The Genetics of Protein Secretion in Escherichia coli

The chapter discusses the methods used in gene fusion to study the export of certain E. coli envelope proteins. Much of the information obtained comes from the work done with two proteins—namely, the periplasmic maltose-binding protein (MBP, product of the malE gene) and the outer membrane bacteriophage λ receptor protein (λrec, product of the lamB gene). The synthesis of both these proteins is inducible by maltose—that is, these proteins are made in high levels only when maltose is present in the growth media. Genetic technique developed by Casadaban is used to fuse the malE and the lamB genes to a gene (lacZ) that codes for the cytoplasmic enzyme, P-galactosidase. The resulting strain produces a hybrid protein in which the amino-terminal sequence of P-galactosidase is replaced by some portion of the amino-terminal sequence of the MBP. The location of the hybrid proteins within the cell is determined by assaying various subcellular fractions for P-galactosidase activity. The chapter discusses the deleterious effects of hybrid protein synthesis on bacterial growth.