Strain and rupture of HIV-1 capsids during uncoating.

SignificanceThe mature capsids of HIV-1 are transiently stable complexes that self-assemble around the viral genome during maturation, and uncoat to release preintegration complexes that archive a double-stranded DNA copy of the virus in the host cell genome. However, a detailed view of how HIV cores rupture remains lacking. Here, we elucidate the physical properties involved in capsid rupture using a combination of large-scale all-atom molecular dynamics simulations and cryo-electron tomography. We find that intrinsic strain on the capsid forms highly correlated patterns along the capsid surface, along which cracks propagate. Capsid rigidity also increases with high strain. Our findings provide fundamental insight into viral capsid uncoating.

Poly(ADP-ribose) potentiates ZAP antiviral activity.

Zinc-finger antiviral protein (ZAP), also known as poly(ADP-ribose) polymerase 13 (PARP13), is an antiviral factor that selectively targets viral RNA for degradation. ZAP is active against both DNA and RNA viruses, including important human pathogens such as hepatitis B virus and type 1 human immunodeficiency virus (HIV-1). ZAP selectively binds CpG dinucleotides through its N-terminal RNA-binding domain, which consists of four zinc fingers. ZAP also contains a central region that consists of a fifth zinc finger and two WWE domains. Through structural and biochemical studies, we found that the fifth zinc finger and tandem WWEs of ZAP combine into a single integrated domain that binds to poly(ADP-ribose) (PAR), a cellular polynucleotide. PAR binding is mediated by the second WWE module of ZAP and likely involves specific recognition of an adenosine diphosphate-containing unit of PAR. Mutation of the PAR binding site in ZAP abrogates the interaction in vitro and diminishes ZAP activity against a CpG-rich HIV-1 reporter virus and murine leukemia virus. In cells, PAR facilitates formation of non-membranous sub-cellular compartments such as DNA repair foci, spindle poles and cytosolic RNA stress granules. Our results suggest that ZAP-mediated viral mRNA degradation is facilitated by PAR, and provides a biophysical rationale for the reported association of ZAP with RNA stress granules.

X-ray structures of the hexameric building block of the HIV capsid.

The mature capsids of HIV and other retroviruses organize and package the viral genome and its associated enzymes for delivery into host cells. The HIV capsid is a fullerene cone: a variably curved, closed shell composed of approximately 250 hexamers and exactly 12 pentamers of the viral CA protein. We devised methods for isolating soluble, assembly-competent CA hexamers and derived four crystallographically independent models that define the structure of this capsid assembly unit at atomic resolution. A ring of six CA N-terminal domains form an apparently rigid core, surrounded by an outer ring of C-terminal domains. Mobility of the outer ring appears to be an underlying mechanism for generating the variably curved lattice in authentic capsids. Hexamer-stabilizing interfaces are highly hydrated, and this property may be key to the formation of quasi-equivalent interactions within hexamers and pentamers. The structures also clarify the molecular basis for capsid assembly inhibition and should facilitate structure-based drug design strategies.

Author Correction: Inositol phosphates are assembly co-factors for HIV-1.

In this Letter, the Protein Data Bank (PDB) accessions were incorrectly listed as '6BH5, 6BHT and 6BHS' instead of '6BHR, 6BHT and 6BHS'; this has been corrected online.

Inositol phosphates are assembly co-factors for HIV-1.

A short, 14-amino-acid segment called SP1, located in the Gag structural protein1, has a critical role during the formation of the HIV-1 virus particle. During virus assembly, the SP1 peptide and seven preceding residues fold into a six-helix bundle, which holds together the Gag hexamer and facilitates the formation of a curved immature hexagonal lattice underneath the viral membrane2,3. Upon completion of assembly and budding, proteolytic cleavage of Gag leads to virus maturation, in which the immature lattice is broken down; the liberated CA domain of Gag then re-assembles into the mature conical capsid that encloses the viral genome and associated enzymes. Folding and proteolysis of the six-helix bundle are crucial rate-limiting steps of both Gag assembly and disassembly, and the six-helix bundle is an established target of HIV-1 inhibitors4,5. Here, using a combination of structural and functional analyses, we show that inositol hexakisphosphate (InsP6, also known as IP6) facilitates the formation of the six-helix bundle and assembly of the immature HIV-1 Gag lattice. IP6 makes ionic contacts with two rings of lysine residues at the centre of the Gag hexamer. Proteolytic cleavage then unmasks an alternative binding site, where IP6 interaction promotes the assembly of the mature capsid lattice. These studies identify IP6 as a naturally occurring small molecule that promotes both assembly and maturation of HIV-1.

