Different sec‐requirements for signal peptide cleavage and protein translocation in a model E. coli protein

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Abstract
We describe a secretory E. coli protein with a novel phenotype: signal peptide cleavage is largely unaffected whereas chain translocation is efficiently blocked under conditions where SecA, a central component of the secretory machinery, is rendered non‐functional, and we have traced this phenotype to the presence of a mildly hydrophobic segment located ~30 residues downstream of the signal peptide. When this segment is deleted, normal SecA‐dependent signal peptide cleavage and chain translocation is observed; when its hydrophobicity is increased, it becomes a permanent membrane anchor with cleavage of the signal peptide and membrane insertion both being SecA‐independent. These findings suggest that the initial insertion of the signal peptide across the membrane can be uncoupled from the translocation process proper.