Mature vaccinia virus enters cells through either fluid-phase endocytosis/macropinocytosis or plasma membrane fusion. This may explain the wide range of host cell susceptibilities to vaccinia virus entry; however, it is not known how vaccinia virus chooses between these two pathways and which viral envelope proteins determine such processes. By screening several recombinant viruses and different strains, we found that mature virions containing the vaccinia virus A25 and A26 proteins entered HeLa cells preferentially through a bafilomycin-sensitive entry pathway, whereas virions lacking these two proteins entered through a bafilomycin-resistant pathway. To investigate whether the A25 and A26 proteins contribute to entry pathway specificity, two mutant vaccinia viruses, WR⌬A25L and WR⌬A26L, were subsequently generated from the wild-type WR strain. In contrast to the WR strain, both the WR⌬A25L and WR⌬A26L viruses became resistant to bafilomycin, suggesting that the removal of the A25 and A26 proteins bypassed the low-pH endosomal requirement for mature virion entry. Indeed, WR⌬A25L and WR⌬A26L virus infections of HeLa, CHO-K1, and L cells immediately triggered cell-to-cell fusion at a neutral pH at 1 to 2 h postinfection (p.i.), providing direct evidence that viral fusion machinery is readily activated after the removal of the A25 and A26 proteins to allow virus entry through the plasma membrane. In summary, our data support a model that on vaccinia mature virions, the viral A25 and A26 proteins are low-pH-sensitive fusion suppressors whose inactivation during the endocytic route results in viral and cell membrane fusion. Our results also suggest that during virion morphogenesis, the incorporation of the A25 and A26 proteins into mature virions may help restrain viral fusion activity until the time of infections.Vaccinia virus has a wide host range and infects many cell lines in cultures and animal species (14). It belongs to the genus Orthopoxvirus of the family Poxviridae and replicates in the cytoplasm of infected cells. Several unique features of vaccinia virus help to maximize its ability to transmit virus infections in different host cells. First, vaccinia virus produces several forms of infectious particles, including mature virus (MV), intracellular wrapped virus (WV), and extracellular enveloped virus (EEV) particles, that are suited for inter-or intrahost cell transmission and dissemination (8). Second, vaccinia virus MV attaches to cell surface components that are commonly expressed on cells. MV contains at least four attachment proteins, of which viral H3, A27, and D8 bind to cell surface glycosaminoglycans (GAGs) (7,18,24) and A26 binds to the extracellular matrix protein laminin (6). Third, MV enters cells through more than one route, since both endocytosis (9, 31) and plasma membrane fusion (1,3,4,10,12,25,39) were reported previously. The endocytosis of vaccinia virus MV is dependent on low pH (4.5 to 5.0) and sensitive to chemicals such as NaF, cytochalasin B (31), as well as bafilomycin A (BFLA),...

Tumor necrosis factor alpha (TNF-␣) activates the nuclear factor B (NF-B) signaling pathway that regulates expression of many cellular factors playing important roles in innate immune responses and inflammation in infected hosts. Poxviruses employ many strategies to inhibit NF-B activation in cells. In this report, we describe a poxvirus host range protein, CP77, which blocked NF-B activation by TNF-␣. Immunofluorescence analyses revealed that nuclear translocation of NF-B subunit p65 protein in TNF-␣-treated HeLa cells was blocked by CP77. CP77 did so without blocking IB␣ phosphorylation, suggesting that upstream kinase activation was not affected by CP77. Using GST pull-down, we showed that CP77 bound to the NF-B subunit p65 through the N-terminal six-ankyrin-repeat region in vitro. CP77 also bound to Cullin-1 and Skp1 of the SCF complex through a C-terminal 13-amino-acid F-box-like sequence. Both regions of CP77 are required to block NF-B activation. We thus propose a model in which poxvirus CP77 suppresses NF-B activation by two interactions: the C-terminal F-box of CP77 binding to the SCF complex and the N-terminal six ankyrins binding to the NF-B subunit p65. In this way, CP77 attenuates innate immune response signaling in cells. Finally, we expressed CP77 or a CP77 F-box deletion protein from a vaccinia virus host range mutant (VV-hr-GFP) and showed that either protein was able to rescue the host range defect, illustrating that the F-box region, which is important for NF-B modulation and binding to SCF complex, is not required for CP77's host range function. Consistently, knocking down the protein level of NF-B did not relieve the growth restriction of VV-hr-GFP in HeLa cells.Vaccinia virus, the prototype of the poxvirus family, infects a wide range of cells in vitro and animal species in vivo (14). Vaccinia virus has a double-stranded DNA genome that encodes 263 open reading frames (ORFs). Vaccinia virus expresses different classes of viral genes in a cascade-regulated manner and completes the virus life cycle in the cytoplasm of infected cells (11).To replicate successfully in infected hosts, poxviruses have evolved various strategies to overcome cellular immune responses (20, 39). Viral infections activate cellular antiviral signaling and inflammatory responses (49), such as NF-B, which plays a critical role in inflammatory signaling and immune activation (23). NF-B contains five different members, NF-B1 (p50/p105), NF-B2 (p52/p100), RelA (p65), RelB, and c-Rel, all of which share a Rel homology domain for DNA binding, dimerization, and interaction with IB (22, 23). The most abundant activated form consists of a p50 or p52 subunit and a p65 subunit (16,26). In the inactive state, dimerized NF-B (such as p65/p50) is bound by IB␣, and the crystal structure of the IB␣/p65/p50 complex shows multiple contact sites between the ankyrin repeats of IB␣ and NF-B (29). In well-characterized canonical NF-B signaling, such as tumor necrosis factor alpha (TNF-␣) treatment, receptor activation sends intracellular sig...

SUMMARY Typhoid toxin is an essential virulence factor of Salmonella Typhi, the cause of typhoid fever. Typhoid toxin is secreted into the lumen of Salmonella-containing vacuole (SCVs), after which it is packaged into vesicle carrier intermediates and released extracellularly through incompletely understood mechanisms. Following export, the toxin targets cells by interacting with human-specific Neu5Ac-terminated glycan receptors. We show that typhoid toxin is sorted from the SCV into vesicle carrier intermediates via interactions of its B subunit, PltB, with specific lumenal sialylated glycan packaging receptors. Cells deficient in N-glycosylation or the synthesis of specific gangliosides, or displaying Neu5Gc-terminated as opposed to Neu5Ac-terminated glycans do not support typhoid toxin export. Additionally, typhoid toxin packaging requires the specific SCV environment as toxin produced by a S. Typhi mutant with impaired trafficking is not properly sorted into vesicles. These results reveal how the exotoxin of an intracellular pathogen engages host pathways for packaging and release.