Domains mediating intramolecular folding and oligomerization of MxA GTPase

Novel DNM1L variants impair mitochondrial dynamics through divergent mechanisms

Imbalances in mitochondrial and peroxisomal dynamics are associated with a spectrum of human neurological disorders. Mitochondrial and peroxisomal fission both involve dynamin-related protein 1 (DRP1) oligomerisation and membrane constriction, although the precise biophysical mechanisms by which distinct DRP1 variants affect the assembly and activity of different DRP1 domains remains largely unexplored. We analysed four unreported de novo heterozygous variants in the dynamin-1-like gene DNM1L affecting different highly conserved DRP1 domains, leading to developmental delay, seizures, hypotonia, and/or rare cardiac complications in infancy. Single-nucleotide DRP1 stalk domain variants were found to correlate with more severe clinical phenotypes, with in vitro recombinant human DRP1 mutants demonstrating greater impairments in protein oligomerisation, DRP1-peroxisomal recruitment, and both mitochondrial and peroxisomal hyperfusion compared to GTPase or GTPase-effector domain variants. Importantly, we identified a novel mechanism of pathogenesis, where a p.Arg710Gly variant uncouples DRP1 assembly from assembly-stimulated GTP hydrolysis, providing mechanistic insight into how assembly-state information is transmitted to the GTPase domain. Together, these data reveal that discrete, pathological DNM1L variants impair mitochondrial network maintenance by divergent mechanisms.

Dynamin-Like Proteins Are Potentially Involved in Membrane Dynamics within Chloroplasts and Cyanobacteria

Dynamin-like proteins (DLPs) are a family of membrane-active proteins with low sequence identity. The proteins operate in different organelles in eukaryotic cells, where they trigger vesicle formation, membrane fusion, or organelle division. As discussed here, representatives of this protein family have also been identified in chloroplasts and DLPs are very common in cyanobacteria. Since cyanobacteria and chloroplasts, an organelle of bacterial origin, have similar internal membrane systems, we suggest that DLPs are involved in membrane dynamics in cyanobacteria and chloroplasts. Here, we discuss the features and activities of DLPs with a focus on their potential presence and activity in chloroplasts and cyanobacteria.

Oligomerization and GTP-binding Requirements of MxA for Viral Target Recognition and Antiviral Activity against Influenza A Virus

The IFN-induced human myxovirus resistance protein A (MxA) exhibits a broad antiviral activity against many viruses, including influenza A virus (IAV). MxA belongs to the family of dynamin-like GTPases and assembles in vitro into dimers, tetramers, and oligomeric ring-like structures. The molecular mechanism of action remains to be elucidated. Furthermore, it is not clear whether MxA exerts its antiviral activity in a monomeric and/or multimeric form. Using a set of MxA mutants that form complexes with defined stoichiometry, we observed that, in the presence of guanosine 5'-O-(thiotriphosphate), purified MxA disassembled into tetramers and dimers. Dimeric forms did not further disassemble into monomers. Infection experiments revealed that besides wild-type MxA, dimeric and monomeric variants of MxA also efficiently restricted IAV at a replication step after primary transcription. Moreover, only dimeric MxA was able to form stable complexes with the nucleoprotein (NP) of IAV. MxA interacted with NP independently of other viral components. Interestingly, the dimeric form of MxA was able to efficiently bind to NP from several MxA-sensitive strains but interacted much more weakly with NP from the MxA-resistant PR8 strain derived from the H1N1 1918 lineage. Taken together, these data suggest that, during infection, a fraction of MxA disassembles into dimers that bind to NP synthesized following primary transcription in the cytoplasm, thereby preventing viral replication.

MxB binds to the HIV-1 core and prevents the uncoating process of HIV-1

Background

The IFN-α-inducible restriction factor MxB blocks HIV-1 infection after reverse transcription but prior to integration. Genetic evidence suggested that capsid is the viral determinant for restriction by MxB. This work explores the ability of MxB to bind to the HIV-1 core, and the role of capsid-binding in restriction.

Results

We showed that MxB binds to the HIV-1 core and that this interaction leads to inhibition of the uncoating process of HIV-1. These results identify MxB as an endogenously expressed protein with the ability to inhibit HIV-1 uncoating. In addition, we found that a benzimidazole-based compound known to have a binding pocket on the surface of the HIV-1 capsid prevents the binding of MxB to capsid. The use of this small-molecule identified the MxB binding region on the surface of the HIV-1 core. Domain mapping experiments revealed the following requirements for restriction: 1) MxB binding to the HIV-1 capsid, which requires the 20 N-terminal amino acids, and 2) oligomerization of MxB, which is mediated by the C-terminal domain provides the avidity for the interaction of MxB with the HIV-1 core.

Conclusions

Overall our work establishes that MxB binds to the HIV-1 core and inhibits the uncoating process of HIV-1. Moreover, we demonstrated that HIV-1 restriction by MxB requires capsid binding and oligomerization.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-014-0068-x) contains supplementary material, which is available to authorized users.

Characterization of four Mx isoforms in the European eel, Anguilla anguilla

Mx protein is known to play an important role in vertebrate immune response to viral infection. In this study, cDNA sequences of four Mx isoforms, designated as MxA, B, C and D were characterized in the European eel, Anguilla anguilla. These sequences contained an open reading frame of 1899, 1896, 1866, 1779 bp, flanked by 95, 53, 138, 69 bp of 5' untranslated region and 389, 241, 136, 124 bp of 3' untranslated region, respectively. A phylogenetic tree constructed with Mx peptide sequences from vertebrates revealed that MxA, C and D in the European eel formed into a clade containing zebrafish MxA and MxB and Mx proteins in other teleosts, whereas MxB in the eel was clustered together with zebrafish MxD, MxG and MxF. The transcription level of all Mx isoforms increased in a poly I:C dose-dependent manner in peripheral blood leukocytes of eels, as revealed by real-time PCR. A further experiment was conducted to reveal the temporal change in expression of these isoforms in various organs/tissues following poly I:C stimulation, and significant increase in expression was observed at various degrees in different organs or in different sampling occasions within the 12 h experimental period. In particular, MxA had the highest level of increase, while MxB had the lowest; and three isoforms, MxA, MxB and MxD had the highest increase in intestine, while the highest increase of MxC expression was observed in liver. These four isoforms of eel Mx are thus expressed differentially, and further work is certainly required to clarify the activity of promoter elements and antiviral activity of these Mx isoforms.

