A pathogenic picornavirus acquires an envelope by hijacking cellular membranes.

Animal viruses are broadly categorized structurally by the presence or absence of an envelope composed of a lipid-bilayer membrane, attributes that profoundly affect stability, transmission and immune recognition. Among those lacking an envelope, the Picornaviridae are a large and diverse family of positive-strand RNA viruses that includes hepatitis A virus (HAV), an ancient human pathogen that remains a common cause of enterically transmitted hepatitis. HAV infects in a stealth-like manner and replicates efficiently in the liver. Virus-specific antibodies appear only after 3-4 weeks of infection, and typically herald its resolution. Although unexplained mechanistically, both anti-HAV antibody and inactivated whole-virus vaccines prevent disease when administered as late as 2 weeks after exposure, when virus replication is well established in the liver. Here we show that HAV released from cells is cloaked in host-derived membranes, thereby protecting the virion from antibody-mediated neutralization. These enveloped viruses ('eHAV') resemble exosomes, small vesicles that are increasingly recognized to be important in intercellular communications. They are fully infectious, sensitive to extraction with chloroform, and circulate in the blood of infected humans. Their biogenesis is dependent on host proteins associated with endosomal-sorting complexes required for transport (ESCRT), namely VPS4B and ALIX. Whereas the hijacking of membranes by HAV facilitates escape from neutralizing antibodies and probably promotes virus spread within the liver, anti-capsid antibodies restrict replication after infection with eHAV, suggesting a possible explanation for prophylaxis after exposure. Membrane hijacking by HAV blurs the classic distinction between 'enveloped' and 'non-enveloped' viruses and has broad implications for mechanisms of viral egress from infected cells as well as host immune responses.

An engineered fibrinogen variant AαQ328,366P does not polymerise normally, but retains the ability to form α cross-links.

A fibrin clot is stabilised through the formation of factor XIIIa-catalysed intermolecular ε-lysyl-γ-glutamyl covalent cross-links between α chains to form α polymers and between γ chains to form γ dimers. In a previous study we characterised fibrinogen Seoul II, a heterozygous dysfibrinogen in which a cross-linking acceptor site in Aα chain, Gln328, was replaced with Pro (AαQ328P). Following on the previous study, we investigated whether the alteration of Gln residues Aα328 and Aα366 affects fibrin polymerisation and α chain cross-linking. We have expressed three recombinant fibrinogens: AαQ328P, AαQ366P, and AαQ328,366P in Chinese hamster ovary cells, purified these fibrinogens from the culture media and performed biochemical tests to see how the introduced changes affect fibrin polymerisation and α chain cross-linking. Thrombin-catalysed fibrin polymerisation of all variants was impaired with the double mutation being the most impaired. In contrast, sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblot analysis showed α polymer formation with all three engineered proteins. This study demonstrates that AαQ328 and AαQ366 are important for normal fibrin clot formation and in the absence of residues AαQ328 and AαQ366, other Gln residues in the α chain can support FXIIIa-catalysed fibrin cross-linking.

Fibrinogen residue γAla341 is necessary for calcium binding and 'A-a' interactions.

The fibrinogen γ-module has several important sites relating to fibrinogen function, which include the high affinity calcium binding site, hole 'a' that binds with knob 'A', and the D:D interface. Residue γAla341, which is located in the vicinity of these sites, is altered in three variant fibrinogens: fibrinogen Seoul (γAla341Asp), Tolaga Bay (γAla341Val), and Lyon III (γAla341Thr). In order to investigate the impaired polymerisation of fibrinogens γAla341Asp and γAla341Val to understand the role of γAla341 in fibrin polymerisation and fibrinogen synthesis, we have expressed γAla341Asp and γAla341Val in Chinese hamster ovary (CHO) cells, purified these fibrinogens from the culture media and performed biochemical tests to elucidate their function. Expression in CHO cells was similar for these variants. For both variants the kinetics of thrombin-catalysed FpA release was not different from normal fibrinogen, while FpB release was slower than that of normal. Thrombin-catalysed polymerisation of both variants was dependent on the calcium concentration. At physiologic calcium (1 mM) the variants showed impaired polymerisation with a longer lag period and a slower Vmax than normal fibrinogen. Scanning electron micrographs showed the clots were less organised than normal, having thicker and more twisted fibers, and larger pores. Analysis by SDS-PAGE showed that factor XIIIa-catalysed γ and α chain cross-linking was delayed, and plasmin-catalysed lysis was not reduced by the presence of 5 mM calcium or 5 mM GPRP (Gly-Pro-Arg-Pro). Our data indicate that fibrinogen residue γAla341 is important for the proper conformation of the γ-module, maintaining calcium-binding site and 'A-a' interactions.