Structural evidence that MOAP1 and PEG10 are derived from retrovirus/retrotransposon Gag proteins.

The Gag proteins of retroviruses play an essential role in virus particle assembly by forming a protein shell or capsid and thus generating the virion compartment. A variety of human proteins have now been identified with structural similarity to one or more of the major Gag domains. These human proteins are thought to have been evolved or "domesticated" from ancient integrations due to retroviral infections or retrotransposons. Here, we report that X-ray crystal structures of stably folded domains of MOAP1 (modulator of apoptosis 1) and PEG10 (paternally expressed gene 10) are highly similar to the C-terminal capsid (CA) domains of cognate Gag proteins. The structures confirm classification of MOAP1 and PEG10 as domesticated Gags, and suggest that these proteins may have preserved some of the key interactions that facilitated assembly of their ancestral Gags into capsids.

Structural Determinants of Virion Assembly and Release in the C Terminus of the Mason-Pfizer Monkey Virus Capsid Protein.

The transition from an immature to a fully infectious mature retrovirus particle is associated with molecular switches that trigger dramatic conformational changes in the structure of the Gag proteins. A dominant maturation switch that stabilizes the immature capsid (CA) lattice is located downstream of the CA protein in many retroviral Gags. The HIV-1 Gag protein contains a stretch of 5 amino acid residues termed the "clasp motif," important for the organization of the hexameric subunits that provide stability to the overall immature HIV-1 shell. Sequence alignment of the CA C-terminal domains (CTDs) of HIV-1 and Mason-Pfizer monkey virus (M-PMV) highlighted a spacer-like domain in M-PMV that may provide a comparable function. The importance of the sequences spanning the CA-nucleocapsid (NC) cleavage has been demonstrated by mutagenesis, but the specific requirements for the clasp motif in several steps of M-PMV particle assembly and maturation have not been determined in detail. In the present study, we report an examination of the role of the clasp motif in the M-PMV life cycle. We generated a series of M-PMV Gag mutants and assayed for assembly of the recombinant proteins in vitro and for the assembly, maturation, release, genomic RNA packaging, and infectivity of the mutant viruses in vivo. The mutants revealed major defects in virion assembly and release in HEK 293T and HeLa cells and even larger defects in infectivity. Our data identify the clasp motif as a fundamental contributor to CA-CTD interactions necessary for efficient retroviral infection. IMPORTANCE The C-terminal domain of the capsid protein of many retroviruses has been shown to be critical for virion assembly and maturation, but the functions of this region of M-PMV are uncertain. We show that a short "clasp" motif in the capsid domain of the M-PMV Gag protein plays a key role in M-PMV virion assembly, genome packaging, and infectivity.

HIV-cell membrane fusion intermediates are restricted by Serincs as revealed by cryo-electron and TIRF microscopy.

To enter a cell and establish infection, HIV must first fuse its lipid envelope with the host cell plasma membrane. Whereas the process of HIV membrane fusion can be tracked by fluorescence microscopy, the 3D configuration of proteins and lipids at intermediate steps can only be resolved with cryo-electron tomography (cryoET). However, cryoET of whole cells is technically difficult. To overcome this problem, we have adapted giant plasma membrane vesicles (or blebs) from native cell membranes expressing appropriate receptors as targets for fusion with HIV envelope glycoprotein-expressing pseudovirus particles with and without Serinc host restriction factors. The fusion behavior of these particles was probed by TIRF microscopy on bleb-derived supported membranes. Timed snapshots of fusion of the same particles with blebs were examined by cryo-ET. The combination of these methods allowed us to characterize the structures of various intermediates on the fusion pathway and showed that when Serinc3 or Serinc5 (but not Serinc2) were present, later fusion products were more prevalent, suggesting that Serinc3/5 act at multiple steps to prevent progression to full fusion. In addition, the antifungal amphotericin B reversed Serinc restriction, presumably by intercalation into the fusing membranes. Our results provide a highly detailed view of Serinc restriction of HIV-cell membrane fusion and thus extend current structural and functional information on Serinc as a lipid-binding protein.