The myxovirus resistance A (MxA) gene -88G>T single nucleotide polymorphism is associated with prostate cancer

BACKGROUND

Myxovirus (influenza virus) resistance A (MxA) is an interferon stimulated antiviral protein that is required for a complete antiviral response. MxA polymorphism (rs2071430) is located within an Interferon Stimulated Response Element (ISRE) at position -88 in the gene's promoter region, and it has been associated with increased susceptibility to infections and various diseases. In general, the low promoter activity genotype (GG) promotes susceptibility, whereas the high promoter activity genotype (TT) confers protection to Hepatitis C viral infection. MxA's role in prostate cancer is not fully understood. Previous literature has shown that MxA may be a mediator of the effect of IFN on normal and tumor cell motility. MxA may act as a tumor suppressor and the level of expression may be a predictor of metastatic potential. Based on this information, in this study we investigated the association of this functional polymorphism (rs2071430) in MxA with prostate cancer.

METHODS

Sample size and power was calculated using the PGA software. Genomic DNA from a controls (n=140) and prostate cancer patients (n=164) were used for genotyping SNP rs2071430 on all samples. Statistical analysis was performed using logistic regression model.

RESULTS

A significant association was observed between rs2071430 genotype GG and prostate cancer. Individuals harboring the GG genotype are at an increased risk of prostate cancer. Data stratification reveals that the mutant GT genotype offers either offers some protection against prostate cancer in Caucasians.

CONCLUSIONS

MxA SNP rs2071430 GG genotype is significantly associated with prostate cancer irrespective of race. However, data stratification also suggests that the GT genotype is under-represented in Caucasian subjects suggesting its role in protection against prostate cancer in Caucasians. Although MxA is primarily implicated in viral infection, but it may be also be associated with prostate cancer. Recent studies have implicated viral and bacterial infections with increased prostate cancer risk. Expression of the high promoter activity genotype may offer resistance to prostate cancer infection and possibly influence clinical outcomes.

Both antiviral activity and intracellular localization of chicken Mx protein depend on a polymorphism at amino acid position 631

The Mx protein is known to inhibit the multiplication of several RNA viruses. In chickens, a polymorphism at amino acid position 631 (631 aa) of Mx protein has been suggested to be involved in the antiviral ability against vesicular stomatitis virus (VSV) and influenza virus, indicating that a Ser-to-Asn substitution at 631 aa is the source of this antiviral ability. However, how the substitution at 631 aa contributes to the antiviral activity remains to be clarified. In this study, we investigated differences in antiviral activity against VSV and intracellular localization between Ser and Asn types at 631 aa of the chicken Mx protein. The results showed that chicken Mx protein with an Asn at 631 aa inhibited VSV multiplication and Mx distribution in a granular-like pattern in the cytoplasm. However, Mx carrying the Ser type did not inhibit viral growth and homogenous spread throughout the cytoplasm. Furthermore, we found that replacing Ser with Asn at 631 aa provided Mx with antiviral activity against VSV, with Mx showing granular-like distribution in the cytoplasm. These results demonstrated that a single amino acid polymorphism at 631 aa of the chicken Mx protein altered both the antiviral activity and intracellular localization.

MxA inhibits hepatitis B virus replication by interaction with hepatitis B core antigen

Human MxA, an interferon-inducible cytoplasmic dynamin-like GTPase, possesses antiviral activity against multiple RNA viruses. Recently, MxA has also been demonstrated to have activity against the hepatitis B virus (HBV), a well-known DNA virus responsible for acute and chronic liver disease in humans. We investigated the molecular mechanism for the anti-HBV activity of MxA. Our results demonstrated that in HepG2.2.15 cells, MxA GTPase independently suppressed the production of hepatitis B surface antigen and HBV DNA without changing the level of hepatitis B core antigen (HBcAg) and the distribution of HBV mRNA. MxA significantly reduced the level of the encapsidated pregenomic RNA. Through its central interactive domain, MxA interacted with HBcAg, causing accumulation of the proteins in perinuclear compartments. MxA-HBcAg interaction significantly affected the dynamics of HBcAg by immobilizing HBcAg in the perinuclear structures.

CONCLUSION

MxA displays antiviral activity against HBV involving a mechanism of MxA-HBcAg interaction that may interfere with core particle formation.

Human/bovine chimeric MxA-like GTPases reveal a contribution of N-terminal domains to the magnitude of anti-influenza A activity

Type I interferons (IFN-α/β) provide powerful and universal innate intracellular defense mechanisms against viruses. Among the antiviral effectors induced by IFN-α/β, Mx proteins of some species appear as key components of defense against influenza A viruses. The body of work published to date suggests that to exert anti-influenza activity, an Mx protein should possess a GTP-binding site, structural bases allowing multimerisation, and a specific C-terminal GTPase effector domain (GED). Both the human MxA and bovine Mx1 proteins meet these minimal requirements, but the bovine protein is more active against influenza viruses. Here, we measured the anti-influenza activity exerted by 2 human/bovine chimeric Mx proteins. We show that substituting the bovine GED for the human one in human MxA does not affect the magnitude of anti-influenza activity. Strikingly, however, substituting the human GED for the bovine one in bovine Mx1 yields a chimeric protein with a much higher anti-influenza activity than the human protein. We conclude, in contradiction to the hypothesis currently in vogue in the literature, that the GED is not the sole determinant controlling the magnitude of the anti-influenza activity exercised by an Mx protein that can bind GTP and multimerise. Our results suggest that 1 or several motifs that remain to be discovered, located N-terminally with regard to the GED, may interact with a viral component or a cellular factor so as to alter the viral cycle. Identifying, in the N-terminal portion of bovine Mx1, the motif(s) responsible for its higher anti-influenza activity could contribute to the development of new anti-influenza molecules.

