RESUMO
Foamy viruses (FVs) are an ancient lineage of retroviruses, with an evolutionary history spanning over 450 million years. Vector systems based on Prototype Foamy Virus (PFV) are promising candidates for gene and oncolytic therapies. Structural studies of PFV contribute to the understanding of the mechanisms of FV replication, cell entry and infection, and retroviral evolution. Here we combine cryoEM and cryoET to determine high-resolution in situ structures of the PFV icosahedral capsid (CA) and envelope glycoprotein (Env), including its type III transmembrane anchor and membrane-proximal external region (MPER), and show how they are organized in an integrated structure of assembled PFV particles. The atomic models reveal an ancient retroviral capsid architecture and an unexpected relationship between Env and other class 1 fusion proteins of the Mononegavirales. Our results represent the de novo structure determination of an assembled retrovirus particle.
Assuntos
Microscopia Crioeletrônica , Spumavirus , Montagem de Vírus , Internalização do Vírus , Spumavirus/genética , Capsídeo/metabolismo , Capsídeo/química , Capsídeo/ultraestrutura , Proteínas do Capsídeo/química , Proteínas do Capsídeo/metabolismo , Proteínas do Capsídeo/genética , Humanos , Evolução Molecular , Proteínas do Envelope Viral/química , Proteínas do Envelope Viral/metabolismo , Proteínas do Envelope Viral/genética , Modelos MolecularesRESUMO
SAMHD1 regulates cellular nucleotide homeostasis, controlling dNTP levels by catalysing their hydrolysis into 2'-deoxynucleosides and triphosphate. In differentiated CD4+ macrophage and resting T-cells SAMHD1 activity results in the inhibition of HIV-1 infection through a dNTP blockade. In cancer, SAMHD1 desensitizes cells to nucleoside-analogue chemotherapies. Here we employ time-resolved cryogenic-EM imaging and single-particle analysis to visualise assembly, allostery and catalysis by this multi-subunit enzyme. Our observations reveal how dynamic conformational changes in the SAMHD1 quaternary structure drive the catalytic cycle. We capture five states at high-resolution in a live catalytic reaction, revealing how allosteric activators support assembly of a stable SAMHD1 tetrameric core and how catalysis is driven by the opening and closing of active sites through pairwise coupling of active sites and order-disorder transitions in regulatory domains. This direct visualisation of enzyme catalysis dynamics within an allostery-stabilised platform sets a precedent for mechanistic studies into the regulation of multi-subunit enzymes.
Assuntos
Domínio Catalítico , Microscopia Crioeletrônica , Proteína 1 com Domínio SAM e Domínio HD , Proteína 1 com Domínio SAM e Domínio HD/metabolismo , Proteína 1 com Domínio SAM e Domínio HD/química , Proteína 1 com Domínio SAM e Domínio HD/genética , Regulação Alostérica , Humanos , Estrutura Quaternária de Proteína , Catálise , Biocatálise , HIV-1/metabolismo , Modelos MolecularesRESUMO
Vaccinia virus assembly in the cytoplasm of infected cells involves the formation of a biconcave viral core inside the maturing viral particle. The boundary of the core is defined by a pseudohexagonal palisade layer, composed of trimers projecting from an inner wall. To understand the assembly of this complex core architecture, we obtained a subnanometer structure of the palisade trimer by cryo-electron tomography and subtomogram averaging of purified intact virions. Using AlphaFold2 structure predictions, we determined that the palisade is formed from trimers of the proteolytically processed form of the viral protein A10. In addition, we found that each A10 protomer associates with an α-helix (residues 24-66) of A4. Cellular localization assays outside the context of infection demonstrate that the A4 N-terminus is necessary and sufficient to interact with A10. The interaction between A4 and A10 provides insights into how the palisade layer might become tightly associated with the viral membrane during virion maturation. Reconstruction of the palisade layer reveals that, despite local hexagonal ordering, the A10/A4 trimers are widely spaced, suggesting that additional components organize the lattice. This spacing would, however, allow the adoption of the characteristic biconcave shape of the viral core. Finally, we also found that the palisade incorporates multiple copies of a hexameric portal structure. We suggest that these portals are formed by E6, a viral protein that is essential for virion assembly and required to release viral mRNA from the core early in infection.IMPORTANCEPoxviruses such as variola virus (smallpox) and monkeypox cause diseases in humans. Other poxviruses, including vaccinia and modified vaccinia Ankara, are used as vaccine vectors. Given their importance, a greater structural understanding of poxvirus virions is needed. We now performed cryo-electron tomography of purified intact vaccinia virions to study the structure of the palisade, a protein lattice that defines the viral core boundary. We identified the main viral proteins that form the palisade and their interaction surfaces and provided new insights into the organization of the viral core.