Biochemical Reconstitution of HIV-1 Assembly and Maturation.

The assembly of an orthoretrovirus such as HIV-1 requires the coordinated functioning of multiple biochemical activities of the viral Gag protein. These activities include membrane targeting, lattice formation, packaging of the RNA genome, and recruitment of cellular cofactors that modulate assembly. In most previous studies, these Gag activities have been investigated individually, which provided somewhat limited insight into how they functionally integrate during the assembly process. Here, we report the development of a biochemical reconstitution system that allowed us to investigate how Gag lattice formation, RNA binding, and the assembly cofactor inositol hexakisphosphate (IP6) synergize to generate immature virus particles in vitro The results identify an important rate-limiting step in assembly and reveal new insights into how RNA and IP6 promote immature Gag lattice formation. The immature virus-like particles can be converted into mature capsid-like particles by the simple addition of viral protease, suggesting that it is possible in principle to fully biochemically reconstitute the sequential processes of HIV-1 assembly and maturation from purified components.IMPORTANCE Assembly and maturation are essential steps in the replication of orthoretroviruses such as HIV-1 and are proven therapeutic targets. These processes require the coordinated functioning of the viral Gag protein's multiple biochemical activities. We describe here the development of an experimental system that allows an integrative analysis of how Gag's multiple functionalities cooperate to generate a retrovirus particle. Our current studies help to illuminate how Gag synergizes the formation of the virus compartment with RNA binding and how these activities are modulated by the small molecule IP6. Further development and use of this system should lead to a more comprehensive understanding of the molecular mechanisms of HIV-1 assembly and maturation and may provide new insights for the development of antiretroviral drugs.

Nup153 Unlocks the Nuclear Pore Complex for HIV-1 Nuclear Translocation in Nondividing Cells.

Human immunodeficiency virus type 1 (HIV-1) displays the unique ability to infect nondividing cells. The capsid of HIV-1 is the viral determinant for viral nuclear import. To understand the cellular factors involved in the ability of HIV-1 to infect nondividing cells, we sought to find capsid mutations that allow the virus to infect dividing but not nondividing cells. Because the interaction of capsid with the nucleoporin protein 153 (Nup153) is important for nuclear import of HIV-1, we solved new crystal structures of hexameric HIV-1 capsid in complex with a Nup153-derived peptide containing a phenylalanine-glycine repeat (FG repeat), which we used to guide structure-based mutagenesis of the capsid-binding interface. HIV-1 viruses with mutations in these capsid residues were tested for their ability to infect dividing and nondividing cells. HIV-1 viruses with capsid N57 substitutions infected dividing but not nondividing cells. Interestingly, HIV-1 viruses with N57 mutations underwent reverse transcription but not nuclear translocation. The mutant capsids also lost the ability to interact with Nup153 and CPSF6. The use of small molecules PF74 and BI-2 prevented the interaction of FG-containing nucleoporins (Nups), such as Nup153, with the HIV-1 core. Analysis of integration sites in HIV-1 viruses with N57 mutations revealed diminished integration into transcriptionally active genes in a manner resembling that of HIV-1 in CPSF6 knockout cells or that of HIV-1-N74D. The integration pattern of the N57 mutant HIV-1 can be explained by loss of capsid interaction with CPSF6, whereas capsid interaction with Nup153 is required for HIV-1 to infect nondividing cells. Additionally, the observed viral integration profiles suggested that integration site selection is a multiparameter process that depends upon nuclear factors and the state of the cellular chromatin.IMPORTANCE One of the key advantages that distinguish lentiviruses, such as HIV-1, from all other retroviruses is its ability to infect nondividing cells. Interaction of the HIV-1 capsid with Nup153 and CPSF6 is important for nuclear entry and integration; however, the contribution of each of these proteins to nuclear import and integration is not clear. Using genetics, we demonstrated that these proteins contribute to different processes: Nup153 is essential for the HIV-1 nuclear import in nondividing cells, and CPSF6 is important for HIV-1 integration. In addition, nuclear factors such as CPSF6 and the state of the chromatin are known to be important for integration site selection; nevertheless, the preferential determinant influencing integration site selection is not known. This work demonstrates that integration site selection is a multiparameter process that depends upon nuclear factors and the state of the cellular chromatin.

