Outer Membrane Proteins: Old and New

Abstract

The outer membrane of Escherichia coli has been studied extensively for almost two decades and much is now known about the composition, structure, function, immunology, biosynthesis and regulation of the proteins which contribute to this outer protective layer (for reviews see refs 1–10). It is known to contain a number of major protein species, several of which e.g. the OmpF, OmpC, PhoE, LamB, Tsx proteins and Protein K are porins involved in the passive accumulation of small hydrophilic solutes. Others, like the iron-regulated outer membrane proteins (the fepA, fhuA, fhuE and fecA gene products) and the vitamin Bi2 receptor (the BtuB protein) are involved in the TonB-dependent uptake of more specific solutes (11–13). Other major proteins appear to play a structural role for the cell envelope. Falling into this category are proteins such as the Braun lipoprotein (the Lpp protein), peptidoglycan-associated lipoprotein (PAL) and the ompA gene product. Several other minor or less well characterized proteins, such as phospholipase A, the OmpT protein (a trypsin-like protease), signal peptidase, the tolC gene product (possibly involved in protein processing). Protein III, and an 83-kilodalton (kDa) iron regulated protein have also been identified (2, 4, 11).