Strength of Envelope Protein Interaction Modulates Cytopathicity of Measles Virus

Abstract

To understand the molecular determinants of measles virus (MV) cytopathicity, we have characterized mutant viruses exhibiting a more-extensive cell-to-cell fusion while maintaining efficient replication to high titers. A virus which is modified by the addition of an 8-amino-acid Flag epitope tag at the cytoplasmic tail of its H (for MV hemagglutinin) envelope glycoprotein replicates efficiently, has an increased cytopathicity, possesses a greater infectivity per particle, and has an altered protein composition compared with that of unmodified MV. The mutant phenotype is not specifically linked to the epitope sequence, since an alternatively added HA (for influenza virus-derived hemagglutinin) epitope tag caused similar effects. We demonstrate that both epitope tags weaken the interaction between the H and fusion (F) glycoproteins in virus-infected cells. This reduction in strength of H/F interaction is independent of the presence of the viral matrix (M) protein. Viruses with this less stable complex are more sensitive to neutralization by a soluble octameric form of the CD46 receptor, consistent with their increased fusogenicity. Similar analyses of glycoproteins derived from MV strains with reduced cytopathicities confirm that the strength of H and F glycoprotein interaction is a modulator of viral fusogenicity.Enveloped viruses that enter target cells through membrane fusion at neutral pH spread through a cell culture both by producing infectious particles and by lateral cell-to-cell fusion. The latter mechanism occurs through binding of viral envelope glycoproteins present at the infected cell surface to a viral receptor(s) on surrounding uninfected cells, thereby inducing membrane fusion, recruiting many cells into a multinucleated syncytium which ultimately dies (16).The determinants of viral infectivity and the balance between lateral spread and release are complex and incompletely understood. Syncytium formation may be important for viral pathogenesis in vivo. For measles virus (MV), a clinically relevant member of the Paramyxoviridae, syncytia have been reported in cells from infected humans (27), primates (29), and transgenic mice (19) expressing one of the virus receptors, CD46 (7, 21). However, syncytia are not produced in every MV-infected tissue, and some primary MV isolates are poorly or nonfusogenic in many cell lines (35). Repeated passaging of the MV-Edmonston (MV-Edm) original isolate on chicken embryo fibroblasts provided the basis for the generation of the MV-Edm B line and the currently used vaccine strains (8, 22, 28), which are no longer pathogenic in vivo. Further adaptation by laboratory passage on cultured primate cells often results in the selection of more fusogenic variants.Recombinant MVs lacking matrix (M) or the cytoplasmic tail of the fusion (F) protein, mutations found in MV variants causing the rare and fatal complication, subacute sclerosing panencephalitis (17), spread more extensively by cell-to-cell fusion but achieve a significantly lower titer, particularly of released vi...

Biochemical, Conformational, and Immunogenic Analysis of Soluble Trimeric Forms of Henipavirus Fusion Glycoproteins

“…The first example is described as a clamp or dissociation model, which adheres to the premise that the attachment glycoprotein restrains F in a nonfusogenic or metastable conformation and that upon receptor engagement the attachment protein dissociates from F, initiating the conformational changes in F leading to its postfusion configuration and in so doing facilitates the membrane merger process. The clamp or dissociation model is largely supported with data from an extensive analysis of measles virus (MeV) (24,60) and also the henipaviruses (1, 7), whereby membrane fusion activity is inversely correlated with the F to H or G avidity. In contrast, a second alternative association, or provocateur, model is suggested, which implies that the F association with its attachment glycoprotein partner, as in the case of HN, forms a complex at the cell surface only in the presence of receptor, and this scenario is supported by data showing that mutations which alter receptor binding decrease both membrane fusion activity and the F-HN interaction.…”

Identification of a Region in the Stalk Domain of the Nipah Virus Receptor Binding Protein That Is Critical for Fusion Activation

“…The role of the receptor binding protein has been considered to be mainly repressive for viruses such as measles virus and NiV, where clamping the F protein prevents it from assuming a postfusion structure before the virus engages its proteinaceous receptor (10,12,36,47,(57)(58)(59)(60)(61). In the case of sialic acid binding receptor binding paramyxoviruses, such as HPIV and NDV, en- viruses engage receptor, the receptor binding proteins stabilize the F protein to prevent premature activation, and the switch to an active role in triggering the F protein to fuse occurs upon receptor engagement (17).…”

CD46- and CD150-independent endothelial cell infection with wild-type measles viruses

“…Various evidence has been accumulated indicating that subtle differences in the envelope proteins of MV can play a role in altering the tropism, virus uptake, cell-to-cell fusion and pathogenicity (Bartz et al, 1996;Bieback et al, 2002;Hsu et al, 1998;Johnston et al, 1999;Lecouturier et al, 1996;Moeller et al, 2001;Moll et al, 2001;Ohgimoto et al, 2001;Plemper et al, 2002;Shibahara et al, 1994;Takeuchi et al, 2002). To investigate the molecular basis for the differential spread of MV wild-types in cultures of HUVECs, we will further analyse the sequences of the envelope genes of the MV wildtypes and intend functional studies with corresponding recombinant viruses.…”