Opening image: The tailspike protein of Salmonella phage P22. The 150-Å-long molecule is a part of the apparatus by which the phage attaches to the bacterial host. Each subunit of the homo-trimer contains a large β helix.

Each subunit consists of 666 amino-acid residues which can be divided into three domains.

Toggle one of the three subunits. Then, color the domains; the N-terminal domain is shown in blue, the central domain in purple, and the C-terminal domain in red.

The three N-terminal domains form the "N-terminal cap." This structure caps a central domain containing three β-helices (one helix per subunit).

Spacefill the central domains.

Now complete the assembly with the C-terminal domains.

One of the central domains is isolated, containing a large β-helix. Let's reassemble the β-helix sheet by sheet:

The first sheet.

Next, the second sheet.

Finally, the third sheet.

Take a moment to sight down the axis of the β-helix before advancing.

Packing of the β helices into a three-helix bundle. Toggle spacefill.

Return to the complete tailspike protein.