A right-handed parallel β-helix of 400 residues in 13 tightly packed coils is a major motif of the chains forming the trimeric P22 tailspike adhesin. The β-helix domains of three identical subunits are side-by-side in the trimer and make predominantly hydrophilic inter-subunit contacts (Steinbacher S et al., 1994, Science 265:383-386). After the 13th coil the three individual β-helices terminate and the chains wrap around each other to form three interdigitated β-sheets organized into the walls of a triangular prism. The β-strands then separate and form antiparallel β-sheets, but still defining a triangular prism in which each side is a β-sheet from a different subunit (Seckler R, 1998, J Struct Biol 122:216–222). The subunit interfaces are buried in the triangular core of the prism, which is densely packed with hydrophobic side chains from the three β-sheets. Examination of this structure reveals that its packed core maintains the same pattern of interior packing found in the left-handed β-helix, a single-chain structure. This packing is maintained in both the interdigitated parallel region of the prism and the following antiparallel sheet section. This oligomerization motif for the tailspike β-helices presumably contributes to the very high thermal and detergent stability that is a property of the native tailspike adhesin.
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