Stalk domain of the dynamin-like MxA GTPase protein mediates membrane binding and liposome tubulation via the unstructured L4 loop

The human MxA protein is an interferon-induced large GTPase with antiviral activity against a wide range of viruses, including influenza viruses. Recent structural data demonstrated that MxA oligomerizes into multimeric filamentous or ring-like structures by virtue of its stalk domain. Here, we show that negatively charged lipid membranes support MxA self-assembly. Like dynamin, MxA assembled around spherical liposomes inducing liposome tubulation. Cryo-transmission electron microscopy revealed that MxA oligomers around liposomes have a "T-bar" shape similar to dynamin. Moreover, biochemical assays indicated that the unstructured L4 loop of the MxA stalk serves as the lipid-binding moiety, and mutational analysis of L4 revealed that a stretch of four lysine residues is critical for binding. The orientation of the MxA molecule within the membrane-associated oligomer is in agreement with the proposed topology of MxA oligomers based on crystallographic data. Although oligomerization of wild-type MxA around liposomes led to the creation of helically decorated tubes similar to those formed by dynamin, this lipid interaction did not stimulate GTPase activity, in sharp contrast to the assembly-stimulated nucleotide hydrolysis observed with dynamin. Moreover, MxA readily self-assembles into rings at physiological conditions, as opposed to dynamin which self-assembles only at low salt conditions or onto lipids. Thus, the present results indicate that the oligomeric structures formed by MxA critically differ from those of dynamin.

Interferon-induced antiviral protein MxA interacts with the cellular RNA helicases UAP56 and URH49

Mx proteins are a family of large GTPases that are induced exclusively by interferon-α/β and have a broad antiviral activity against several viruses, including influenza A virus (IAV). Although the antiviral activities of mouse Mx1 and human MxA have been studied extensively, the molecular mechanism of action remains largely unsolved. Because no direct interaction between Mx proteins and IAV proteins or RNA had been demonstrated so far, we addressed the question of whether Mx protein would interact with cellular proteins required for efficient replication of IAV. Immunoprecipitation of MxA revealed its association with two closely related RNA helicases, UAP56 and URH49. UAP56 and its paralog URH49 play an important role in IAV replication and are involved in nuclear export of IAV mRNAs and prevention of dsRNA accumulation in infected cells. In vitro binding assays with purified recombinant proteins revealed that MxA formed a direct complex with the RNA helicases. In addition, recombinant mouse Mx1 was also able to bind to UAP56 or URH49. Furthermore, the complex formation between cytoplasmic MxA and UAP56 or URH49 occurred in the perinuclear region, whereas nuclear Mx1 interacted with UAP56 or URH49 in distinct dots in the nucleus. Taken together, our data reveal that Mx proteins exerting antiviral activity can directly bind to the two cellular DExD/H box RNA helicases UAP56 and URH49. Moreover, the observed subcellular localization of the Mx-RNA helicase complexes coincides with the subcellular localization, where human MxA and mouse Mx1 proteins act antivirally. On the basis of these data, we propose that Mx proteins exert their antiviral activity against IAV by interfering with the function of the RNA helicases UAP56 and URH49.

A lethal de novo mutation in the middle domain of the dynamin-related GTPase Drp1 impairs higher order assembly and mitochondrial division

Mitochondria dynamically fuse and divide within cells, and the proper balance of fusion and fission is necessary for normal mitochondrial function, morphology, and distribution. Drp1 is a dynamin-related GTPase required for mitochondrial fission in mammalian cells. It harbors four distinct domains: GTP-binding, middle, insert B, and GTPase effector. A lethal mutation (A395D) within the Drp1 middle domain was reported in a neonate with microcephaly, abnormal brain development, optic atrophy, and lactic acidemia (Waterham, H. R., Koster, J., van Roermund, C. W., Mooyer, P. A., Wanders, R. J., and Leonard, J. V. (2007) N. Engl. J. Med. 356, 1736-1741). Mitochondria within patient-derived fibroblasts were markedly elongated, but the molecular mechanisms underlying these findings were not demonstrated. Because the middle domain is particularly important for the self-assembly of some dynamin superfamily proteins, we tested the hypothesis that this A395D mutation, and two other middle domain mutations (G350D, G363D) were important for Drp1 tetramerization, higher order assembly, and function. Although tetramerization appeared largely intact, each of these mutations compromised higher order assembly and assembly-dependent stimulation of Drp1 GTPase activity. Moreover, mutant Drp1 proteins exhibited impaired localization to mitochondria, indicating that this higher order assembly is important for mitochondrial recruitment, retention, or both. Overexpression of these middle domain mutants markedly inhibited mitochondrial division in cells. Thus, the Drp1 A395D lethal defect likely resulted in impaired higher order assembly of Drp1 at mitochondria, leading to decreased fission, elongated mitochondria, and altered cellular distribution of mitochondria.

Truncation of the NS1 protein converts a low pathogenic avian influenza virus into a strong interferon inducer in duck cells

The NS1 protein of influenza A viruses is known as a nonessential virulence factor inhibiting type I interferon (IFN) production in mammals and in chicken cells. Whether NS1 inhibits the induction of type I IFNs in duck cells is currently unknown. In order to investigate this issue, we used reverse genetics to generate a virus expressing a truncated NS1 protein. Using the low pathogenic avian influenza virus A/turkey/Italy/977/1999 (H7N1) as a backbone, we were able to rescue a virus expressing a truncated NS1 protein of 99 amino acids in length. The truncated virus replicated poorly in duck embryonic fibroblasts, but reached high titers in the mammalian IFN-deficient Vero cell line. Using a gene reporter system to measure duck type I IFN production, we showed that the truncated virus is a potent inducer of type I IFN in cell culture. These results show that the NS1 protein functions to prevent the induction of IFN in duck cells and underline the need for a functional NS1 protein in order for the virus to express its full virulence.

Mx gene diversity and influenza association among five wild dabbling duck species (Anas spp.) in Alaska

Mx (myxovirus-resistant) proteins are induced by interferon and inhibit viral replication as part of the innate immune response to viral infection in many vertebrates. Influenza A virus appears to be especially susceptible to Mx antiviral effects. We characterized exon 13 and the 3' UTR of the Mx gene in wild ducks, the natural reservoir of influenza virus and explored its potential relevance to influenza infection. We observed a wide range of intra- and interspecies variations. Total nucleotide diversity per site was 0.0014, 0.0027, 0.0044, 0.0051, and 0.0061 in mallards, northern shovelers, northern pintails, American wigeon, and American green-winged teals, respectively. There were 61 haplotypes present across all five species and four were shared among species. Additionally, we observed a significant association between Mx haplotype and influenza infection status in northern shovelers. However, we found no evidence of balancing or diversifying selection in this region of the Mx gene. Characterization of the duck Mx gene is an important step in understanding how the gene may affect disease resistance or susceptibility in wild populations. Furthermore, given that waterfowl act as a natural reservoir for influenza virus, the Mx gene could be an important determinant in the ecology of the virus.

