Human T-cell leukaemia virus types-1 (HTLV-1) and bovine leukaemia virus (BLV) are divergent blood borne viruses that cause hematological malignancies in humans and cattle respectively. In common with other enveloped viruses, infection of cells by HTLV-1 and BLV is dependent on the membrane fusion properties of the viral envelope glycoproteins. Here we have solved the crystal structure of the BLV transmembrane glycoprotein, and, through a functional and comparative analysis with HTLV-1, we have identified features that are critical to fusion protein function. In particular, we demonstrate that electrostatic interactions with small ions dramatically stabilize the assembly and fusion-associated forms of the BLV TM, but are not required for the cell surface display of native prefusogenic envelope. Moreover, we show that charged residues that border a deep hydrophobic pocket contribute directly to appropriate folding of fusion-active envelope and are critical to membrane fusion. Importantly, the charged residues that border the pocket are key features that determine the specificity and activity of peptide inhibitors of envelope function. Our study demonstrates that charge-surrounded pockets and electrostatic interactions with small ions are significant leitmotifs of diverse class-1 fusion proteins and that these elements represent ideal targets for novel small-molecule inhibitors of viral entry. binds (Fig. 3A ). In the context of envelope the T353V substitution, which replaces threonine with a non-polar residue of equivalent size, completely abrogates envelope-mediated membrane fusion (Fig. 2C) . Moreover, the introduction of the T353V substitution into the pMBP-BLVhairpin vector yields a recombinant protein that completely fails to trimerise (Fig. 3E) . Interestingly, the data demonstrate that substitutions at Thr353 do not affect expression, processing, or surface expression of native envelope but severely impair the fusogenic activity of envelope by preventing assembly of the trimer-of-hairpins. Our data therefore provides further evidence that the trimeric coiled coil likely forms during the fusion process.