ROLE OF THE CARBOXY-TERMINAL PHENYLALANINE IN THE BIOGENESIS OF OUTER-MEMBRANE PROTEIN PHOE OF ESCHERICHIA-COLI K-12

Most bacterial outer membrane proteins contain a phenylalanine at thei r C terminus. It has been shown that this residue has an important rol e in the efficient and correct assembly of PhoE protein into the Esche richia coli outer membrane, since its substitution or deletion resulte d in the accumulation of trypsin-sensitive monomers of this normally t rimeric protein. Here, the role of the C-terminal Phe in the assembly of PhoE was studied in further detail. Immunocytochemical labelling on ultrathin cryosections revealed that a mutant PhoE protein that lacks the C-terminal Phe accumulates in the periplasm. However, when the ex pression levels of the altered species were reduced, the efficiency of outer membrane incorporation was increased and the lethal effects wer e alleviated. The role of the C-terminal Phe in protein folding, trime rization and outer membrane incorporation was further studied in vitro . Deletion of this residue interfered with the efficiency of the forma tion of an assembly-competent folded monomer, and the stability of thi s PhoE form was affected. The in vitro trimerization and insertion int o outer membranes were not affected by the mutation. (C) 1997 Academic Press Limited.