Herpes simplex virus-1 induces expression of a novel MxA isoform that enhances viral replication

MxA is an antiviral protein induced by interferon (IFN)-α/β that is known to inhibit the replication of many RNA viruses. In these experiments, the 76-kDa MxA protein expressed in IFN-α-treated cells was shown to have antiviral activity against herpes simplex virus-1 (HSV-1), a human DNA virus. However, MxA was expressed as a 56-kDa protein in HSV-1-infected cells in the absence of IFN-α. This previously unrecognized MxA isoform was produced from an alternatively spliced MxA transcript that had a deletion of Exons 14-16 and a frame shift altering the C-terminus. The variant MxA (varMxA) isoform was associated with HSV-1 regulatory proteins and virions in nuclear replication compartments. varMxA expression enhanced HSV-1 infection as shown by a reduction in infectious virus titers from cells in which MxA had been inhibited by RNA interference and by an increase in HSV-1 titers when the 56-kDa varMxA was expressed constitutively. Thus, the human MxA gene encodes two MxA isoforms, which are expressed differentially depending on whether the stimulus is IFN-α or HSV-1. These findings show that alternative splicing of cellular mRNA can result in expression of a novel isoform of a host defense gene that supports instead of restricting viral infection.

Dynamin-like MxA GTPase: structural insights into oligomerization and implications for antiviral activity

The interferon-inducible MxA GTPase is a key mediator of cell-autonomous innate immunity against a broad range of viruses such as influenza and bunyaviruses. MxA shares a similar domain structure with the dynamin superfamily of mechanochemical enzymes, including an N-terminal GTPase domain, a central middle domain, and a C-terminal GTPase effector domain. Recently, crystal structures of a GTPase domain dimer of dynamin 1 and of the oligomerized stalk of MxA (built by the middle and GTPase effector domains) were determined. These data provide exciting insights into the architecture and antiviral function of the MxA oligomer. Moreover, the structural knowledge paves the way for the development of novel antiviral drugs against influenza and other highly pathogenic viruses.

Polymorphisms of the chicken antiviral MX gene

The Mx gene was originally found in laboratory mice in an infection experiment using influenza virus (Lindermann, 1962). Almost all of the mouse strains in that experiment died from the infection, and only the A2G strain had resistance to the virus. This resistant character was shown to be inherited as a single autosomal dominant trait (Lindermann et al., 1963; Lindermann, 1964; Haller et al., 1979). A congenic mouse strain was established by introducing the Mx+ allele of the A2G resistant strain into the Mx- sensitive inbred strain BALB/c (Staeheli et al., 1984). By immunizing parental BALB/c mice with extracts of interferon (IFN)-treated cultured cells from congenic BALB/c-Mx+ mice, a specific antibody against Mx protein was obtained (Horisberger et al., 1983; Staeheli et al., 1985). The Mx protein was detected in the nucleus of IFN-alpha/beta-treated mouse cells by immunofluorescence using the anti-Mx antibody (Dreiding et al., 1985). Thereafter, by using the antibody as an indicator, cDNA encoding the Mx protein was cloned from a cDNA library constructed from IFN-treated cells of congenic BALB/c-Mx+ mice (Staeheli et al., 1986a). IFN-treated Mx+ mouse cells contained a 3.5-kb Mx mRNA in the Northern blot, while Mx- cells failed to express the transcript. The functional Mx+ gene from an A2G mouse was found to contain 14 exons and encode 631 amino acids. The Mx- allelic mouse strains were found to be missing sequence of exons 9 through 11 or to contain a point mutation that converts lysine at position 389 to a stop codon (Staeheli et al., 1988). If these polymorphisms of the Mx gene could be detected in domestic animals, it would be possible to produce breeds that show resistance to infectious diseases.

Cyclic AMP-dependent protein kinase phosphorylation of Drp1 regulates its GTPase activity and mitochondrial morphology

Mitochondria in cells comprise a tubulovesicular reticulum shaped by dynamic fission and fusion events. The multimeric dynamin-like GTPase Drp1 is a critical protein mediating mitochondrial division. It harbors multiple motifs including GTP-binding, middle, and GTPase effector (GED) domains that are important for both intramolecular and intermolecular interactions. As for other members of the dynamin superfamily, such interactions are critical for assembly of higher-order structures and cooperative increases in GTPase activity. Although the functions of Drp1 in cells have been extensively studied, mechanisms underlying its regulation remain less clear. Here, we have identified cAMP-dependent protein kinase-dependent phosphorylation of Drp1 within the GED domain at Ser(637) that inhibits Drp1 GTPase activity. Mechanistically, this change in GTPase activity likely derives from decreased interaction of GTP-binding/middle domains with the GED domain since the phosphomimetic S637D mutation impairs this intramolecular interaction but not Drp1-Drp1 intermolecular interactions. Using the phosphomimetic S637D substitution, we also demonstrate that mitochondrial fission is prominently inhibited in cells. Thus, protein phosphorylation at Ser(637) results in clear alterations in Drp1 function and mitochondrial morphology that are likely involved in dynamic regulation of mitochondrial division in cells.

Interferon-induced Mx proteins in antiviral host defense

Mx proteins are key components of the antiviral state induced by interferons in many species. They belong to the class of dynamin-like large guanosine triphosphatases (GTPases) known to be involved in intracellular vesicle trafficking and organelle homeostasis. Mx GTPases share structural and functional properties with dynamin, such as self-assembly and association with intracellular membranes. A unique property of some Mx GTPases is their antiviral activity against a wide range of RNA viruses, including influenza viruses and members of the bunyavirus family. These viruses are inhibited at an early stage in their life cycle, soon after host cell entry and before genome amplification. The mouse Mx1 GTPase accumulates in the cell nucleus where it associates with components of the PML nuclear bodies and inhibits influenza and Thogoto viruses known to replicate in the nucleus. The human MxA GTPase accumulates in the cytoplasm and is partly associated with a COP-I-positive subcompartment of the endoplasmic reticulum. This membrane compartment seems to provide an interaction platform that facilitates viral target recognition. In the case of bunyaviruses, MxA recognizes the viral nucleocapsid protein and interferes with its role in viral genome replication. In the case of Thogoto virus, MxA recognizes the viral nucleoprotein and prevents the incoming viral nucleocapsids from being transported into the nucleus, the site of viral transcription and replication. In both cases, GTP-binding and carboxy-terminal effector functions of MxA are required for target recognition. In general, Mx GTPases appear to detect viral infection by sensing nucleocapsid-like structures. As a consequence, these viral components are trapped and sorted to locations where they become unavailable for the generation of new virus particles.