Assuntos
Benzenoacetamidas , Piperidonas , Vaccinia virus , Vacínia , Humanos , Vaccinia virus/química , Montagem de Vírus , Vírion/genética , Proteínas Virais/metabolismoRESUMO
The subunits of the influenza hemagglutinin (HA) trimer are synthesized as single-chain precursors (HA0s) that are proteolytically cleaved into the disulfide-linked polypeptides HA1 and HA2. Cleavage is required for activation of membrane fusion at low pH, which occurs at the beginning of infection following transfer of cell-surface-bound viruses into endosomes. Activation results in extensive changes in the conformation of cleaved HA. To establish the overall contribution of cleavage to the mechanism of HA-mediated membrane fusion, we used cryogenic electron microscopy (cryo-EM) to directly image HA0 at neutral and low pH. We found extensive pH-induced structural changes, some of which were similar to those described for intermediates in the refolding of cleaved HA at low pH. They involve a partial extension of the long central coiled coil formed by melting of the preexisting secondary structure, threading it between the membrane-distal domains, and subsequent refolding as extended helices. The fusion peptide, covalently linked at its N terminus, adopts an amphipathic helical conformation over part of its length and is repositioned and packed against a complementary surface groove of conserved residues. Furthermore, and in contrast to cleaved HA, the changes in HA0 structure at low pH are reversible on reincubation at neutral pH. We discuss the implications of covalently restricted HA0 refolding for the cleaved HA conformational changes that mediate membrane fusion and for the action of antiviral drug candidates and cross-reactive anti-HA antibodies that can block influenza infectivity.
Assuntos
Glicoproteínas de Hemaglutininação de Vírus da Influenza , Fusão de Membrana , Orthomyxoviridae , Internalização do Vírus , Glicoproteínas de Hemaglutininação de Vírus da Influenza/química , Humanos , Concentração de Íons de Hidrogênio , Orthomyxoviridae/fisiologia , Conformação ProteicaRESUMO
Malaria is responsible for half a million deaths annually and poses a huge economic burden on the developing world. The mosquito-borne parasites (Plasmodium spp.) that cause the disease depend upon an unconventional actomyosin motor for both gliding motility and host cell invasion. The motor system, often referred to as the glideosome complex, remains to be understood in molecular terms and is an attractive target for new drugs that might block the infection pathway. Here, we present the high-resolution structure of the actomyosin motor complex from Plasmodium falciparum. The complex includes the malaria parasite actin filament (PfAct1) complexed with the class XIV myosin motor (PfMyoA) and its two associated light-chains. The high-resolution core structure reveals the PfAct1:PfMyoA interface in atomic detail, while at lower-resolution, we visualize the PfMyoA light-chain binding region, including the essential light chain (PfELC) and the myosin tail interacting protein (PfMTIP). Finally, we report a bare PfAct1 filament structure at improved resolution.
Assuntos
Malária , Parasitos , Citoesqueleto de Actina/metabolismo , Actomiosina/metabolismo , Animais , Malária/metabolismo , Miosinas/metabolismo , Parasitos/metabolismo , Proteínas de Protozoários/metabolismoRESUMO
The lipid-enveloped influenza C virus contains a single surface glycoprotein, the haemagglutinin-esterase-fusion (HEF) protein, that mediates receptor binding, receptor destruction, and membrane fusion at the low pH of the endosome. Here we apply electron cryotomography and subtomogram averaging to describe the structural basis for hexagonal lattice formation by HEF on the viral surface. The conformation of the glycoprotein in situ is distinct from the structure of the isolated trimeric ectodomain, showing that a splaying of the membrane distal domains is required to mediate contacts that form the lattice. The splaying of these domains is also coupled to changes in the structure of the stem region which is involved in membrane fusion, thereby linking HEF's membrane fusion conformation with its assembly on the virus surface. The glycoprotein lattice can form independent of other virion components but we show a major role for the matrix layer in particle formation.
