The Ebola virus envelope glycoprotein (GP) derived from the pathogenic Zaire subtype mediates cell rounding and detachment from the extracellular matrix in 293T cells. In this study we provide evidence that GPs from the other pathogenic subtypes, Sudan and Côte d'Ivoire, as well as from Reston, a strain thought to be nonpathogenic in humans, also induced cell rounding, albeit at lower levels than Zaire GP. Sequential removal of regions of potential O-linked glycosylation at the C terminus of GP1 led to a step-wise reduction in cell detachment without obviously affecting GP function, suggesting that such modifications are involved in inducing the detachment phenotype. While causing cell rounding and detachment in 293T cells, Ebola virus GP did not cause an increase in cell death. Indeed, following transient expression of GP, cells were able to readhere and continue to divide. Also, the rounding effect was not limited to 293T cells. Replication-deficient adenovirus vectors expressing Ebola virus GP induced the loss of cell adhesion in a range of cell lines and primary cell types, including those with proposed relevance to Ebola virus infection in vivo, such as endothelial cells and macrophages. In both transfected 293T and adenovirus-infected Vero cells, a reduction in cell surface expression of adhesion molecules such as integrin ␤1 concurrent with the loss of cell adhesion was observed. A number of other cell surface molecules, however, including major histocompatibility complex class I and the epidermal growth factor receptor, were also down-modulated, suggesting a global mechanism for surface molecule down-regulation.

Studies analyzing Ebola virus replication have been severely hampered by the extreme pathogenicity of this virus. To permit analysis of the host range and function of the Ebola virus glycoprotein (Ebo-GP), we have developed a system for pseudotyping these glycoproteins into murine leukemia virus (MLV). This pseudotyped virus, MLV(Ebola), can be readily concentrated to titers which exceed 5 × 106 infectious units/ml and is effectively neutralized by antibodies specific for Ebo-GP. Analysis of MLV(Ebola) infection revealed that the host range conferred by Ebo-GP is very broad, extending to cells of a variety of species. Notably, all lymphoid cell lines tested were completely resistant to infection; we speculate that this is due to the absence of a cellular receptor for Ebo-GP on B and T cells. The generation of high-titer MLV(Ebola) pseudotypes will be useful for the analysis of immune responses to Ebola virus infection, development of neutralizing antibodies, analysis of glycoprotein function, and isolation of the cellular receptor(s) for the Ebola virus.

Proteolytic processing is required for the activation of numerous viral glycoproteins. Here we show that the envelope glycoprotein from the Zaire strain of Ebola virus (Ebo-GP) is proteolytically processed into two subunits, GP1 and GP2, that are likely covalently associated through a disulfide linkage. Murine leukemia virions pseudotyped with Ebo-GP contain almost exclusively processed glycoprotein, indicating that this is the mature form of Ebo-GP. Mutational analysis identified a dibasic motif, reminiscent of furin-like protease processing sites, as the Ebo-GP cleavage site. However, analysis of Ebo-GP processing in LoVo cells that lack the proprotein convertase furin demonstrated that furin is not required for processing of Ebo-GP. In sharp contrast to other viral systems, we found that an uncleaved mutant of Ebo-GP was able to mediate infection of various cell lines as efficiently as the wild-type, proteolytically cleaved glycoprotein, indicating that cleavage is not required for the activation of Ebo-GP despite the conservation of a dibasic cleavage site in all filoviral envelope glycoproteins.

Tva is the cellular receptor for subgroup A avian leukosis and sarcoma virus (ALSV-A). The viral interaction domain of Tva is determined by a 40-residue, cysteine-rich module closely related to the ligand binding domain of the human low-density lipoprotein receptor (LDLR). In this report, we examined the role of the LDLR-like module of Tva in envelope binding and viral infection by mutational analysis. We found that the entire LDLR module in Tva is essential for efficient binding to the viral envelope protein. However, the 17 N-terminal residues of this module can be deleted without affecting receptor function, suggesting that the major determinants for viral entry are located at the C terminus of the module. The effect on viral infection of many amino acid substitutions and deletions in the LDLR module is context dependent, suggesting that the residues important for viral entry are dispersed throughout the LDLR module. In addition, we found that all 27 mutations at residues D46, E47, and W48 greatly reduced envelope binding. These results are discussed in relation to a recently elucidated structure for an LDLR module.

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