The Interferon-Inducible GTPases

Mammalian cells respond to interferons (IFNs) secreted during infection by the transcriptional upregulation of as many as a thousand genes. This remarkable transition prepares cells and organisms for resistance to infection, and many IFN-regulated gene products are players in well-understood resistance programs. Oddly, however, many of the most abundantly induced proteins are GTPases whose functions are not well understood. Here we review the progress that has been made toward understanding the roles of individual GTPase families in disease resistance and the hints of common mechanisms that are now available.

Interferon-Induced, Antiviral Human MxA Protein Localizes to a Distinct Subcompartment of the Smooth Endoplasmic Reticulum

Human MxA protein belongs to the superfamily of dynamin-like large GTPases that are involved in intracellular membrane trafficking. MxA is induced by interferons-alpha/beta (IFN-alpha/beta) and is a key component of the antiviral response against RNA viruses. Here, we show that MxA localizes to membranes that are positive for specific markers of the smooth endoplasmic reticulum, such as Syntaxin17, but is excluded from other membrane compartments. Overexpression of MxA leads to a characteristic reorganization of the associated membranes. Interestingly, Hook3, mannose-6-phosphate receptor, and Lamp-1, which normally accumulate in cis- Golgi, endosomes, and lysosomes, respectively, also colocalized with MxA, indicating that these markers were redistributed to the MxA-positive compartment. Functional assays, however, did not show any effect of MxA on endocytosis or the secretory pathway. The present results demonstrate that MxA is an IFN-induced antiviral effector protein that resembles the constitutively expressed large GTPase family members in its capacity to localize to and reorganize intracellular membranes.

Investigations into the electrooxidation of guanosine-5'-triphosphate at the pyrolytic graphite electrode

The electrochemical oxidation of guanosine-5'-triphosphate has been investigated in phosphate-containing electrolytes in the pH range 1.5-10.9 at a pyrolytic graphite electrode by cyclic sweep voltammetry, spectral studies, bulk electrolysis and related techniques. In this pH range, the oxidation occurred in a single well-defined peak (Ia). The peak potential of oxidation peaks (Ep) was found to be dependent on pH, concentration and sweep rate. The kinetics of the UV-absorbing intermediates was followed spectrophotometrically and the decay of the intermediate occurred in a pseudo-first-order reaction. The first-order rate constants for the disappearance of the UV-absorbing intermediate have also been calculated. The products of the electrode reaction were characterized by HPLC and GC/MS. A tentative mechanism for the formation of the products has also been suggested.

Intracellular localization and antiviral property of canine Mx proteins

Mx is an interferon (IFN)-induced protein that shows antiviral activities against RNA viruses. We examined an expression of mRNA, an intracellular localization of protein, and an antiviral property of canine Mx1 and Mx2. Both Mx1 and Mx2 mRNAs were induced in a canine kidney cell line Madin-Darby canine kidney (MDCK), stimulated with an IFN-inducer, poly(I) x poly(C) for 12 h, suggesting the presence of regulatory mechanisms consistent with Mx genes in other species. By immunostaining BALB/3T3 fibroblasts transiently transfected FLAG epitope-tagged canine Mx1 and Mx2 cDNAs with an anti-FLAG tag, it was revealed that both Mx1 and Mx2 proteins are localized in cytoplasm. BALB/3T3 fibroblasts expressing stably Mx2 but not Mx1 had an antiviral activity against recombinant vesicular stomatitis virus (VSV). This is the first report demonstrating the functional analysis of canine Mx proteins.

MxA, a member of the dynamin superfamily, interacts with the ankyrin-like repeat domain of TRPC

Mammalian transient receptor potential canonical channels have been proposed as the molecular entities associated with calcium entry activity in nonexcitable cells. Amino acid sequence analyses of TRPCs revealed the presence of ankyrin-like repeat domains, one of the most common protein-protein interaction motifs. Using a yeast two-hybrid interaction assay, we found that the second ankyrin-like repeat domain of TRPC6 interacted with MxA, a member of the dynamin superfamily. Using a GST pull-down and co-immunoprecipitation assay, we showed that MxA interacted with TRPC1, -3, -4, -5, -6, and -7. Overexpression of MxA in HEK293T cells slightly increased endogenous calcium entry subsequent to stimulation of G(q) protein-coupled receptors or store depletion by thapsigargin. Co-expression of MxA with TRPC6 enhanced agonist-induced or OAG-induced calcium entry activity. GTP binding-defective MxA mutants had only a minor potentiating effect on OAG-induced TRPC6 activity. However, a MxA mutant that could bind GTP but that lacked GTPase activity produced the same effect as MxA on OAG-induced TRPC6 activity. These results indicated that MxA interacted specifically with the second ankyrin-like repeat domain of TRPCs and suggested that monomeric MxA regulated the activity of TRPC6 by a mechanism requiring GTP binding. Additional results showed that an increase in the endogenous expression of MxA, induced by a treatment with interferon alpha, regulated the activity of TRPC6. The study clearly identified MxA as a new regulatory protein involved in Ca2+ signaling.

Assay and Functional Analysis of Dynamin‐Like Mx Proteins

Mx proteins are interferon-induced large guanosine triphosphatases (GTPases) that share structural and functional properties with dynamin and dynamin-like proteins, such as self-assembly and association with intracellular membranes. A unique property of some Mx proteins is their antiviral activity against a range of RNA viruses, including influenza viruses and members of the bunyavirus family. These viruses are inhibited at an early stage in their life cycle, soon after host cell entry and before genome amplification. The association of the human MxA GTPase with membranes of the endoplasmic reticulum seems to support its antiviral function by providing an interaction platform that facilitates viral target recognition, MxA oligomerization, and missorting of the resulting multiprotein complex into large intracellular aggregates.