Assuntos
Gammainfluenzavirus/metabolismo , Glicoproteínas de Membrana/química , Glicoproteínas de Membrana/metabolismo , Animais , Cães , Hemaglutininas Virais/química , Hemaglutininas Virais/metabolismo , Gammainfluenzavirus/ultraestrutura , Células Madin Darby de Rim Canino , Fusão de Membrana , Modelos Moleculares , Multimerização Proteica , Proteínas Virais de Fusão/química , Proteínas Virais de Fusão/metabolismo , Vírion/ultraestruturaRESUMO
Infection by enveloped viruses involves fusion of their lipid envelopes with cellular membranes to release the viral genome into cells. For HIV, Ebola, influenza and numerous other viruses, envelope glycoproteins bind the infecting virion to cell-surface receptors and mediate membrane fusion. In the case of influenza, the receptor-binding glycoprotein is the haemagglutinin (HA), and following receptor-mediated uptake of the bound virus by endocytosis1, it is the HA that mediates fusion of the virus envelope with the membrane of the endosome2. Each subunit of the trimeric HA consists of two disulfide-linked polypeptides, HA1 and HA2. The larger, virus-membrane-distal, HA1 mediates receptor binding; the smaller, membrane-proximal, HA2 anchors HA in the envelope and contains the fusion peptide, a region that is directly involved in membrane interaction3. The low pH of endosomes activates fusion by facilitating irreversible conformational changes in the glycoprotein. The structures of the initial HA at neutral pH and the final HA at fusion pH have been investigated by electron microscopy4,5 and X-ray crystallography6-8. Here, to further study the process of fusion, we incubate HA for different times at pH 5.0 and directly image structural changes using single-particle cryo-electron microscopy. We describe three distinct, previously undescribed forms of HA, most notably a 150 Å-long triple-helical coil of HA2, which may bridge between the viral and endosomal membranes. Comparison of these structures reveals concerted conformational rearrangements through which the HA mediates membrane fusion.
Assuntos
Microscopia Crioeletrônica , Glicoproteínas de Hemaglutininação de Vírus da Influenza/química , Glicoproteínas de Hemaglutininação de Vírus da Influenza/metabolismo , Vírus da Influenza A Subtipo H3N2 , Fusão de Membrana , Endossomos/metabolismo , Glicoproteínas de Hemaglutininação de Vírus da Influenza/ultraestrutura , Concentração de Íons de Hidrogênio , Vírus da Influenza A Subtipo H3N2/química , Vírus da Influenza A Subtipo H3N2/metabolismo , Vírus da Influenza A Subtipo H3N2/ultraestrutura , Modelos Moleculares , Conformação Proteica , Fatores de TempoRESUMO
The HML2 (HERV-K) group constitutes the most recently acquired family of human endogenous retroviruses, with many proviruses less than one million years old. Many maintain intact open reading frames and provirus expression together with HML2 particle formation are observed in early stage human embryo development and are associated with pluripotency as well as inflammatory disease, cancers and HIV-1 infection. Here, we reconstruct the core structural protein (CA) of an HML2 retrovirus, assemble particles in vitro and employ single particle cryogenic electron microscopy (cryo-EM) to determine structures of four classes of CA Fullerene shell assemblies. These icosahedral and capsular assemblies reveal at high-resolution the molecular interactions that allow CA to form both pentamers and hexamers and show how invariant pentamers and structurally plastic hexamers associate to form the unique polyhedral structures found in retroviral cores.
Assuntos
Proteínas do Capsídeo/ultraestrutura , Capsídeo/ultraestrutura , Retrovirus Endógenos/ultraestrutura , Fulerenos/química , Estrutura Quaternária de Proteína , Proteínas do Capsídeo/genética , Proteínas do Capsídeo/isolamento & purificação , Microscopia Crioeletrônica/métodos , Cristalografia por Raios X , Ressonância Magnética Nuclear Biomolecular , Proteínas Recombinantes/genética , Proteínas Recombinantes/isolamento & purificação , Proteínas Recombinantes/ultraestrutura , Imagem Individual de Molécula/métodosRESUMO
Weibel-Palade bodies (WPBs) are secretory granules that contain von Willebrand factor and P-selectin, molecules that regulate hemostasis and inflammation, respectively. The presence of CD63/LAMP3 in the limiting membrane of WPBs has led to their classification as lysosome-related organelles. Many lysosome-related organelles contain intraluminal vesicles (ILVs) enriched in CD63 that are secreted into the extracellular environment during cell activation to mediate intercellular communication. To date, there are no reports that WPBs contain or release ILVs. By light microscopy and live-cell imaging, we show that CD63 is enriched in microdomains within WPBs. Extracellular antibody recycling studies showed that CD63 in WPB microdomains can originate from the plasma membrane. By cryo-electron tomography of frozen-hydrated endothelial cells, we identify internal vesicles as novel structural features of the WPB lumen. By live-cell fluorescence microscopy, we directly observe the exocytotic release of EGFP-CD63 ILVs as discrete particles from individual WPBs. WPB exocytosis provides a novel route for release of ILVs during endothelial cell stimulation.
