Besides PDI, we also identified three of the seven known coatomer subunits of the COPI vesicle transport system. As shown in Table 1 , more peptides corresponding to the alpha-COPI, beta-COPI, beta'-COPI, subunits were identified in the microsomal fractions isolated from unfed mosquitoes than fed mosquitoes ( Table 1 ), indicating that endosomal membranes in these fractions were depleted of COPI subunits. Since COPI subunits are soluble proteins that transiently cycle between vesicle membrane bound and unbound forms as a function of interactions with Arf proteins [29, 30] , the observed differential abundance of COPI subunits in the two midgut microsomal fractions suggests that they could be associated with RER whorls. We tested this idea by knocking down alpha-COPI expression using RNAi to determine if this COPI coatomer subunit is required for RER whorl structures in mosquito midgut epithelial cells.