Truncated DENV E proteins (Fig. 1) were obtained by inducible expression in Drosophila S2 cells and purified by affinity chromatography as previously described in detail [29, 30] . The H244A mutation was introduced into the DI/II protein by in vitro mutagenesis, and S2 cell expression and purification were performed as above. DENV2 NGC DIII (Fig. 1) was previously referred to as LDIIIH1CS [30] , and contains domain III, the linker between domain I and domain III, and the H1 and CS regions of the stem domain. DIII was expressed in E.coli and refolded as previously described [41] . SFV E1 DI/II protein was produced as previously described [29] . All purified proteins were stored in TAN buffer (20 mM Triethanolamine[TEA], pH 8.0; 130 mM NaCl) at 280uC.