Assuntos
Micropartículas Derivadas de Células/metabolismo , Exocitose/fisiologia , Corpos de Weibel-Palade/metabolismo , Micropartículas Derivadas de Células/ultraestrutura , Células Cultivadas , Microscopia Crioeletrônica , Células Endoteliais/metabolismo , Células Endoteliais/ultraestrutura , Humanos , Tetraspanina 30/metabolismo , Corpos de Weibel-Palade/ultraestruturaRESUMO
Gyrase is a unique type IIA topoisomerase that uses ATP hydrolysis to maintain the negatively supercoiled state of bacterial DNA. In order to perform its function, gyrase undergoes a sequence of conformational changes that consist of concerted gate openings, DNA cleavage, and DNA strand passage events. Structures where the transported DNA molecule (T-segment) is trapped by the A subunit have not been observed. Here we present the cryoEM structures of two oligomeric complexes of open gyrase A dimers and DNA. The protein subunits in these complexes were solved to 4 Å and 5.2 Å resolution. One of the complexes traps a linear DNA molecule, a putative T-segment, which interacts with the open gyrase A dimers in two states, representing steps either prior to or after passage through the DNA-gate. The structures locate the T-segment in important intermediate conformations of the catalytic cycle and provide insights into gyrase-DNA interactions and mechanism.
Assuntos
Trifosfato de Adenosina/química , DNA Girase/química , DNA Bacteriano/química , Escherichia coli/genética , Subunidades Proteicas/química , Streptococcus pneumoniae/genética , Trifosfato de Adenosina/metabolismo , Sítios de Ligação , Clonagem Molecular , Microscopia Crioeletrônica , Clivagem do DNA , DNA Girase/metabolismo , DNA Bacteriano/metabolismo , Escherichia coli/metabolismo , Expressão Gênica , Vetores Genéticos/química , Vetores Genéticos/metabolismo , Isoenzimas/química , Isoenzimas/metabolismo , Modelos Moleculares , Oligonucleotídeos/química , Oligonucleotídeos/metabolismo , Ligação Proteica , Conformação Proteica em alfa-Hélice , Conformação Proteica em Folha beta , Domínios e Motivos de Interação entre Proteínas , Multimerização Proteica , Subunidades Proteicas/metabolismo , Proteínas Recombinantes/química , Proteínas Recombinantes/genética , Proteínas Recombinantes/metabolismo , Streptococcus pneumoniae/metabolismoRESUMO
BRCA1 is a tumor suppressor found to be mutated in hereditary breast and ovarian cancer and plays key roles in the maintenance of genomic stability by homologous recombination repair. It is recruited to damaged chromatin as a component of the BRCA1-A deubiquitinase, which cleaves K63-linked ubiquitin chains attached to histone H2A and H2AX. BRCA1-A contributes to checkpoint regulation, repair pathway choice, and HR repair efficiency through molecular mechanisms that remain largely obscure. The structure of an active core complex comprising two Abraxas/BRCC36/BRCC45/MERIT40 tetramers determined by negative-stain electron microscopy (EM) reveals a distorted V-shape architecture in which a dimer of Abraxas/BRCC36 heterodimers sits at the base, with BRCC45/Merit40 pairs occupying each arm. The location and ubiquitin-binding activity of BRCC45 suggest that it may provide accessory interactions with nucleosome-linked ubiquitin chains that contribute to their efficient processing. Our data also suggest how ataxia telangiectasia mutated (ATM)-dependent BRCA1 dimerization may stabilize self-association of the entire BRCA1-A complex.
