Figure 3. NS1 tubular structure.

a. A thin slice of the crystal packing of H5N1 NS1 molecules (orange) as viewed along the crystallographic c-axis. The crystallographic unit cell (solid black diamond) along with the a and b axes are shown for reference. One of the tubular structures formed by the interaction of three NS1 chains related by the crystallographic 3₁-screw axis (perpendicular to the plane of the paper) is shown surrounded by a broken circle. b. NS1 tubular structure (diameter of ~65 Å) as viewed down its long axis showing the ~20 Å wide tunnel. Each of the three chains is colored differently in green, orange and light blue. The residues 38 and 41 critical for dsRNA binding¹⁶ are shown as blue spheres, demonstrating their location within the central tunnel, the residues critical to CPSF binding¹⁴,¹⁵,¹⁹ are colored red. c. A surface representation of the tubular structure (diameter ~65 Å) viewed across its long axis. The CSPF binding sites colored in red are surface exposed. Each chain is colored as in panel b. d. A cryo-electron micrograph of native H1N1 NS1 in the presence of dsRNA. Long tubular structures of diameter ~70 Å can be clearly visualized. Scale bar, ~500 Å.