The in vitro interaction of pr with various truncated forms of E protein was strongly pH-dependent, with a pH optimum of ,6.25. In situ measurements indicate that the pH of the TGN is ,6 [53] , while the pH optimum of DENV2 NGC fusion is ,6.2 [41] . The low pH of the TGN is critical for the rearrangement of immature DENV to allow furin cleavage, but once the virus is processed it becomes fusion-active in this same pH range. Thus the pH dependence of the pr-E interaction appears optimized to protect DENV during its continued transit through the secretory pathway. Pr's inhibition of E membrane insertion was less efficient at a pH value (pH 5.0) similar to that in the late endocytic pathway (Fig. 3D-E) . This loss of pr inhibition at more acidic pH could help to explain the recent finding that infection by immature DENV is enhanced by antibodies to prM [42] . The antibodybound immature virus is likely to be endocytosed and processed by cellular furin in the endocytic pathway [54] . The lower pH conditions of the late endocytic pathway could then cause the loss of pr inhibition and allow virus fusion.