E9 protein efficiently inserted into membranes over a wide range of pH values from 6.25-4.5 ( Fig. 3D-E) . However, pr's inhibition of E membrane insertion was less efficient in the pH range (pH 5.0) present in the late endocytic pathway (Fig. 3D-E) . This loss of pr inhibition at more acidic pH may be relevant to recent studies of infection by immature DENV [42] , as mentioned in the discussion section below.