Conservative mutations in the immunosuppressive region of the bovine leukemia virus transmembrane protein affect fusion but not infectivity in vivo.
Many retroviruses, including bovine leukemia virus (BLV), contain a highly conserved region located about 40 amino acids downstream from the fusion peptide within the sequence of the external domain of the transmembrane (TM) protein. This region is notably thought to be involved in the presentation of the NH2-terminal peptide to allow cell fusion. By using hydrophobic cluster analysis and by analogy with the influenza A hemagglutinin structures, the core of the TM structure including this particular region was predicted to consist, in the BLV and other retroviral envelope proteins, of an alpha-helix followed by a loop region, both docked against a subsequent alpha-helix that forms a triple-stranded coiled coil. The loop region could undergo, as in hemagglutinin, a major refolding into an alpha-helix integrating the coiled coil structure and putting the fusion peptide to one tip of the molecule. Based on this model, we have identified amino acids that may be essential to the BLV TM structure, and a series of mutations were introduced in the BLV env gene of an infectious molecular clone. A first series of mutations was designed to disturb the coiled coil structure (substitutions with proline residues), whereas others would maintain the general TM structure. When expressed by Semliki Forest virus recombinants, all the mutated envelope proteins were stable and efficiently synthesized in baby hamster kidney cells. Both proline-substituted and conservative mutants were strongly affected in their capacity to fuse to CC81 indicator cells. In addition, it appeared that the integrity of the TM coiled coil structure is essential for envelope protein multimerization, as analyzed by metrizamide gradient centrifugation. Finally, to gain insight into the role of this coiled coil in the infectious potential of BLV in vivo, the mutated TM genes were introduced in an infectious and pathogenic molecular clone and injected into sheep. It appeared that only the conservative mutations (A60V and A64S) allowed maintenance of viral infectivity in vivo. Since these mutations destroyed the ability to induce syncytia, we conclude that efficient fusion capacity of the recombinant envelopes is not a prerequisite for the infectious potential of BLV in vivo. Viral propagation of these mutants was strongly affected in some of the infected sheep. However, the proviral loads within half of the infected animals (2 out of 2 for A60V and 1 out of 4 for A64S) were close to the wild-type levels. In these sheep, it thus appears that the A60V and A64S mutants propagate efficiently despite being unable to induce syncytia in cell culture.