Intra- and intermolecular domain interactions of the C-terminal GTPase effector domain of the multimeric dynamin-like GTPase Drp1

Mammalian Drp1 is a dynamin-like GTPase required for mitochondrial fission. Although it exists primarily as a cytosolic homo-tetramer in vivo, it can also self-assemble into higher order structures on the mitochondrial outer membrane, where it is required for proper mitochondrial division. Functional studies and sequence comparisons have revealed four different structural domains in Drp1, comprising N-terminal GTP-binding, middle, insert B, and C-terminal GTPase effector (GED) domains. Here we describe an intramolecular interaction within Drp1 between the GED and the N-terminal GTP-binding and middle domains. A point mutation (K679A) within the C-terminal GED domain inhibits this intramolecular association, without affecting the formation of Drp1 tetramers or the intermolecular associations among isolated C-terminal domains. Mutant Drp1 K679A exhibits impaired GTPase activity, and when overexpressed in mammalian cells it decreases mitochondrial division. Sedimentation experiments indicate that the K679A mutation either increases Drp1 complex formation or, more likely, decreases complex disassembly as compared with wild-type Drp1. Taken together, these data suggest that the C-terminal GED domain is important for stimulation of GTPase activity, formation and stability of higher order complexes, and efficient mitochondrial division.

The dynamin superfamily: universal membrane tubulation and fission molecules?

Dynamins are large GTPases that belong to a protein superfamily that, in eukaryotic cells, includes classical dynamins, dynamin-like proteins, OPA1, Mx proteins, mitofusins and guanylate-binding proteins/atlastins. They are involved in many processes including budding of transport vesicles, division of organelles, cytokinesis and pathogen resistance. With sequenced genomes from Homo sapiens, Drosophila melanogaster, Caenorhabditis elegans, yeast species and Arabidopsis thaliana, we now have a complete picture of the members of the dynamin superfamily from different organisms. Here, we review the superfamily of dynamins and their related proteins, and propose that a common mechanism leading to membrane tubulation and/or fission could encompass their many varied functions.

Nuclear MxA proteins form a complex with influenza virus NP and inhibit the transcription of the engineered influenza virus genome

Mx proteins belong to the dynamin superfamily of high molecular weight GTPases and interfere with multiplication of a wide variety of viruses. Earlier studies show that nuclear mouse Mx1 and human MxA designed to be localized in the nucleus inhibit the transcription step of the influenza virus genome. Here we set a transient influenza virus transcription system using luciferase as a reporter gene and cells expressing the three RNA polymerase subunits, PB1, PB2 and PA, and NP. We used this reporter assay system and nuclear-localized MxA proteins to get clues for elucidating the anti-influenza virus activity of MxA. Nuclear-localized VP16-MxA and MxA-TAg NLS strongly interfered with the influenza virus transcription. Over-expression of PB2 led to a slight resumption of the transcription inhibition by nuclear MxA, whereas over-expression of PB1 and PA did not affect the MxA activity. Of interest is that the inhibitory activity of the nuclear MxA was markedly neutralized by over-expression of NP. An NP devoid of its C-terminal region, but containing the N-terminal RNA binding domain, also neutralized the VP16-MxA activity in a dose-dependent manner, whereas an NP lacking the N-terminal region did not affect the VP16-MxA activity. Further, not only VP16-MxA but also the wild-type MxA was found to interact with NP in influenza virus-infected cells. This indicates that the nuclear MxA suppresses the influenza virus transcription by interacting with not only PB2 but also NP.

Cellular localization, oligomerization, and membrane association of the hereditary spastic paraplegia 3A (SPG3A) protein atlastin

Hereditary spastic paraplegias comprise a group of clinically heterogeneous syndromes characterized by lower extremity spasticity and weakness, with distal axonal degeneration in the long ascending and descending tracts of the spinal cord. The early onset hereditary spastic paraplegia SPG3A is caused by mutations in the atlastin/human guanylate-binding protein-3 gene (renamed here atlastin-1), which codes for a 64-kDa member of the dynamin/Mx/guanylate-binding protein superfamily of large GTPases. The atlastin-1 protein is localized predominantly in brain, where it is enriched in pyramidal neurons in the cerebral cortex and hippocampus. In cultured cortical neurons, atlastin-1 co-localized most prominently with markers of the Golgi apparatus, and immunogold electron microscopy revealed a predominant localization of atlastin-1 to the cis-Golgi. Yeast two-hybrid analyses and co-immunoprecipitation studies demonstrated that atlastin-1 can self-associate, and gel-exclusion chromatography and chemical cross-linking studies indicated that atlastin-1 exists as an oligomer in vivo, most likely a tetramer. Membrane fractionation and protease protection assays revealed that atlastin-1 is an integral membrane protein with two predicted transmembrane domains; both the N-terminal GTP-binding and C-terminal domains are exposed to the cytoplasm. Together, these findings indicate that the SPG3A protein atlastin-1 is a multimeric integral membrane GTPase that may be involved in Golgi membrane dynamics or vesicle trafficking.

Polymorphisms and the antiviral property of porcine Mx1 protein

We determined the cDNA sequences of the type I interferon-inducible proteins, pig Mx1 from PK(15) and LLC-PK1 cells, and compared the antiviral activities of both Mx proteins, including Mx1 polymorphisms against vesicular stomatitis virus (VSV). Mx1 cDNA derived from PK(15) cells had an 11 bp-deletion in the 3' end of the coding region, and was estimated to encode 8 amino acid substitutions and a 23 amino acid extension compared to that from LLC-PK1 cells. VSV replication was inhibited in the 3T3 cells expressing Mx1 mRNA after the cDNA was transfected. However, the efficiency of this inhibition was not different between the cells expressing Mx1 mRNA from both PK and LLC. These results indicate that pig Mx1 protein confers resistance to VSV.

Interferon-induced mx proteins: dynamin-like GTPases with antiviral activity

Mx proteins are interferon-induced GTPases that belong to the dynamin superfamily of large GTPases. Similarities include a high molecular weight, a propensity to self-assemble, a relatively low affinity for GTP, and a high intrinsic rate of GTP hydrolysis. A unique property of Mx GTPases is their antiviral activity against a wide range of RNA viruses, including bunya- and orthomyxoviruses. The human MxA GTPase accumulates in the cytoplasm of interferon-treated cells, partly associating with the endoplasmic reticulum. In the case of bunyaviruses, MxA interferes with transport of the viral nucleocapsid protein (N) to the Golgi compartment, the site of virus assembly. In the case of Thogoto virus (an orthomyxovirus), MxA prevents the incoming viral nucleocapsids from being transported into the nucleus, the site of viral transcription and replication. In both cases, the GTP-binding and carboxy-terminal effector functions of MxA are required for target recognition. In general, Mx GTPases appear to detect viral infection by sensing nucleocapsid-like structures. As a consequence, these viral components are trapped and sorted to locations where they become unavailable for the generation of new virus particles.