General Model for Retroviral Capsid Pattern Recognition by TRIM5 Proteins.

Restriction factors are intrinsic cellular defense proteins that have evolved to block microbial infections. Retroviruses such as HIV-1 are restricted by TRIM5 proteins, which recognize the viral capsid shell that surrounds, organizes, and protects the viral genome. TRIM5α uses a SPRY domain to bind capsids with low intrinsic affinity (KD of >1 mM) and therefore requires higher-order assembly into a hexagonal lattice to generate sufficient avidity for productive capsid recognition. TRIMCyp, on the other hand, binds HIV-1 capsids through a cyclophilin A domain, which has a well-defined binding site and higher affinity (KD of ∼10 µM) for isolated capsid subunits. Therefore, it has been argued that TRIMCyp proteins have dispensed with the need for higher-order assembly to function as antiviral factors. Here, we show that, consistent with its high degree of sequence similarity with TRIM5α, the TRIMCyp B-box 2 domain shares the same ability to self-associate and facilitate assembly of a TRIMCyp hexagonal lattice that can wrap about the HIV-1 capsid. We also show that under stringent experimental conditions, TRIMCyp-mediated restriction of HIV-1 is indeed dependent on higher-order assembly. Both forms of TRIM5 therefore use the same mechanism of avidity-driven capsid pattern recognition.IMPORTANCE Rhesus macaques and owl monkeys are highly resistant to HIV-1 infection due to the activity of TRIM5 restriction factors. The rhesus macaque TRIM5α protein blocks HIV-1 through a mechanism that requires self-assembly of a hexagonal TRIM5α lattice around the invading viral core. Lattice assembly amplifies very weak interactions between the TRIM5α SPRY domain and the HIV-1 capsid. Assembly also promotes dimerization of the TRIM5α RING E3 ligase domain, resulting in synthesis of polyubiquitin chains that mediate downstream steps of restriction. In contrast to rhesus TRIM5α, the owl monkey TRIM5 homolog, TRIMCyp, binds isolated HIV-1 CA subunits much more tightly through its cyclophilin A domain and therefore was thought to act independently of higher-order assembly. Here, we show that TRIMCyp shares the assembly properties of TRIM5α and that both forms of TRIM5 use the same mechanism of hexagonal lattice formation to promote viral recognition and restriction.

TRIM5α SPRY/coiled-coil interactions optimize avid retroviral capsid recognition.

Restriction factors are important components of intrinsic cellular defense mechanisms against viral pathogens. TRIM5α is a restriction factor that intercepts the incoming capsid cores of retroviruses such as HIV and provides an effective species-specific barrier to retroviral infection. The TRIM5α SPRY domain directly binds the capsid with only very weak, millimolar-level affinity, and productive capsid recognition therefore requires both TRIM5α dimerization and assembly of the dimers into a multivalent hexagonal lattice to promote avid binding. Here, we explore the important unresolved question of whether the SPRY domains are flexibly linked to the TRIM lattice or more precisely positioned to maximize avidity. Biochemical and biophysical experiments indicate that the linker segment connecting the SPRY domain to the coiled-coil domain adopts an α-helical fold, and that this helical portion mediates interactions between the two domains. Targeted mutations were generated to disrupt the putative packing interface without affecting dimerization or higher-order assembly, and we identified mutant proteins that were nevertheless deficient in capsid binding in vitro and restriction activity in cells. Our studies therefore support a model wherein substantial avidity gains during assembly-mediated capsid recognition by TRIM5α come in part from tailored spacing of tethered recognition domains.

Structure and catalytic activation of the TRIM23 RING E3 ubiquitin ligase.

Tripartite motif (TRIM) proteins comprise a large family of RING-type ubiquitin E3 ligases that regulate important biological processes. An emerging general model is that TRIMs form elongated antiparallel coiled-coil dimers that prevent interaction of the two attendant RING domains. The RING domains themselves bind E2 conjugating enzymes as dimers, implying that an active TRIM ligase requires higher-order oligomerization of the basal coiled-coil dimers. Here, we report crystal structures of the TRIM23 RING domain in isolation and in complex with an E2-ubiquitin conjugate. Our results indicate that TRIM23 enzymatic activity requires RING dimerization, consistent with the general model of TRIM activation.

