Conformational and sequence signatures in beta helix proteins.

beta helix proteins are characterized by a repetitive fold, in which the repeating unit is a beta-helical coil formed by three strand segments linked by three loop segments. Using a data set of left- and right-handed beta helix proteins, we have examined conformational features at equivalent positions in successive coils. This has provided insights into the conformational rules that the proteins employ to fold into beta helices. Left-handed beta helices attain their equilateral prism fold by incorporating "corners" with the conformational sequence P(II)-P(II)-alpha(L)-P(II), which imposes sequence restrictions, resulting in the first and third P(II) residues often being G and a small, uncharged residue (V, A, S, T, C), respectively. Right-handed beta helices feature mid-sized loops (4, 5, or 6 residues) of conserved conformation, but not of conserved sequence; they also display an alpha-helical residue at the C-terminal end of L2 loops. Backbone conformational parameters (phi,psi) that permit the formation of continuous, loopless beta helices (Perutz nanotubes) have also been investigated.