In order to investigate mechanisms controlling RER whorl unwinding and the initiation of blood meal digestion in female Ae. aegypti mosquitoes, we performed proteomic studies using mass spectrometry and identified midgut proteins associated with microsomal fractions in unfed and amino acid fed mosquitoes. The 139 kDa alpha subunit of the COPI coatomer complex was selected as one of eight candidate RER whorl-forming proteins based on bioinformatic analyses. We used RNAi-mediated knockdown to determine if loss of alpha-COPI expression had any effect on RER whorl formation in unfed mosquitoes or blood meal digestion in fed mosquitoes. Our data show that alpha-COPI functions are indeed required for RER whorl formation in unfed mosquitoes, and moreover, that a deficiency in alpha-COPI blocked the expression of three late phase midgut proteases in Figure 1 . Amino acid feeding is sufficient to induce RER whorl unwinding. Three-day-old Ae. aegypti females were kept unfed (sugar fed), or fed an amino acid meal for 30 min., and then sacrificed and dissected 30 min. or 120 min. later. The dissected midguts were fixed for electron microscopy preparation as described in Materials and Methods and representative electron micrographs of midgut sections are shown here. A) Unfed mosquito (25,0006magnification) . B) Unfed mosquito (8,8006magnification) . C) Amino acid fed mosquito dissected at 30 min. post-feeding (8,8006 magnification). D) Amino acid fed mosquito dissected at 120 min. post-feeding (8,8006 magnification). doi:10.1371/journal.pone.0018150.g001 blood fed mosquitoes (AaSPVI, AaSPVII, AaLT). Interestingly, the early phase of blood meal digestion, which is characterized by the synthesis and secretion of the early trypsin protease (AaET), was not dependent on alpha-COPI expression.