Han et al. identified 72 proteins in the periplasm of E. coli K-12, designated in this geneset (Han, 2013).
Han et el. identified the periplasmic osmotically-shocked proteins of both E. coli K-12 and E. coli B using mass spectrometry after separation with two-dimensional gel electrophoresis. Some proteins that were found only in the E. coli B periplasm are also found encoded in E. coli genome and are likely to be periplasmic proeins in K-12 also, but either have not yet been detected; perhaps due to problems with gene expression or export to the periplasmic.