An inner membrane protein N-terminal signal sequence is able to promote efficient localisation of an outer membrane protein in Escherichia coli.

To test the importance of N-terminal pre-sequences in translocation of different classes of membrane proteins, we exchanged the normal signal sequence of an Escherichia coli outer membrane protein, OmpF, for the pre-sequence of the inner membrane protein, DacA. The DacA-OmpF hybrid was efficiently assembled into the outer membrane in a functionally active form. Thus the pre-sequence of DacA, despite its relatively low hydrophobicity compared with that of OmpF, contains all the essential information necessary to initiate the translocation of OmpF to the outer membrane. Since processing of DacA was also shown to be dependent upon SecA we conclude that the initiation of translocation of this inner membrane polypeptide across the envelope occurs by the same mechanism as outer membrane and periplasmic proteins. The N-terminal 11 amino acids of mature OmpF, which in the hybrid are replaced by the N-terminal nine amino acids of DacA, carry no essential assembly signals since the hybrid protein is apparently assembled with equal efficiency to OmpF.