Mutations affecting localization of an Escherichia coli outer membrane protein, the bacteriophage λ receptor

Mutants have been isolated in which the cellular location of the phage λ receptor protein, an Escherichia coli outer membrane protein, is altered. The mutations were selected for in a strain containing a lamB-lacZ‡ gene fusion. The fused gene codes for a hybrid protein comprised of λ receptor sequences at its aminoterminal end and active β-galactosidase at its carboxy-terminal end. In the parent strain the hybrid protein is localized, at least in part, to the outer membrane of the cell. The mutations described here cause the hybrid protein to be located in the cytoplasm. These mutations (both point mutations and deletions) have been found to lie very early in the lamB gene. When these mutations are incorporated into an otherwise wild-type lamB gene, most also prevent export of λ receptor and cause, instead, the precursor form of the λ receptor protein to accumulate in the cell cytoplasm. It is suggested that this group of mutations leads to alterations in the λ receptor signal sequence and thereby prevents export of this protein. For reasons that are as yet unclear, another group of mutants that prevent export of the hybrid protein does not drastically affect localization of the λ receptor protein. Also described is an unusual deletion mutation that was obtained by a similar selection procedure. This deletion results in the fusion of malK (promoter proximal to lamB) to lamB, which in turn is fused to lacZ. This double fusion codes for a MalK-LamB-LacZ tribrid protein that exhibits both MalK and LacZ activities and is localized almost exclusively to the inner membrane of the cell.