Human metapneumovirus (HMPV) is a major causative agent of severe bronchiolitis and pneumonia. Its fusion (F) protein must be cleaved by host proteases to cause membrane fusion, a critical step for virus infection. By generating Vero cells constitutively expressing the transmembrane serine protease TMPRSS2 and green fluorescent protein-expressing recombinant HMPV, we show that TMPRSS2, which is expressed in the human lung epithelium, cleaves the HMPV F protein efficiently and supports HMPV multiplication. The results indicate that TMPRSS2 is a possible candidate protease involved in the development of lower respiratory tract illness in HMPV-infected patients.

Measles is an acute febrile infectious disease with high morbidity and mortality. The genome of measles virus (MV), the causative agent, encodes two accessory products, V and C proteins, that play important roles in MV virulence. The V but not the C protein of the IC-B strain (a well-characterized virulent strain of MV) has been shown to block the Jak/Stat signaling pathway and counteract the cellular interferon (IFN) response. We have recently shown that a recombinant IC-B strain that lacks C protein expression replicates poorly in certain cell lines, and its growth defect is related to translational inhibition and strong IFN induction. Here, we show that the V protein of the MV IC-B strain also blocks the IFN induction pathway mediated by the melanoma differentiation-associated gene 5 product, thus actively interfering with the host IFN response at two different steps. On the other hand, the C protein per se possesses no activity to block the IFN induction pathway. Our data indicate that the C protein acts as a regulator of viral RNA synthesis, thereby acting indirectly to suppress IFN induction. Since recombinant MVs with C protein defective in modulating viral RNA synthesis or lacking C protein expression strongly stimulate IFN production, in spite of V protein production, both the C and V proteins must be required for MV to fully circumvent the host IFN response.Measles is an acute febrile infectious disease with high morbidity and mortality and accounts for ϳ4% of deaths in children aged Ͻ5 years worldwide (5). Measles virus (MV), the causative agent, belongs to the genus Morbillivirus of the family Paramyxoviridae and has a nonsegmented negative-strand RNA genome of ϳ16 kb in length. The genome has six genes that encode the phosphoprotein (P) and the nucleocapsid (N), matrix (M), fusion (F), hemagglutinin (H), and large (L) proteins (24). The P gene encodes two additional proteins, V and C, by a process of RNA editing and an alternative translational initiation in a different reading frame, respectively (4, 9). The V and C proteins are nonessential products (58, 62) but play important roles in MV virulence (12,54,70,74,75). Although the V protein has been shown to counteract the cellular interferon (IFN) response (7,13,50,52,69), molecular mechanisms by which the C protein contributes to virus virulence are poorly understood. We have recently shown that a recombinant MV that lacks C protein expression (MV⌬C) replicates poorly in certain cell lines, and its growth defect is related to translational inhibition and strong IFN induction (47).Host antiviral responses are initiated by detecting pathogenassociated molecular patterns, such as cytoplasmic singlestranded RNA (ssRNA) bearing a 5Ј triphosphate and doublestranded RNA (dsRNA) (19, 71). The retinoic acid inducible gene I (RIG-I) and melanoma differentiation-associated gene 5 (mda-5) products are known to be intracellular receptors (sensors) for these virus-derived RNA molecules (32). Recently, some RNAs processed by RNase L have also been shown to be re...

