The differential effect on two hybrid proteins of deletion mutations within the hydrophobic region of the Escherichia coli OmpA signal peptide.

Oligonucleotide-directed site-specific mutagenesis was used to systematically shorten the hydrophobic region within the signal peptide of the Escherichia coli outer membrane protein OmpA. DNA encoding the wild type and mutant OmpA signal peptides were then fused in frame to DNA encoding the mature regions of Staphylococcus aureus nuclease A and TEM beta-lactamase. The ability of these signal peptides to direct processing of the resulting hybrid proteins was dependent on both their length and the protein to which they were fused. Deletion of two or more residues progressively slowed processing of pro-OmpA-nuclease. By contrast, pro-OmpA-beta-lactamase was less sensitive to the length of the hydrophobic region than to the nature of the deleted residue(s). Deletion of an Ala residue tended to reduce processing efficiency of pro-OmpA-beta-lactamase, while deletion of an Ile residue, together with the Ala residue, resulted in improvement. The loss of either 3 or 4 residues abolished processing of both hybrids. These data indicate that both the length as well as the identity of residues in the hydrophobic region are important. The relative importance of these two factors depends on the mature region of the protein being secreted.