Crystal structures of various HN ectodomain conformations.

<p>A) Cartoon representation of the NDV HN crystal structure (PDB ID: 3TIE) in the “4-heads-down” conformation. Active site residues (E400, R415, and Y525) are shown as yellow, blue and red spheres. The locations of the second sialic acid binding sites are not shown. B) Model of the “4-heads-up” conformation of PIV5 HN comprised of separate previously solved crystal structures of the head domain (PDB ID: 1Z4X) and the 4HB stalk (PDB ID: 3TSI). Sialyllactose in the active site is shown as yellow and red spheres. Ordered glycan chains are shown as sticks. Coloring as in A. C–E) Front, side, and top views, respectively, of the “2-heads-up/2-heads-down” conformation revealed by the PIV5-HN ectodomain crystal structure. A sulfate ion coordinated by R405, R495, and Y523 in each active site, which partially overlaps with sialyllactose in the head domain of the 1Z4X structure, is shown as orange and red spheres. Ordered glycan chains are shown as sticks. Coloring as in A–B. F) Sigma-A weighted 2Fo-Fc electron density map rendered at 1.0 sigma showing the fit of the final PIV5-HN ectodomain model. D105 and E108, project charged sidechains toward the central 4HB axis and are marked with an asterisk. The region shown is the continuous density that links the stalk of chain B to its globular head domain. The PIV5-HN ectodomain structure was used to position head domains relative to the stalk in the 4-heads-up model in B.