unusual extended signal peptide region is not required for secretion and function of an Escherichia coli autotransporter

The plasmid-encoded toxin, Pet, a prototypical member of the serine protease autotransporters of the Enterobacteriaceae, possesses an unusually long signal peptide, which can be divided into five regions termed N1 (charged), H1 (hydrophobic), N2, H2 and C (cleavage site) domains. The N1 and H1 regions correspond to a conserved N-terminal extension previously designated the extended signal peptide region (ESPR), while the N2, H2 and C regions resemble typical Sec-dependent signal sequences and exhibit considerable sequence variability. We have shown previously that the ESPR directs Sec-dependent, post-translational translocation of Pet across the bacterial inner membrane. In this study, we demonstrate that the ESPR is not essential for the secretion or the function of Pet.