Atomic-level modelling of the HIV capsid.

The mature capsids of human immunodeficiency virus type 1 (HIV-1) and other retroviruses are fullerene shells, composed of the viral CA protein, that enclose the viral genome and facilitate its delivery into new host cells. Retroviral CA proteins contain independently folded amino (N)- and carboxy (C)-terminal domains (NTD and CTD) that are connected by a flexible linker. The NTD forms either hexameric or pentameric rings, whereas the CTD forms symmetric homodimers that connect the rings into a hexagonal lattice. We previously used a disulphide crosslinking strategy to enable isolation and crystallization of soluble HIV-1 CA hexamers. Here we use the same approach to solve the X-ray structure of the HIV-1 CA pentamer at 2.5 Å resolution. Two mutant CA proteins with engineered disulphides at different positions (P17C/T19C and N21C/A22C) converged onto the same quaternary structure, indicating that the disulphide-crosslinked proteins recapitulate the structure of the native pentamer. Assembly of the quasi-equivalent hexamers and pentamers requires remarkably subtle rearrangements in subunit interactions, and appears to be controlled by an electrostatic switch that favours hexamers over pentamers. This study completes the gallery of substructures describing the components of the HIV-1 capsid and enables atomic-level modelling of the complete capsid. Rigid-body rotations around two assembly interfaces appear sufficient to generate the full range of continuously varying lattice curvature in the fullerene cone.

Structural basis of HIV-1 capsid recognition by PF74 and CPSF6.

Upon infection of susceptible cells by HIV-1, the conical capsid formed by ∼250 hexamers and 12 pentamers of the CA protein is delivered to the cytoplasm. The capsid shields the RNA genome and proteins required for reverse transcription. In addition, the surface of the capsid mediates numerous host-virus interactions, which either promote infection or enable viral restriction by innate immune responses. In the intact capsid, there is an intermolecular interface between the N-terminal domain (NTD) of one subunit and the C-terminal domain (CTD) of the adjacent subunit within the same hexameric ring. The NTD-CTD interface is critical for capsid assembly, both as an architectural element of the CA hexamer and pentamer and as a mechanistic element for generating lattice curvature. Here we report biochemical experiments showing that PF-3450074 (PF74), a drug that inhibits HIV-1 infection, as well as host proteins cleavage and polyadenylation specific factor 6 (CPSF6) and nucleoporin 153 kDa (NUP153), bind to the CA hexamer with at least 10-fold higher affinities compared with nonassembled CA or isolated CA domains. The crystal structure of PF74 in complex with the CA hexamer reveals that PF74 binds in a preformed pocket encompassing the NTD-CTD interface, suggesting that the principal inhibitory target of PF74 is the assembled capsid. Likewise, CPSF6 binds in the same pocket. Given that the NTD-CTD interface is a specific molecular signature of assembled hexamers in the capsid, binding of NUP153 at this site suggests that key features of capsid architecture remain intact upon delivery of the preintegration complex to the nucleus.

Helical Conformation in the CA-SP1 Junction of the Immature HIV-1 Lattice Determined from Solid-State NMR of Virus-like Particles.

Maturation of HIV-1 requires disassembly of the Gag polyprotein lattice, which lines the viral membrane in the immature state, and subsequent assembly of the mature capsid protein lattice, which encloses viral RNA in the mature state. Metastability of the immature lattice has been proposed to depend on the existence of a structurally ordered, α-helical segment spanning the junction between capsid (CA) and spacer peptide 1 (SP1) subunits of Gag, a segment that is dynamically disordered in the mature capsid lattice. We report solid state nuclear magnetic resonance (ssNMR) measurements on the immature lattice in noncrystalline, spherical virus-like particles (VLPs) derived from Gag. The ssNMR data provide definitive evidence for this critical α-helical segment in the VLPs. Differences in ssNMR chemical shifts and signal intensities between immature and mature lattice assemblies also support a major rearrangement of intermolecular interactions in the maturation process, consistent with recent models from electron cryomicroscopy and X-ray crystallography.

Hexagonal assembly of a restricting TRIM5alpha protein.

