôle des GTPases, et ce que leur connaissance ouvre comme perspectives pour comprendre comment mTORC1 fonctionne à la membrane du lysosome.