The antiviral dynamin family member, MxA, tubulates lipids and localizes to the smooth endoplasmic reticulum

Mx proteins are induced by type I interferon and inhibit a broad range of viruses by undefined mechanisms. They are included within the dynamin family of large GTPases, which are involved in vesicle trafficking and share common biophysical features. These properties include the propensity to self-assemble, an affinity for lipids, and the ability to tubulate membranes. In this report we establish that human MxA, despite sharing only 30% homology with conventional dynamin, possesses many of these properties. We demonstrate for the first time that MxA self-assembles into rings that tubulate lipids in vitro, and associates with a specific membrane compartment in cells, the smooth endoplasmic reticulum.

Self-assembly of human MxA GTPase into highly ordered dynamin-like oligomers

Human MxA protein is a member of the interferon-induced Mx protein family and an important component of the innate host defense against RNA viruses. The Mx family belongs to a superfamily of large GTPases that also includes the dynamins and the interferon-regulated guanylate-binding proteins. A common feature of these large GTPases is their ability to form high molecular weight oligomers. Here we determined the capacity of MxA to self-assemble into homo-oligomers in vitro. We show that recombinant MxA protein assembles into long filamentous structures with a diameter of about 20 nm at physiological salt concentration as demonstrated by sedimentation assays and electron microscopy. In the presence of guanosine nucleotides the filaments rearranged into rings and more compact helical arrays. Our data indicate that binding and hydrolysis of GTP induce conformational changes in MxA that may be essential for viral target recognition and antiviral activity.

Antivirally active MxA protein sequesters La Crosse virus nucleocapsid protein into perinuclear complexes

Bunyaviruses replicate in the cytoplasm of infected cells. New viral particles are formed by budding of nucleocapsids into the Golgi apparatus. We have previously shown that the IFN-induced human MxA protein inhibits bunyavirus replication by an unknown mechanism. Here we demonstrate that MxA binds to the nucleocapsid protein of La Crosse virus (LACV) and colocalizes with the viral protein in cytoplasmic complexes. Electron microscopy revealed that these complexes accumulated in the perinuclear area and consisted of highly ordered fibrillary structures. A similar MxA-mediated redistribution of viral nucleocapsid proteins was detected with other bunyaviruses, such as Bunyamwera virus and Rift Valley fever virus. MxA(E645R), a carboxy-terminal mutant of MxA without antiviral activity against LACV, did not lead to complex formation. Wild-type MxA, but not MxA(E645R), was able to bind to LACV nucleocapsid protein in coimmunoprecipitation assays, demonstrating the importance of the carboxy-terminal effector domain of MxA. These results illustrate an efficient mechanism of IFN action whereby an essential virus component is trapped in cytoplasmic inclusions and becomes unavailable for the generation of new virus particles.

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.

Oligomerization of Rac1 gtpase mediated by the carboxyl-terminal polybasic domain

The Rho family GTPase Rac1 mediates a variety of signal transduction processes leading to activation of NADPH oxidase, actin cytoskeleton reorganization, transcription activation, and stimulation of DNA synthesis. In this study, Rac1 was found to form a reversible monomer and oligomer in both the GDP- and GTP-bound states in vitro and in cells. Mutational analysis and peptide competition experiments showed that the unique C-terminal domain of Rac1 consisting of six consecutive basic residues (amino acids 183-188) is required for the homophilic interaction. Oligomerization of Rac1-GTP led to a self-stimulatory GTPase-activating protein (GAP) activity, resulting in a significantly enhanced intrinsic GTP hydrolysis rate of Rac1-GTP. Deletion or mutation of the polybasic residues drastically decreased its intrinsic GTPase activity and resulted in a loss of the self-stimulatory GAP activity. In the oligomeric state, Rac1 became insensitive to the RhoGAP stimulation, albeit maintaining the responsiveness to the guanine nucleotide exchange factor. The ability of the Rac1 C-terminal mutants to activate the effector p21(cdc42/rac)-activated kinase-1 correlated with their oligomerization states, suggesting that oligomer formation potentiates effector activation. Furthermore, the oligomer-to-monomer transition of Rac1-GDP could be driven effectively by interaction with the Rho guanine nucleotide dissociation inhibitor. Building on previous characterizations of Rac1 interaction with regulatory proteins and effectors, these results suggest that Rac1 may employ yet another means of regulation by cycling between the monomeric and oligomeric states to effectively generate a transient and augmented signal.

A monomeric GTPase-negative MxA mutant with antiviral activity

MxA is a large, interferon-induced GTPase with antiviral activity against RNA viruses. It forms large oligomers, but whether oligomerization and GTPase activity are important for antiviral function is not known. The mutant protein MxA(L612K) carries a lysine-for-leucine substitution at position 612 and fails to form oligomers. Here we show that monomeric MxA(L612K) lacks detectable GTPase activity but is capable of inhibiting Thogoto virus in transiently transfected Vero cells or in a Thogoto virus minireplicon system. Likewise, MxA(L612K) inhibited vesicular stomatitis virus multiplication. These findings indicate that MxA monomers are antivirally active and suggest that GTP hydrolysis may not be required for antiviral activity. MxA(L612K) is rapidly degraded in cells, whereas wild-type MxA is stable. We propose that high-molecular-weight MxA oligomers represent a stable intracellular pool from which active MxA monomers are recruited.

Inhibition of influenza C viruses by human MxA protein

Human MxA protein was analyzed for its ability to inhibit the replication of different influenza C viruses. Three laboratory derivatives of viral strain C/Ann Arbor/1/50 were investigated, namely the parental wild-type virus C/AA-wt, the persistent variant C/AA-pi and the highly cytopathogenic variant C/AA-cyt. In addition, strain C/Paris/214/91 isolated from an influenza patient was used. Multiplication of all four viruses was suppressed in MxA-expressing Vero cells, as indicated by a decrease in viral RNA synthesis, viral protein synthesis, virion production and induction of a cytopathic effect. Inhibition correlated with the level of MxA expression. Furthermore, inhibition was independent of cell clone-specific differences in expression of virus receptors, as demonstrated by receptor reconstitution experiments. Thus, human MxA protein has antiviral activity against influenza C viruses.

