Alanine-insertion mutagenesis of the TM regions of prM protein: effect on SP formation. Recombinant SPs secreted from Sf9 cells co-infected with with Bac-E and (A) Bac-prM; (B) Bac-prM A139; (C) Bac-prM A143; (D) Bac-prM A147; (E) Bac-prM A157; (F) Bac-prM A161, and (G) Bac-prM A165 were quantified by ELISA. The size and shape of the secreted recombinant SP were observed by TEM using immuno-gold labeling with (H) mAb E3.3 and (I) and mAb 5B1. (J) The total E protein in Sf9 cells was determined in the presence of Brefeldin A, a fungal a fungal macrolide antibiotic which blocks E protein secretion. GAPDH was taken as an internal loading control.