Flaviviruses affect hundreds of millions of people each year causing tremendous morbidity and mortality worldwide. This genus includes significant human pathogens such as dengue, West Nile, yellow fever, tick-borne encephalitis and Japanese encephalitis virus among many others. The disease caused by these viruses can range from febrile illness to hemorrhagic fever and encephalitis. A deeper understanding of the virus life cycle is required to foster development of antivirals and vaccines, which are an urgent need for many flaviviruses, especially dengue. The focus of this review is to summarize our current knowledge of flaviviral replication and assembly, the proteins and lipids involved therein, and how these processes are coordinated for efficient virus production.

Background: A homology model of the trimeric measles virus fusion protein predicts a cavity in the base of the head. Results: Hydrophobic residues required for interactions with the hemagglutinin map to this cavity. Conclusion:The base of the measles virus fusion protein trimer head receives the signal that triggers membrane fusion. Significance: Emerging, re-emerging, and prevalent paramyxoviruses may operate based on similar signal transmission mechanisms.The measles virus (MV) fusion (F) protein trimer executes membrane fusion after receiving a signal elicited by receptor binding to the hemagglutinin (H) tetramer. Where and how this signal is received is understood neither for MV nor for other paramyxoviruses. Because only the prefusion structure of the parainfluenza virus 5 (PIV5) F-trimer is available, to study signal receipt by the MV F-trimer, we generated and energy-refined a homology model. We used two approaches to predict surface residues of the model interacting with other proteins. Both approaches measured interface propensity values for patches of residues. The second approach identified, in addition, individual residues based on the conservation of physical chemical properties among F-proteins. Altogether, about 50 candidate interactive residues were identified. Through iterative cycles of mutagenesis and functional analysis, we characterized six residues that are required specifically for signal transmission; their mutation interferes with fusion, although still allowing efficient F-protein processing and cell surface transport. One residue is located adjacent to the fusion peptide, four line a cavity in the base of the F-trimer head, while the sixth residue is located near this cavity. Hydrophobic interactions in the cavity sustain the fusion process and contacts with H. The cavity is flanked by two different subunits of the F-trimer. Tetrameric H-stalks may be lodged in apposed cavities of two F-trimers. Because these insights are based on a PIV5 homology model, the signal receipt mechanism may be conserved among paramyxoviruses.Measles virus (MV), 2 an enveloped nonsegmented negative strand RNA virus, remains a significant public health problem (1). Although targeted for eradication (2), MV still caused 139,000 worldwide deaths in 2010 (3). In addition, relaxed vaccination discipline favored recent measles re-emergence in Europe and North America, now reporting small but costly epidemics (4, 5).MV is a member of the family Paramyxoviridae that includes other deadly emerging viruses such as Hendra and Nipah and prevalent human pathogens such as mumps, parainfluenza, and respiratory syncytial viruses that still cause significant morbidity and mortality (6). Although many other enveloped viruses take advantage of low pH (7) or proteases (8) in the endosomal compartment to trigger membrane fusion, most paramyxoviruses including MV fuse directly with the plasma membrane (6, 9).An accurate mechanism must be in place to secure timely and efficient MV cell entry at the plasma membrane. It is known tha...

The measles virus (MeV) membrane fusion apparatus consists of a fusion protein trimer and an attachment protein tetramer. To trigger membrane fusion, the heads of the MeV attachment protein, hemagglutinin (H), bind cellular receptors while the 96-residue-long H stalk transmits the triggering signal. Structural and functional studies of the triggering mechanism of other paramyxoviruses suggest that receptor binding to their hemagglutinin-neuraminidase (HN) results in signal transmission through the central segments of their stalks. To gain insight into H-stalk structure and function, we individually replaced its residues with cysteine. We then assessed how stable the mutant proteins are, how efficiently they can be cross-linked by disulfide bonds, whether cross-linking results in loss of function, and, in this case, whether disulfide bond reduction restores function. While many residues in the central segment of the stalk and in the spacer segment above it can be efficiently cross-linked by engineered disulfide bonds, we report here that residues 59 to 79 cannot, suggesting that the 20 membrane-proximal residues are not engaged in a tetrameric structure. Rescue-of-function studies by disulfide bond reduction resulted in the redefinition and extension of the central fusion-activation segment as covering residues 84 to 117. In particular, we identified four residues located between positions 92 and 99, the function of which cannot be restored by disulfide bond reduction after cysteine mutagenesis. These mutant H proteins reached the cell surface as complex oligomers but could not trigger membrane fusion. We discuss these observations in the context of the stalk exposure model of membrane fusion triggering by paramyxoviruses. IMPORTANCEMeasles virus, while being targeted for eradication, still causes significant morbidity and mortality. Here, we seek to understand how it enters cells by membrane fusion. Two viral integral membrane glycoproteins (hemagglutinin tetramers and fusion protein trimers) mediate the concerted receptor recognition and membrane fusion processes. Since previous studies have suggested that the hemagglutinin stalk transmits the triggering signal to the fusion protein trimer, we completed an analysis of its structure and function by systematic Cys mutagenesis. We report that while certain residues of the central stalk segment confer specificity to the interaction with the fusion protein trimer, others are necessary to allow folding of the H-oligomer in a standard conformation conducive to fusion triggering, and still other residues sustain the conformational change that transmits the fusiontriggering signal.

The pH-independent measles virus membrane fusion process begins when the attachment protein H binds to a receptor. Knowing that the central segment of the tetrameric H stalk transmits the signal to the fusion protein trimer, we investigated how. We document that exact conservation of most residues in the 92 through 99 segment is essential for function. In addition, hydrophobic and charged residues in the 104 through 125 segment, arranged with helical periodicity, are critical for F protein interactions and signal transmission.