9] Proteins forming large channels from bacterial and mitochondrial outer membranes: Porins and phage lambda receptor protein

Abstract

This chapter focuses on the proteins that form large channels across the membrane—porins. Diffusion through the porin channel is too slow for solutes of certain type. In this case, the bacteria may produce a specific diffusion channel—lambda receptor protein (LamB protein)—for one or more of these solutes. The chapter describes the purification and in vitro reconstitution of lambda receptor protein into lipid bilayer membranes. Properties of porins from Escherichia coli—la , coded by the ompF gene and lb coded by the ompC gene—and Salmonella typhimurium are discussed in this chapter. They are all acidic proteins and contain a very large proportion of β-sheet structure. They form trimers that are stable even in sodium dodecyl sulfate (SDS); the trimers are associated with the underlying peptidoglycan sheets through noncovalent interactions. Unless E. coli B or its derivative is used as the starting material, the outer membrane contains multiple species of porin, and separation of one porin from another is difficult. The best strategy is to start from mutant strains that produce only one species of porin: such strains are available both in E. coli K l2 and in Salmonella typhimurium LT2.