Assuntos
Proteína BRCA1/química , Proteínas de Transporte/química , Enzimas Desubiquitinantes/química , Histonas/química , Complexos Multiproteicos/química , Proteínas Mutadas de Ataxia Telangiectasia/química , Proteínas Mutadas de Ataxia Telangiectasia/genética , Proteína BRCA1/genética , Proteína BRCA1/ultraestrutura , Neoplasias da Mama/genética , Neoplasias da Mama/patologia , Proteínas de Transporte/genética , Proteínas de Transporte/ultraestrutura , Cromatina/química , Cromatina/genética , Dano ao DNA/genética , Reparo do DNA/genética , Enzimas Desubiquitinantes/genética , Enzimas Desubiquitinantes/ultraestrutura , Instabilidade Genômica , Histonas/genética , Humanos , Complexos Multiproteicos/genética , Complexos Multiproteicos/ultraestrutura , Ligação Proteica , Conformação Proteica , Multimerização Proteica , Reparo de DNA por Recombinação/genética , Ubiquitina/genéticaRESUMO
The Spumaretrovirinae, or foamy viruses (FVs) are complex retroviruses that infect many species of monkey and ape. Despite little sequence homology, FV and orthoretroviral Gag proteins perform equivalent functions, including genome packaging, virion assembly, trafficking and membrane targeting. However, there is a paucity of structural information for FVs and it is unclear how disparate FV and orthoretroviral Gag molecules share the same function. To probe the functional overlap of FV and orthoretroviral Gag we have determined the structure of a central region of Gag from the Prototype FV (PFV). The structure comprises two all α-helical domains NtDCEN and CtDCEN that although they have no sequence similarity, we show they share the same core fold as the N- (NtDCA) and C-terminal domains (CtDCA) of archetypal orthoretroviral capsid protein (CA). Moreover, structural comparisons with orthoretroviral CA align PFV NtDCEN and CtDCEN with NtDCA and CtDCA respectively. Further in vitro and functional virological assays reveal that residues making inter-domain NtDCEN-CtDCEN interactions are required for PFV capsid assembly and that intact capsid is required for PFV reverse transcription. These data provide the first information that relates the Gag proteins of Spuma and Orthoretrovirinae and suggests a common ancestor for both lineages containing an ancient CA fold.
Assuntos
Proteínas do Capsídeo/genética , Produtos do Gene gag/química , Produtos do Gene gag/genética , Spumavirus/genética , Montagem de Vírus/fisiologia , Sequência de Aminoácidos , Animais , Western Blotting , Capsídeo , Linhagem Celular , Humanos , Ressonância Magnética Nuclear Biomolecular , Conformação Proteica , Reação em Cadeia da Polimerase em Tempo RealRESUMO
Images of radiation-sensitive specimens obtained by electron microscopy suffer a reduction in quality beyond that expected from radiation damage alone due to electron beam-induced charging or movement of the specimen. For biological specimens, charging and movement are most severe when they are suspended in an insulating layer of vitreous ice, which is otherwise optimal for preserving hydrated specimens in a near native state. We image biological specimens, including a single particle protein complex and a lipid-enveloped virus in thin, vitreous ice films over suspended sheets of unmodified graphene. We show that in such preparations, the charging of ice, as assessed by electron-optical perturbation of the imaging beam, is eliminated. We also use the same specimen supports to record high resolution images at liquid nitrogen temperature of monolayer paraffin crystals grown over graphene.
Assuntos
Microscopia Crioeletrônica/métodos , Criopreservação/métodos , Grafite/química , Apoferritinas/ultraestrutura , Cristalografia , Microscopia Eletrônica de Transmissão/métodos , Orthomyxoviridae/ultraestrutura , Parafina/química , Vírion/ultraestruturaRESUMO
Interpretation of the structural information in cryomicroscopy images recorded on film or CCD camera requires a precise knowledge of the electron microscope parameters that affect image features such as magnification and defocus. Magnification must be determined in order to combine data from different images in a three-dimensional reconstruction and to accurately scale reconstructions for fitting with atomic resolution models. A method is described for estimating the absolute magnification of an electron micrograph of a frozen-hydrated specimen using horse spleen apoferritin as a standard. Apoferritin is a widely available protein complex of known structure that may be included with the specimen of interest and imaged under conditions identical to those used for imaging other biological specimens by cryomicroscopy. The sum of the structure factor intensities of images of randomly-oriented apoferritin particles shows three low resolution peaks to 25Å that arise from the hollow ball structure of apoferritin. Comparison of peak positions of the experimental intensities with structure factor intensities of an atomic model of apoferritin determined by X-ray crystallography provides a scale factor for estimating the absolute magnification of the micrograph. We compare the magnification estimate using apoferritin to that obtained with tobacco mosaic virus, another common magnification standard for cryomicroscopy. We verify the precision of the method by acquiring images with a systematic variation of magnification.
