Membrane lipid raft domains are thought to be sites of assembly for many enveloped viruses. The roles of both classical lipid rafts and lipid rafts associated with the membrane cytoskeleton in the assembly of Newcastle disease virus (NDV) were investigated. The lipid raft-associated proteins caveolin-1, flotillin-2, and actin were incorporated into virions, while the non-lipid raft-associated transferrin receptor was excluded. Kinetic analyses of the distribution of viral proteins in lipid rafts, as defined by detergent-resistant membranes (DRMs), in non-lipid raft membranes, and in virions showed an accumulation of HN, F, and NP viral proteins in lipid rafts early after synthesis. Subsequently, these proteins exited the DRMs and were recovered quantitatively in purified virions, while levels of these proteins in detergent-soluble cell fractions remained relatively constant. Cholesterol depletion of infected cells drastically altered the association of viral proteins with DRMs and resulted in an enhanced release of virus particles with reduced infectivity. Decreased infectivity was not due to effects on subsequent virus entry, since the extraction of cholesterol from intact virus did not significantly reduce infectivity. Particles released from cholesterol-depleted cells had very heterogeneous densities and altered ratios of NP and glycoproteins, demonstrating structural abnormalities which potentially contributed to their lowered infectivity. Taken together, these results indicate that lipid rafts, including cytoskeletonassociated lipid rafts, are sites of NDV assembly and that these domains are important for ordered assembly and release of infectious Newcastle disease virus particles.

For many viruses, one or two proteins allow cell attachment and entry, which occurs through the plasma membrane or following endocytosis at low pH. In contrast, vaccinia virus (VACV) enters cells by both neutral and low pH routes; four proteins mediate cell attachment and twelve that are associated in a membrane complex and conserved in all poxviruses are dedicated to entry. The aim of the present study was to determine the roles of cellular and viral proteins in initial stages of entry, specifically fusion of the membranes of the mature virion and cell. For analysis of the role of cellular components, we used well characterized inhibitors and measured binding of a recombinant VACV virion containing Gaussia luciferase fused to a core protein; viral and cellular membrane lipid mixing with a self-quenching fluorescent probe in the virion membrane; and core entry with a recombinant VACV expressing firefly luciferase and electron microscopy. We determined that inhibitors of tyrosine protein kinases, dynamin GTPase and actin dynamics had little effect on binding of virions to cells but impaired membrane fusion, whereas partial cholesterol depletion and inhibitors of endosomal acidification and membrane blebbing had a severe effect at the later stage of core entry. To determine the role of viral proteins, virions lacking individual membrane components were purified from cells infected with members of a panel of ten conditional-lethal inducible mutants. Each of the entry protein-deficient virions had severely reduced infectivity and except for A28, L1 and L5 greatly impaired membrane fusion. In addition, a potent neutralizing L1 monoclonal antibody blocked entry at a post-membrane lipid-mixing step. Taken together, these results suggested a 2-step entry model and implicated an unprecedented number of viral proteins and cellular components involved in signaling and actin rearrangement for initiation of virus-cell membrane fusion during poxvirus entry.

Entry of vaccinia virus (VACV) into cells occurs by fusion with the plasma membrane and via a low pH-dependent endosomal pathway, presumably involving unidentified cellular receptors. In addition to Ϸ25 viral proteins, the membrane of VACV mature virions contains several phospholipids including phosphatidylserine (PS). A recent model posits that PS flags virions as apoptotic debris to activate a common cellular uptake pathway to gain cell entry, perhaps through an interaction with a PS-specific cell surface receptor. To evaluate the apoptotic mimicry model, we reconstituted the membrane of detergent-extracted virions with several different phospholipids. Although the ability of the L-stereoisomer of PS to reconstitute infectivity was confirmed, the nonbiologically relevant D-stereoisomer of PS, and phosphatidylglycerol, which are not normally present in the virion membrane, functioned as well. Regardless of which phospholipid reconstituted infectivity, virus entry was inhibited by a neutralizing monoclonal antibody to a virion surface protein and by the drugs blebbistatin and bafilomycin A1, suggesting that in each case virus uptake was specific and occurred by a similar mechanism involving macropinocytosis and a low-pH endocytic pathway. Lipid-reconstituted and nonreconstituted, membrane-extracted virions were equally capable of binding to cells. However, the physical association of phospholipids with virus particles during membrane reconstitution correlated directly with rescue of particle infectivity and cell entry capability. Our results support a role for PS in poxvirus entry, but demonstrate that other phospholipids, not known to signal uptake of apoptotic debris, can function similarly.blebbing ͉ endocytosis ͉ macropinocytosis ͉ phosphatidylserine ͉ vaccinia virus