Following in-gel trypsin digestion, each of the eight protein samples was analyzed by LC-MS/MS. Bioinformatic analysis of the mass spectrometry data led to the identification of 127 proteins using the Ae. aegypti genome database in a Sequest-based search (Table S1 ). Gene Ontology (GO) analysis revealed that ,30% of Figure 2 . Quantitation of whorl size and number in unfed and amino acid fed mosquitoes. Whorls with a minimum of five stacked ER membranes were counted and the total area of each whorl was determined using NIH Image software. Data are shown for whorl size and number in 20 random EM fields covering 3335 mm 2 of mosquito midgut epithelial cell area. A) Size and number of RER whorls in 20 random fields from unfed and amino acid fed mosquitoes after 30 min. and 120 min. B) Total whorl in the same sample as shown in A. Statistical analysis using Pearson Chisquare test revealed that the total whorl area in amino acid fed mosquitoes at both 30 min and 120 min post-feeding was significantly less than the total whorl area in unfed mosquitoes (bars with different letters signify significance at p,0.001 comparing 30 min and 120 min to unfed mosquitoes). doi:10.1371/journal.pone.0018150.g002 the identified proteins could be assigned to microsomes (including ribosomal proteins and protein synthesis related proteins), ,19% were mitochondrial proteins, ,37% were cytoplasmic proteins, and ,14% were from other cellular fractions, including the nucleus. Eight of the microsomal proteins that were present in the protein sample from unfed mosquitoes, were chosen as candidate whorl-associated proteins based on their known role in ER or Golgi processes (Table 1 ). Based on peptide frequency, the most often represented protein was SND1 (Staphylococcal nuclease domain-containing protein 1), which is also known as Tudor domain-containing protein 11 [27] . Although SND1 is assigned to the Golgi apparatus cellular component by GO analysis, its primary function is gene regulation and RNA processing [28] . The peptide frequency of SND1 was similar in protein samples from unfed and fed mosquitoes, even though EM analysis revealed that the majority of RER whorl unwinding had already occurred by 30 min post amino acid feeding ( Figure 1C ). This result suggests that SND1 is most likely not associated with whorl maintenance since its presence in microsomal fractions is independent of whorl formation.