Hydrophobic content and lipid interactions of wild-type and mutant OmpA signal peptides correlate with their in vivo function.

Peptides corresponding to the wild-type signal sequence of the Escherichia coli outer membrane protein OmpA and several mutants have been synthesized and characterized biophysically. The mutations were designed collaboratively with Inouye and co-workers to test the understanding of the critical characteristics of signal sequences required for their functions. The in vivo results for these mutants have been reported [Lehnhardt, S., Pollitt, S., & Inouye, M. (1987) J. Biol. Chem. 262, 1716-1719; Goldstein, J., Lehnhardt, S., & Inouye, M. (1990) J. Bacteriol. 172, 1225-1231; Goldstein, J., Lehnhardt, S., & Inouye, M. (1991) J. Biol. Chem. 266, 14413-14417], and the present paper compares the conformational and membrane-interactive properties of six of the OmpA signal peptides. Peptides corresponding to functional OmpA signal sequences in vivo are predominantly alpha-helical in membrane-mimetic environments and insert readily into phospholipid bilayers. Nonfunctional OmpA signal peptides may have high helical content but do not penetrate deeply into the acyl chain region of bilayers. The ability of the signal peptides to insert into membranes and their in vivo function correlate with the residue-average hydrophobicity of their hydrophobic cores. The results obtained on OmpA signal peptides parallel closely our previous observations on peptides corresponding to the LamB signal sequence and mutants, arguing that the critical biophysical properties of signal sequences are general despite their lack of primary sequence identity.