Myristoylation, a protruding loop, and structural plasticity are essential features of a nonenveloped virus fusion peptide motif

Members of the fusion-associated small transmembrane (FAST) protein family are a distinct class of membrane fusion proteins encoded by nonenveloped fusogenic reoviruses. The 125-residue p14 FAST protein of reptilian reovirus has an approximately 38-residue myristoylated N-terminal ectodomain containing a moderately apolar N-proximal region, termed the hydrophobic patch. Mutagenic analysis indicated sequence-specific elements in the N-proximal portion of the p14 hydrophobic patch affected cell-cell fusion activity, independent of overall effects on the relative hydrophobicity of the motif. Circular dichroism (CD) of a myristoylated peptide representing the majority of the p14 ectodomain suggested this region is mostly disordered in solution but assumes increased structure in an apolar environment. From NMR spectroscopic data and simulated annealing, the soluble nonmyristoylated p14 ectodomain peptide consists of an N-proximal extended loop flanked by two proline hinges. The remaining two-thirds of the ectodomain peptide structure is disordered, consistent with predictions based on CD spectra of the myristoylated peptide. The myristoylated p14 ectodomain peptide, but not a nonmyristoylated version of the same peptide nor a myristoylated scrambled peptide, mediated extensive lipid mixing in a liposome fusion assay. Based on the lipid mixing activity, structural plasticity, environmentally induced conformational changes, and kinked structures predicted for the p14 ectodomain and hydrophobic patch (all features associated with fusion peptides), we propose that the majority of the p14 ectodomain is composed of a fusion peptide motif, the first such motif dependent on myristoylation for membrane fusion activity.