DISSECTION OF A RETROVIRUS ENVELOPE PROTEIN REVEALS STRUCTURAL SIMILARITY TO INFLUENZA HEMAGGLUTININ

Background: The amino-acid sequences of retroviral envelope proteins c ontain a '4-3 hydrophobic repeat', with hydrophobic amino acids spaced every four and then every three residues, characteristic of sequences that form coiled coils. The 4-3 hydrophobic repeat is located in the transmembrane subunit (TM) of the retroviral envelope protein, adjacen t to the fusion peptide, a region that inserts into the host bilayer d uring the membrane-fusion process. A 4-3 hydrophobic repeat region in an analogous position of the influenza hemagglutinin protein is recrui ted to extend a three-stranded coiled coil during the conformational c hange to the fusion-competent state. To determine the conformation of the retroviral TM subunit and the role of the 4-3 hydrophobic repeat, we constructed soluble peptide models of the envelope protein of Molon ey murine leukemia virus (MMLV). Results: The region of the MMLV TM pr otein external to the lipid envelope (the ectodomain) contains a stabl y folded, trimeric, protease-resistant core. As predicted, an alpha-he lical segment spans the 4-3 repeat. A cysteine-rich region carboxy-ter minal to the 4-3 repeat confers a dramatic increase in stability and d isplays a unique disulfide bonding pattern. Conclusions: Our results d emonstrate that the MMLV TM subunit can fold into a stable and distinc t species in the absence of the receptor-binding 'surface' co-subunit (SU) of the envelope complex. As the SU subunit is readily shed fi-om the surface of the virus, we conclude that the TM subunit structure fo rms the core of the MMLV membrane-fusion machinery, and that this stru cture, like the fusion-active conformation of influenza hemagglutinin, contains a three-stranded coiled coil adjacent to the fusion peptide.