Fig 9.

Variation in epitope E alters capsid structure. Epitope E was defined by variable residues 407, 412, and 413 (406, 411, and 412 for GII.4.1987) and all residues within 8 Å of these sites, as these additional residues are likely to be impacted by the structural differences (driven by the variable sites) that contribute to MAb recognition differences. GII.4.1987 epitope E (A) differs from GII.4.2002 epitope E (B) at positions 406/407 and 355. The N-to-S difference at 406/407 likely alters the rotameric position of R411 in the GII.4.2002 VLP, allowing it to extend further from the surface. (C) GII.4.1987 (yellow) and GII.4.2002 (blue) superimposed. Because R411 is more buried in 1987, the MAb likely cannot interact as strongly with this residue in the GII.4.1987 VLP. GII.4-2002 (D) differs from GII.4-2006 (E) at five positions in the expanded epitope (positions 355 to 357, 412, and 413). (F) Superimposition suggests that R411 of GII.4.2006 (teal) is buried, and variation at 355 to 357 may alter key interactions involving H357 and D355. Red, differences between epitopes; yellow, key residues. (G) GII.4-2006/N412T (orange) superimposed upon GII.4.2006. Two potentially important residues are R411, which is more surface exposed in GII.4-2006/N412T, and T412, which is buried. (H) GII.4-2002 (blue) superimposed on GII.4-2006/N412T (orange). R411 is nearly identical, suggesting that N412T frees the R411 side chain to extend away from the surface, where it likely interacts with the MAb. Resides that regulate R411 make up site 1, and 87% of binding can be recovered with structural modifications to R411. A second site, site 2, is composed of residues 355 to 357, and particularly D355, which adds negative potential to the second site.