Bovine Leukemia Virus SU Protein Interacts with Zinc, and Mutations within Two Interacting Regions Differently Affect Viral Fusion and Infectivity In Vivo

Abstract

Bovine leukemia virus (BLV) and human T-cell lymphotropic virus type 1 (HTLV-1) belong to the genus of deltaretroviruses. Their entry into the host cell is supposed to be mediated by interactions of the extracellular (SU) envelope glycoproteins with cellular receptors. To gain insight into the mechanisms governing this process, we investigated the ability of SU proteins to interact with specific ligands. In particular, by affinity chromatography, we have shown that BLV SU protein specifically interacted with zinc ions. To identify the protein domains involved in binding, 16 peptides distributed along the sequence were tested. Two of them appeared to be able to interact with zinc. To unravel the role of these SU regions in the biology of the virus, mutations were introduced into the env gene of a BLV molecular clone in order to modify residues potentially interacting with zinc. The fusogenic capacity of envelope mutated within the first zinc-binding region (104 to 123) was completely abolished. Furthermore, the integrity of this domain was also required for in vivo infectivity. In contrast, mutations within the second zinc-binding region (218 to 237) did not hamper the fusogenic capacity; indeed, the syncytia were even larger. In sheep, mutations in region 218 to 237 did not alter infectivity or viral spread. Finally, we demonstrated that the envelope of the related HTLV-1 was also able to bind zinc. Interestingly, zinc ions were found to be associated with the receptor-binding domain (RBD) of Friend murine leukemia virus (Fr-MLV) SU glycoprotein, further supporting their relevance in SU structure. Based on the sequence similarities shared with the Fr-MLV RBD, whose three-dimensional structure has been experimentally determined, we located the BLV zinc-binding peptide 104-123 on the opposite side of the potential receptor-binding surface. This observation supports the hypothesis that zinc ions could mediate interactions of the SU RBD either with the C-terminal part of SU, thereby contributing to the SU structural integrity, or with a partner(s) different from the receptor.Bovine leukemia virus (BLV) and human T-cell lymphotropic virus type 1 (HTLV-1) belong to the genus of deltaretroviruses (70). HTLV-1 is the etiologic agent of adult T-cell leukemia (40,58,76,83) and HTLV-1-associated myelopathy or tropical spastic paraparesis (36,43,56). BLV causes enzootic bovine leukemia, a chronic and contagious disease that evolves over an extended period of time, with tumors developing in a small number of infected cattle but in the majority of infected sheep (12, 46, 52, 54). One of the main interests of the BLV system is that it allows direct experimentation in cattle and sheep as a model for HTLV-1-induced pathogenesis (77,78).Like other retroviruses, HTLV-1 and BLV enter the host cell through interactions of their envelope glycoproteins with specific cellular receptors, a process leading to fusion of the virus and cell membranes and followed by delivery of the viral genome into the cytoplasm. To date, these receptors r...

Genetic variation of bovine leukemia virus (BLV) after replication in cell culture and experimental animals

“…Experimental variant gp51 sequences showed some amino acid substitutions compared to the original wild strains, with these sequences being mainly located in the second neutralizing domain (Figure 3), which is related to T-cell proliferation in infected cows (Callebaut et al, 1993). The amino acid substitution N107D observed in both the rabbit and CC81 strains is located in a region associated with receptor binding (Gatot et al, 2002), and might influence virus fusion and infectivity. These amino acid substitutions might cause antigenic differences, reducing the efficiency of diagnostic methods and failure to detect infected animals, mainly through serological techniques (Fechner et al, 1997).…”

Molecular characterization of the env gene from Brazilian field isolates of Bovine Leukemia Virus

“…This region may be critical for the interaction with the cell receptor and for infection [36]. There was only one aa substitution (385, P fi S) in the highly immunogenic gp30 protein epitope GD21 (aa 351-398).…”

Mutation of a Single Envelope N-Linked Glycosylation Site Enhances the Pathogenicity of Bovine Leukemia Virus

“…Proline residues are shown as black stars, glycine residues are shown as black diamonds, threonine residues are shown as empty squares, and serine residues are shown as squares containing a black dot in the center. The regular secondary structures and variable regions of the N-terminal portion of the protein were deduced by comparison with the SU protein of the murine virus Fr-MLV, whose 3D structure has been solved (61). The predicted ␤-strands of the C-terminal part are shown in gray.…”