Phragmoplastin polymerizes into spiral coiled structures via intermolecular interaction of two self-assembly domains

Phragmoplastin, a high molecular weight GTPase belonging to the dynamin superfamily of proteins, becomes associated with the cell plate during cytokinesis in plants. Growth of the cell plate requires continuous fusion of vesicles, and phragmoplastin appears to play a role in the formation of vesicle-tubule-vesicle structures at the cell plate. In this study, we have demonstrated that two self-assembly domains (SA1 and SA2) are involved in polymerization of phragmoplastin. SA1 is about 42 amino acids long and is located near the N terminus overlapping with the GTP-binding region. SA2, containing at least 24 amino acids, is located in the middle of the molecule outside the GTP-binding domain. Peptides containing either SA1 or SA2 interact efficiently with the full-length phragmoplastin. The SA1 domain of one phragmoplastin molecule also binds to SA2 of another as confirmed in vitro by using radiolabeled peptides. This interaction leads to the formation of polymers with a staggered contoured spiral structure. Electron microscopy studies revealed that helical arrays of phragmoplastin can be induced by reducing salt concentration. Our results suggest that phragmoplastin may assemble into helical arrays that wrap around and squeeze vesicles into vesicle-tubule-vesicle structures observed on the forming cell plate.

Structure of human guanylate-binding protein 1 representing a unique class of GTP-binding proteins

Interferon-gamma is an immunomodulatory substance that induces the expression of many genes to orchestrate a cellular response and establish the antiviral state of the cell. Among the most abundant antiviral proteins induced by interferon-gamma are guanylate-binding proteins such as GBP1 and GBP2. These are large GTP-binding proteins of relative molecular mass 67,000 with a high-turnover GTPase activity and an antiviral effect. Here we have determined the crystal structure of full-length human GBP1 to 1.8 A resolution. The amino-terminal 278 residues constitute a modified G domain with a number of insertions compared to the canonical Ras structure, and the carboxy-terminal part is an extended helical domain with unique features. From the structure and biochemical experiments reported here, GBP1 appears to belong to the group of large GTP-binding proteins that includes Mx and dynamin, the common property of which is the ability to undergo oligomerization with a high concentration-dependent GTPase activity.

The central interactive region of human MxA GTPase is involved in GTPase activation and interaction with viral target structures

To define domains of the human MxA GTPase involved in GTP hydrolysis and antiviral activity, we used two monoclonal antibodies (mAb) directed against different regions of the molecule. mAb 2C12 recognizes an epitope in the central interactive region of MxA, whereas mAb M143 is directed against the N-terminal G domain. mAb 2C12 greatly stimulated MxA GTPase activity, suggesting that antibody-mediated crosslinking enhances GTP hydrolysis. In contrast, monovalent Fab fragments of 2C12 abolished GTPase activity, most likely by blocking intramolecular interactions required for GTPase activation. Interestingly, intact IgG molecules and Fab fragments of 2C12 both prevented association of MxA with viral nucleocapsids and neutralized MxA antiviral activity in vivo. mAb M143 had no effect on MxA function, indicating that this antibody binds outside functional regions. These data demonstrate that the central region recognized by 2C12 is critical for regulation of GTPase activity and viral target recognition.

Intramolecular backfolding of the carboxyl-terminal end of MxA protein is a prerequisite for its oligomerization

Mx proteins are large GTPases, which play a pivotal role in the interferon type I-mediated response against viral infections. The human MxA inhibits the replication of several RNA viruses and is organized in oligomeric structures. Using two different experimental approaches, the mammalian two-hybrid system and an interaction dependent nuclear translocation approach, three domains in the carboxyl-terminal moiety were identified that are involved in the oligomerization of MxA. The first consists of a carboxyl-terminal amphipathic helix (LZ1), which binds to a more proximal part of the same molecule. This intramolecular backfolding is a prerequisite for the formation of an intermolecular complex. This intermolecular interaction is mediated by two domains, a poorly defined region generated by the intramolecular interaction and a domain located between amino acids 363 and 415. Co-expression of wild-type MxA with various mutant fragments thereof revealed that the presence of the carboxyl-terminal region comprising the amphipathic helices LZ1 and LZ2 is necessary and sufficient to exert a dominant negative effect. This finding suggests that the functional interference of the carboxyl-terminal region is due to competition for binding of an as yet unidentified cellular or viral target molecules.

A model for dynamin self-assembly based on binding between three different protein domains

Dynamin is a 100-kDa GTPase that assembles into multimeric spirals at the necks of budding clathrin-coated vesicles. We describe three different intramolecular binding interactions that may account for the process of dynamin self-assembly. The first binding interaction is the dimerization of a 100-amino acid segment in the C-terminal half of dynamin. We call this segment the assembly domain, because it appears to be critical for multimerization. The second binding interaction occurs between the assembly domain and the N-terminal GTPase domain. The strength of this interaction is controlled by the nucleotide-bound state of the GTPase domain, as shown with mutations in GTP binding motifs and in vitro binding experiments. The third binding interaction occurs between the assembly domain and a segment that we call the middle domain. This is the segment between the N-terminal GTPase domain and the pleckstrin homology domain. The three different binding interactions suggest a model in which dynamin molecules first dimerize. The dimers are then linked into a chain by a second binding reaction. The third binding interaction might connect adjacent rungs of the spiral.

Functional diversity in the dynamin family

The function of the GTPase dynamin has been discussed for several years. It clearly plays a role in vesicle budding, but, despite recent insights, precisely how it functions in this process is still a matter of debate. In addition, it is now clear that dynamin is a member of a large protein family, present in a variety of cellular locations where members apparently perform a range of functions. This article describes current understanding of the structure and function of the various dynamin family members.

GTP-bound human MxA protein interacts with the nucleocapsids of Thogoto virus (Orthomyxoviridae)

Human MxA protein is an interferon-induced member of the dynamin superfamily of large GTPases. MxA inhibits the multiplication of several RNA viruses, including Thogoto virus, an influenza virus-like orthomyxovirus transmitted by ticks. Previous studies have indicated that GTP binding is required for antiviral activity, but the mechanism of action is still unknown. Here, we have used an in vitro cosedimentation assay to demonstrate, for the first time, a GTP-dependent interaction between MxA GTPase and a viral target structure. The assay is based on highly active MxA GTPase as effector molecules, Thogoto virus nucleocapsids as viral targets, and guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) as a stabilizing factor. We show that MxA tightly interacts with viral nucleocapsids by binding to the nucleoprotein component. This interaction requires the presence of GTPgammaS and is mediated by domains in the carboxyl-terminal moiety of MxA. We propose that GTP-bound MxA adopts an antivirally active conformation that allows interaction with viral nucleocapsids, thereby impairing their normal function.