Assuntos
Apoferritinas/ultraestrutura , Microscopia Crioeletrônica/normas , Algoritmos , Animais , Apoferritinas/química , Capsídeo/ultraestrutura , Microscopia Crioeletrônica/métodos , Cavalos , Modelos Moleculares , Estrutura Quaternária de Proteína , Padrões de Referência , Software , Vírus do Mosaico do Tabaco/ultraestruturaRESUMO
The comparison of a pair of electron microscope images recorded at different specimen tilt angles provides a powerful approach for evaluating the quality of images, image-processing procedures, or three-dimensional structures. Here, we analyze tilt-pair images recorded from a range of specimens with different symmetries and molecular masses and show how the analysis can produce valuable information not easily obtained otherwise. We show that the accuracy of orientation determination of individual single particles depends on molecular mass, as expected theoretically since the information in each particle image increases with molecular mass. The angular uncertainty is less than 1° for particles of high molecular mass (~50 MDa), several degrees for particles in the range 1-5 MDa, and tens of degrees for particles below 1 MDa. Orientational uncertainty may be the major contributor to the effective temperature factor (B-factor) describing contrast loss and therefore the maximum resolution of a structure determination. We also made two unexpected observations. Single particles that are known to be flexible showed a wider spread in orientation accuracy, and the orientations of the largest particles examined changed by several degrees during typical low-dose exposures. Smaller particles presumably also reorient during the exposure; hence, specimen movement is a second major factor that limits resolution. Tilt pairs thus enable assessment of orientation accuracy, map quality, specimen motion, and conformational heterogeneity. A convincing tilt-pair parameter plot, where 60% of the particles show a single cluster around the expected tilt axis and tilt angle, provides confidence in a structure determined using electron cryomicroscopy.
Assuntos
Adenosina Trifosfatases/ultraestrutura , Microscopia Crioeletrônica , Processamento de Imagem Assistida por Computador , Vírion/ultraestrutura , Leveduras/ultraestrutura , beta-Galactosidase/ultraestrutura , Animais , Bovinos , Rotavirus/químicaRESUMO
The restriction factor Fv1 confers resistance to murine leukemia virus (MLV), blocking progression of the viral life cycle after reverse transcription, but before integration into the host chromosome. It is known that the specificity of restriction is determined by both the restriction factor and the viral capsid (CA), but a direct interaction between Fv1 and MLV CA has not yet been demonstrated. With the development of a previously unexplored method for in vitro polymerization of MLV CA, it has now been possible to display a binding interaction between Fv1 and MLV CA. C-terminally His-tagged CA molecules were assembled on Ni-chelating lipid nanotubes, and analysis by electron microscopy revealed the formation of a regular lattice. Comparison of binding data with existing restriction data confirmed the specificity of the binding interaction, with multiple positions of both Fv1 and CA shown to influence binding specificity.
Assuntos
Proteínas do Capsídeo/metabolismo , Vírus da Leucemia Murina/metabolismo , Nanotubos/virologia , Ligação Proteica , Proteínas/metabolismo , Animais , Proteínas do Capsídeo/genética , Primers do DNA/genética , Processamento de Imagem Assistida por Computador , Metabolismo dos Lipídeos , Camundongos , Microscopia Eletrônica , Mutagênese , Nanotubos/ultraestrutura , Plasmídeos/genéticaRESUMO
The bulk alignment of actin filament sliding movement, powered by randomly oriented myosin molecules, has been observed and studied using an in vitro motility assay. The well established, actin filament gliding assay is a minimal experimental system for studying actomyosin motility. Here, we show that when the assay is performed at densities of actin filaments approaching those found in living cells, filament gliding takes up a preferred orientation. The oriented patterns of movement that we have observed extend over a length scale of 10-100 microm, similar to the size of a mammalian cell. We studied the process of filament alignment and found that it depends critically upon filament length and density. We developed a simple quantitative measure of filament sliding orientation and this enabled us to follow the time course of alignment and the formation and disappearance of oriented domains. Domains of oriented filaments formed spontaneously and were separated by distinct boundaries. The pattern of the domain structures changed on the time scale of several seconds and the collision of neighboring domains led to emergence of new patterns. Our results indicate that actin filament crowding may play an important role in structuring the leading edge of migrating cells. Filament alignment due to near-neighbor mechanical interactions can propagate over a length scale of several microns; much greater than the size of individual filaments and analogous to a log drive. Self-alignment of actin filaments may make an important contribution to cell polarity and provide a mechanism by which cell migration direction responds to chemical cues.