Radiation inactivation analysis of fusion and hemolysis by vesicular stomatitis virus

Initial size and dynamics of viral fusion pores are a function of the fusion protein mediating membrane fusion

BACKGROUND INFORMATION

Protein-mediated merger of biological membranes, membrane fusion, is an important process. To investigate the role of fusogenic proteins in the initial size and dynamics of the fusion pore (a narrow aqueous pathway, which widens to finalize membrane fusion), two different fusion proteins expressed in the same cell line were investigated: the major glycoprotein of baculovirus Autographa californica (GP64) and the HA (haemagglutinin) of influenza X31.

RESULTS

The host Sf9 cells expressing these viral proteins, irrespective of protein species, fused to human RBCs (red blood cells) upon acidification of the medium. A high-time-resolution electrophysiological study of fusion pore conductance revealed fundamental differences in (i) the initial pore conductance; pores created by HA were smaller than those created by GP64; (ii) the ability of pores to flicker; only HA-mediated pores flickered; and (iii) the time required for pore formation; HA-mediated pores took much longer to form after acidification.

CONCLUSION

HA and GP64 have divergent electrophysiological phenotypes even when they fuse identical membranes, and fusion proteins play a crucial role in determining initial fusion pore characteristics. The structure of the initial fusion pore detected by electrical conductance measurements is sensitive to the nature of the fusion protein.

Membrane fusion activity of vesicular stomatitis virus glycoprotein G is induced by low pH but not by heat or denaturant

The fusogenic envelope glycoprotein G of the rhabdovirus vesicular stomatitis virus (VSV) induces membrane fusion at acidic pH. At acidic pH the G protein undergoes a major structural reorganization leading to the fusogenic conformation. However, unlike other viral fusion proteins, the low-pH-induced conformational change of VSV G is completely reversible. As well, the presence of an alpha-helical coiled-coil motif required for fusion by a number of viral and cellular fusion proteins was not predicted in VSV G protein by using a number of algorithms. Results of pH dependence of the thermal stability of G protein as determined by intrinsic Trp fluorescence and circular dichroism (CD) spectroscopy show that the G protein is equally stable at neutral or acidic pH. Destabilization of G structure at neutral pH with either heat or urea did not induce membrane fusion or conformational change(s) leading to membrane fusion. Taken together, these data suggest that the mechanism of VSV G-induced fusion is distinct from the fusion mechanism of fusion proteins that involve a coiled-coil motif.

The role of IFN-gamma in immune responses to viral infections of the central nervous system

Interferon (IFN)-gamma, is not only a marker of T(H)1 CD4, CD8 and natural killer (NK) cells, it is also a critical antiviral mediator which is central to the elimination of viruses from the CNS. In this review, we describe IFN-gamma, its receptor, signal transduction from receptor engagement, and antiviral downstream mediators. We demonstrate that although neurons are post-mitotic and non-renewing, they respond to IFN-gamma in a fashion similar to peripheral fibroblasts or lymphocytes. We have illustrated this review with details about studies on the role(s) of IFN-gamma in the pathogenesis of measles virus (MV), herpes simplex virus (HSV) type 1, and vesicular stomatitis virus (VSV) infections of the CNS. For VSV infection, IFN-gamma signals through Jaks 1 and 2 and STAT1 to activate (interferon regulatory factor) IRF-1; although viral protein synthesis is inhibited, PKR is not a critical mediator in the antiviral response to VSV in murine neurons. In contrast, induction of nitric oxide synthase (NOS) type 1 and its production of nitric oxide is essential in the elimination of viruses from neurons.

Comparison of gamma and neutron radiation inactivation of influenza A virus

Radiation inactivation of viral pathogens has potential application in sterilization and in the manufacture of biological reagents, including the production of non-infectious viral antigens. Viral inactivation by gamma radiation has been extensively investigated, but few direct comparisons to other qualities of radiation have been explored. Experiments were designed to examine direct radiation damage by both gamma photons (gamma) and neutrons (n) while minimizing methodological differences. Frozen samples of influenza A X31/H3N2 and PR8/H1N1 were exposed to gamma and n at doses between 0 and 15.6 kGy. Other experimental parameters, including dose-rate, were not varied. Virus titers were determined by tissue culture infectious dose (TCID(50)) and plaque forming unit (PFU) assays. D(10) values, kGy per log reduction, were calculated from these assays. PR8 D(10) values based on PFU assays were approximately 2 and 5 kGy for gamma and n exposures, respectively, and those based on TCID(50) were approximately 6 and 14 kGy. Similar results were obtained for the A/X31 strain. The data demonstrate that gamma was 2-3-fold more effective than n, with a relative biological effectiveness (RBE) range of 0.43-0.65. These neutron results are likely the first reported for a medically relevant virus. PAGE analysis of viral proteins and RNAs failed to show macromolecular damage. D(10) values were found to be similar to a broad summary of previously reported gamma inactivation values for other virus types. The dependence of the magnitudes of D(10) on titer assay in this study suggests that more than one titer method should be used to determine if complete inactivation has occurred.