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Measles is one of the most contagious human infectious diseases and remains a major cause of childhood morbidity and mortality worldwide. The signaling lymphocyte activation molecule (SLAM), also called CD150, is a cellular receptor for measles virus (MV), presumably accounting for its tropism for immune cells and its immunosuppressive properties. On the other hand, pathological studies have shown that MV also infects epithelial cells at a later stage of infection, although its mechanism has so far been unknown. In this study, we show that wild-type MV can infect and produce syncytia in human polarized epithelial cell lines independently of SLAM and CD46 (a receptor for the vaccine strains of MV). Progeny viral particles are released exclusively from the apical surface of these polarized epithelial cell lines. We have also identified amino acid residues on the MV attachment protein that are likely to interact with a putative receptor on epithelial cells. All of these residues have aromatic side chains and may form a receptor-binding pocket located in a different position from the putative SLAM-and CD46-binding sites on the MV attachment protein. Thus, our results indicate that MV has an intrinsic ability to infect both polarized epithelial and immune cells by using distinctive receptorbinding sites on the attachment protein corresponding to each of their respective receptors. The ability of MV to infect polarized epithelial cells and its exclusive release from the apical surface may facilitate its efficient transmission via aerosol droplets, resulting in its highly contagious nature.Measles remains a major cause of childhood morbidity and mortality worldwide despite the availability of efficacious vaccines. Measles virus (MV), an enveloped RNA virus belonging to the genus Morbillivirus in the family Paramyxoviridae, is transmitted via aerosol droplets and considered to be one of the most contagious human pathogens. MV has two envelope glycoproteins, the hemagglutinin (H) and fusion (F) proteins, which are responsible for receptor binding and membrane fusion, respectively (18). MV enters a cell by membrane fusion at the cell surface. The attachment of the H protein to a cellular receptor is believed to induce the conformational change of the H protein, as well as that of the F protein, which promotes the fusion of the viral envelope with the host cell membrane. MV also causes cell-cell fusion in susceptible cells. The signaling lymphocyte activation molecule (SLAM), also known as CD150, has been identified as a receptor for MV (10,21,47). SLAM is expressed on immune cells, such as activated lymphocytes, mature dendritic cells, and macrophages, providing a good explanation for the lymphotropism and immunosuppressive nature of MV (4, 51). Although CD46, a ubiquitously expressed complement regulatory molecule, functions as a receptor for the vaccine strains of MV (8, 30), a great majority of viruses circulating in measles patients use SLAM, but not CD46, as a receptor (35,51). A recent study of MV infection in macaq...

The genome of measles virus (MV) is encapsidated by the nucleocapsid (N) protein and associates with RNA-dependent RNA polymerase to form the ribonucleoprotein complex. The matrix (M) protein is believed to play an important role in MV assembly by linking the ribonucleoprotein complex with envelope glycoproteins. Analyses using a yeast two-hybrid system and coimmunoprecipitation in mammalian cells revealed that the M protein interacts with the N protein and that two leucine residues at the carboxyl terminus of the N protein (L523 and L524) are critical for the interaction. In MV minigenome reporter gene assays, the M protein inhibited viral RNA synthesis only when it was able to interact with the N protein. The N protein colocalized with the M protein at the plasma membrane when the proteins were coexpressed in plasmid-transfected or MV-infected cells. In contrast, the N protein formed small dots in the perinuclear area when it was expressed without the M protein, or it was incapable of interacting with the M protein. Furthermore, a recombinant MV possessing a mutant N protein incapable of interacting with the M protein grew much less efficiently than the parental virus. Since the M protein has an intrinsic ability to associate with the plasma membrane, it may retain the ribonucleoprotein complex at the plasma membrane by binding to the N protein, thereby stopping viral RNA synthesis and promoting viral particle production. Consequently, our results indicate that the M protein regulates MV RNA synthesis and assembly via its interaction with the N protein.Measles is an acute contagious disease characterized by high fever and a maculopapular rash (15). Measles virus (MV), the causative agent, is an enveloped virus classified as a member of the genus Morbillivirus in the family Paramyxoviridae. The virus has a nonsegmented negative-sense RNA genome, which contains six genes encoding single structural proteins, designated the nucleocapsid (N), phospho-(P), matrix (M), fusion (F), hemagglutinin (H), and large (L) proteins. The P gene encodes additional gene products, termed the V and C proteins, via an RNA editing process and an alternative translational initiation in a different reading frame, respectively (4, 9). The genome is encapsidated by the N protein and forms a nucleocapsid that exhibits helical symmetry. The amino-terminal region of the N protein (N CORE ; amino acids [aa] 1 to 400) constitutes the core region of the helical nucleocapsid while the remaining carboxylterminal region (N TAIL ; aa 401 to 525) is intrinsically disordered and located outside of the helical nucleocapsid core (36). A viral RNA-dependent RNA polymerase composed of the L and P proteins associates with the nucleocapsid, thereby forming the ribonucleoprotein (RNP) complex. The L protein possesses enzymatic activities that are required for nucleotide polymerization and the capping and polyadenylation of viral mRNAs while the P protein acts as an essential cofactor for the RNA-dependent RNA polymerase functions (15). The P protein direct...