Figure 4. The S54N^Vβ Mutation Modulates the Conformation of the Invariant Residue Glu56 to Facilitate an Extensive Water-Mediated Hydrogen Bonding Network with SEC3.

(A) An intramolecular hydrogen bonding interaction between S54^Vβ and E56^Vβ prevents interactions and intermolecular contacts with SEC3.

(B) The analogous region of the CDR2 loop of wild-type apo Vβ.

(C) The mutation S54N^Vβ extends both N54^Vβ and E56^Vβ side chains toward SEC3, resulting in the recruitment of numerous ordered water molecules to the molecular interface.

(D) The relative conformations of the N54^Vβ and E56^Vβ side chains are ordered prior to binding SEC3, as seen in all apo Vβ variants containing the A52V^Vβ mutation for which we have determined crystal structure. Hydrogen bonds are shown as black, dotted lines.