TRIM5α proteins are restriction factors that protect mammalian cells from retroviral infections by binding incoming viral capsids, accelerating their dissociation, and preventing reverse transcription of the viral genome. Individual TRIM5 isoforms can often protect cells against a broad range of retroviruses, as exemplified by rhesus monkey TRIM5α and its variant, TRIM5-21R, which recognize HIV-1 as well as several distantly related retroviruses. Although capsid recognition is not yet fully understood, previous work has shown that the C-terminal SPRY/B30.2 domain of dimeric TRIM5α binds directly to viral capsids, and that higher-order TRIM5α oligomerization appears to contribute to the efficiency of capsid recognition. Here, we report that recombinant TRIM5-21R spontaneously assembled into two-dimensional paracrystalline hexagonal lattices comprising open, six-sided rings. TRIM5-21R assembly did not require the C-terminal SPRY domain, but did require both protein dimerization and a B-box 2 residue (Arg121) previously implicated in TRIM5α restriction and higher-order assembly. Furthermore, TRIM5-21R assembly was promoted by binding to hexagonal arrays of the HIV-1 CA protein that mimic the surface of the viral capsid. We therefore propose that TRIM5α proteins have evolved to restrict a range of different retroviruses by assembling a deformable hexagonal scaffold that positions the capsid-binding domains to match the symmetry and spacing of the capsid surface lattice. Capsid recognition therefore involves a synergistic combination of direct binding interactions, avidity effects, templated assembly, and lattice complementarity.

Molecular Determinants of PQBP1 Binding to the HIV-1 Capsid Lattice.

Human immunodeficiency virus type 1 (HIV-1) stimulates innate immune responses upon infection, including cyclic GMP-AMP synthase (cGAS) signaling that results in type I interferon production. HIV-1-induced activation of cGAS requires the host cell factor polyglutamine binding protein 1 (PQBP1), an intrinsically disordered protein that bridges capsid recognition and cGAS recruitment. However, the molecular details of PQBP1 interactions with the HIV-1 capsid and their functional implications remain poorly understood. Here, we show that PQBP1 binds to HIV-1 capsids through charge complementing contacts between acidic residues in the N-terminal region of PQBP1 and an arginine ring in the central channel of the HIV-1 CA hexamer that makes up the viral capsid. These studies reveal the molecular details of PQBP1's primary interaction with the HIV-1 capsid and suggest that additional elements are likely to contribute to stable capsid binding.

Arg18 Substitutions Reveal the Capacity of the HIV-1 Capsid Protein for Non-Fullerene Assembly.

In the fullerene cone HIV-1 capsid, the central channels of the hexameric and pentameric capsomers each contain a ring of arginine (Arg18) residues that perform essential roles in capsid assembly and function. In both the hexamer and pentamer, the Arg18 rings coordinate inositol hexakisphosphate, an assembly and stability factor for the capsid. Previously, it was shown that amino-acid substitutions of Arg18 can promote pentamer incorporation into capsid-like particles (CLPs) that spontaneously assemble in vitro under high-salt conditions. Here, we show that these Arg18 mutant CLPs contain a non-canonical pentamer conformation and distinct lattice characteristics that do not follow the fullerene geometry of retroviral capsids. The Arg18 mutant pentamers resemble the hexamer in intra-oligomeric contacts and form a unique tetramer-of-pentamers that allows for incorporation of an octahedral vertex with a cross-shaped opening in the hexagonal capsid lattice. Our findings highlight an unexpected degree of structural plasticity in HIV-1 capsid assembly.

Purification and structure of luminal domain C of human Niemann-Pick C1 protein.

Niemann-Pick C1 protein (NPC1) is a membrane protein that primarily resides in late endosomes and lysosomes, and plays an important role in cholesterol homeostasis in the cell. The second luminal domain of NPC1 (NPC1-C) serves as the intracellular receptor for Ebola and Marburg viruses. Here, the recombinant production of nonglycosylated and glycosylated NPC1-C and a new crystal form of the nonglycosylated protein are reported. The crystals belonged to space group P21 and diffracted to 2.3 Šresolution. The structure is similar to other reported structures of NPC1-C, with differences observed in the protruding loops when compared with NPC1-C in complex with Ebola virus glycoprotein or NPC2.