Although the interaction of pr with DENV can clearly prevent virus-membrane interaction and fusion (this study and [35] ), the importance of pr in protecting DENV during exocytic transport has not been defined. The binding interface between prM and E contains three complementary electrostatic patches containing 11 residues [34] (see also Fig. S1 ). Sequence analysis shows that these 11 residues (Fig. 5A , numbered residues) are highly conserved among the 4 DENV serotypes, and that D63 and D65 of pr, and the complementary H244 on E protein are conserved among all reported flavivirus sequences [34] . Optimal pr-E binding in vitro occurred at ,pH 6.25 (Fig. 2) , suggesting that protonation of H244 could be involved in this pH-dependence. To test this we substituted alanine for H244 in the DI/II protein. DI/II H244A was produced in highly purified form with electrophoretic mobility similar to that of the wild type (WT) protein in reducing and nonreducing SDS-PAGE (Fig. 1C) .