Fig. 4. Side-chain steric packing at PL5’s symmetric helix-helix interface.

(A) The pairwise interaction of helices, symmetrically repeated, provides the primary stabilization for PL5. (B) High geometric complementarity of interacting residues across the helix-helix interface, roughly in layers: the a/g and e/g layers. (C and D) Axial view of side-chain packing of individual layers. (E) A potential stereochemical code required for pentameric assembly. At the e/d layer, the C_α-C_β bond vector of each amino acid points outward from the helix-helix interface (β_out), whereas the C_β-C_γ bond vector faces inward (γ_in). This suggests that a heavy atom (e.g., N, C, O, S) at the gauche⁺ position is required for tight interhelical packing. (F) In the a/g layer, the opposite is true; the C_α-C_β bond vector points inward (β_in) and the C_β-C_γ bond vector faces outward (γ_out).