Substitution of the human αC region with the analogous chicken domain generates a fibrinogen with severely impaired lateral aggregation: fibrin monomers assemble into protofibrils but protofibrils do not assemble into fibers.

Fibrin polymerization occurs in two steps: the assembly of fibrin monomers into protofibrils and the lateral aggregation of protofibrils into fibers. Here we describe a novel fibrinogen that apparently impairs only lateral aggregation. This variant is a hybrid, where the human αC region has been replaced with the homologous chicken region. Several experiments indicate this hybrid human-chicken (HC) fibrinogen has an overall structure similar to normal. Thrombin-catalyzed fibrinopeptide release from HC fibrinogen was normal. Plasmin digests of HC fibrinogen produced fragments that were similar to normal D and E; further, as with normal fibrinogen, the knob 'A' peptide, GPRP, reversed the plasmin cleavage associated with addition of EDTA. Dynamic light scattering and turbidity studies with HC fibrinogen showed polymerization was not normal. Whereas early small increases in hydrodynamic radius and absorbance paralleled the increases seen during the assembly of normal protofibrils, HC fibrinogen showed no dramatic increase in scattering as observed with normal lateral aggregation. To determine whether HC and normal fibrinogen could form a copolymer, we examined mixtures of these. Polymerization of normal fibrinogen was markedly changed by HC fibrinogen, as expected for mixed polymers. When the mixture contained 0.45 µM normal and 0.15 µM HC fibrinogen, the initiation of lateral aggregation was delayed and the final fiber size was reduced relative to normal fibrinogen at 0.45 µM. Considered altogether, our data suggest that HC fibrin monomers can assemble into protofibrils or protofibril-like structures, but these either cannot assemble into fibers or assemble into very thin fibers.

Evidence that αC region is origin of low modulus, high extensibility, and strain stiffening in fibrin fibers.

Fibrin fibers form the structural scaffold of blood clots and perform the mechanical task of stemming blood flow. Several decades of investigation of fibrin fiber networks using macroscopic techniques have revealed remarkable mechanical properties. More recently, the microscopic origins of fibrin's mechanics have been probed through direct measurements on single fibrin fibers and individual fibrinogen molecules. Using a nanomanipulation system, we investigated the mechanical properties of individual fibrin fibers. The fibers were stretched with the atomic force microscope, and stress-versus-strain data was collected for fibers formed with and without ligation by the activated transglutaminase factor XIII (FXIIIa). We observed that ligation with FXIIIa nearly doubled the stiffness of the fibers. The stress-versus-strain behavior indicates that fibrin fibers exhibit properties similar to other elastomeric biopolymers. We propose a mechanical model that fits our observed force extension data, is consistent with the results of the ligation data, and suggests that the large observed extensibility in fibrin fibers is mediated by the natively unfolded regions of the molecule. Although some models attribute fibrin's force-versus-extension behavior to unfolding of structured regions within the monomer, our analysis argues that these models are inconsistent with the measured extensibility and elastic modulus.

Analysis of engineered fibrinogen variants suggests that an additional site mediates platelet aggregation and that "B-b" interactions have a role in protofibril formation.

The C-terminal domain of the fibrinogen gamma-chain includes multiple functional sites that have been defined in high-resolution structures and biochemical assays. Calcium binds to this domain through the side chains of gammaD318 and gammaD320 and the backbone carbonyls of gammaF322 and gammaG324. We have examined variant fibrinogens with alanine at position gamma318 and/or gamma320 and found that calcium binding, fibrin polymerization, and fibrinogen-mediated platelet aggregation, but not FXIIIa-catalyzed cross-linking, were abnormal. When measured by turbidity, thrombin-catalyzed polymerization was severely reduced, and batroxobin-catalyzed polymerization was completely obliterated. Moreover, thrombin-catalyzed polymerization was abolished by the peptide GHRP, which binds to the polymerization site in the beta-chain but does not inhibit polymerization of normal fibrinogen. ADP-induced platelet aggregation was also severely impaired. In contrast, as measured by SDS-PAGE, FXIIIa introduced cross-links between gamma-chains for all three variants, as expected if the gamma-chain C-terminal sites were normal. In addition, binding of the monoclonal antibody 4A5, which recognizes the C-terminal residues, was not different from normal. These data suggest two specific conclusions: (1) a site in the gamma-module other than the C-terminus is critical for platelet aggregation and (2) "B-b" interactions have a role in protofibril formation.