Rabies virus-induced membrane fusion pathway

Fusion of rabies virus with membranes is triggered at low pH and is mediated by the viral glycoprotein (G). The rabies virus-induced fusion pathway was studied by investigating the effects of exogenous lipids having various dynamic molecular shapes on the fusion process. Inverted cone-shaped lysophosphatidylcholines (LPCs) blocked fusion at a stage subsequent to fusion peptide insertion into the target membrane. Consistent with the stalk-hypothesis, LPC with shorter alkyl chains inhibited fusion at lower membrane concentrations and this inhibition was compensated by the presence of oleic acid. However, under suboptimal fusion conditions, short chain LPCs, which were translocated in the inner leaflet of the membranes, considerably reduced the lag time preceding membrane merging, resulting in faster kinetics of fusion. This indicated that the rate limiting step for fusion is the formation of a fusion pore in a diaphragm of restricted hemifusion. The previously described cold-stabilized prefusion complex was also characterized. This intermediate is at a well-advanced stage of the fusion process when the hemifusion diaphragm is destabilized, but lipid mixing is still restricted, probably by a ring-like complex of glycoproteins. I provide evidence that this state has a dynamic character and that its lipid organization can reverse back to two lipid bilayers.

Rabies virus-induced membrane fusion

Rabies virus is a member of the rhabdovirus family. It enters cells by a process of receptor mediated endocytosis. Following this step, the viral envelope fuses with the endosomal membrane to allow release of the viral nucleocapsid into the cytoplasm. Fusion is induced by the low pH of the endosomal compartment and is mediated by the single viral glycoprotein G, a homotrimeric integral membrane protein. Rabies virus fusion properties are related to different conformational states of G. By different biochemical and biophysical approaches, it has been demonstrated that G can assume at least three different states: the native (N) state detected at the viral surface above pH 7, the activated (A) hydrophobic state which interacts with the target membrane as a first step of the fusion process, and the fusion inactive (I) conformation. Differently from other fusogenic viruses for which low pH-induced conformational changes are irreversible, there is a pH dependent equilibrium between these states, the equilibrium being shifted toward the I-state at low pH. The objective of this review is to detail recent findings on rhabdovirus-induced membrane fusion and to underline the differences that exist between this viral family and influenza virus which is the best known fusogenic virus. These differences have to be taken into consideration if one wants to have a global understanding of virus-induced membrane fusion.

Identification of amino acids controlling the low-pH-induced conformational change of rabies virus glycoprotein

The glycoprotein (G) of rabies virus assumes at least three different conformations: the native state detected at the viral surface above pH 7, the activated state involved in the first step of the fusion process, and the fusion-inactive conformation (I). A new category of monoclonal antibodies (MAbs) which recognized specifically the I conformation at the viral surface has recently been described. These MAbs (17A4 and 29EC2) became neutralizing when the virus was preincubated at acidic pH to induce the conformational change toward the I state of G. Mutants escaping neutralization were then selected. In this study, we have investigated the fusion and the low-pH-induced fusion inactivation properties of these mutants. All of these mutants have fusion properties similar to those of the CVS parental strain, but five mutants (E282K, M44I, M44V, V392G, and M396T) were considerably slowed in their conformational change leading to the I state. These mutants allow us to define regions that control this conformational change. These results also reinforce the idea that structural transition toward the I state is irrelevant to the fusion process. Other mutations in amino acids 10, 13, and 15 are probably located in the epitopes of selecting MAbs. Furthermore, in electron microscopy, we observed a hexagonal lattice of glycoproteins at the viral surface of mutants M44I and V392G as well as strong cooperativity in the conformational change toward the I state. This finding demonstrates the existence of lateral interactions between the spikes of a rhabdovirus.

Low-pH conformational changes of rabies virus glycoprotein and their role in membrane fusion

Fusion of rabies virus with membranes occurs at acidic pH and is mediated by the viral spike glycoprotein (G). In this paper, we provide the basis for a description of structural transitions associated with exposure to low pH and of their role in membrane fusion. First, we have extended previous studies of fusion kinetics and we have shown that low-pH inhibition of fusion is detectable at 0.5 pH units higher than fusion. Second, low-pH-induced conformational changes were analyzed by using electron microscopy and monoclonal antibody binding assays. The existence of a pH-dependent equilibrium between the native and a low-pH inactive conformation was demonstrated. Third, besides these two conformations, we, using the fluorescent probe ANS (8-anilino-1-naphthalenesulfonic acid), provide evidence for the existence of a transient third state which appears to be more hydrophobic than the native state. Our results suggest that this transient state is responsible for viral aggregation at low pH and could play a role in the first steps of the fusion mechanism.

Damage to proteins due to the direct action of ionizing radiation

Proteins exposed to ionizing radiation suffer both reversible and irreversible effects. Reversible effects are defined as those which disappear in a short period of time after the removal of the radiation field and without further treatment of the sample. Irreversible effects are those which cause a permanent alteration in the structure of a protein.

Entry mechanisms of enveloped viruses. Implications for fusion of intracellular membranes

Enveloped viruses infect cells by a mechanism involving membrane fusion. This process is mediated and triggered by specific viral membrane glycoproteins. Evidence is accumulating that fusion of intracellular membranes, as occurs during endocytosis and transport between intracellular organelles, also requires the presence of specific proteins. The relevance of elucidating the mechanisms of virus fusion for a better understanding of fusion of intracellular membranes is discussed.