Diverse effects of mutations in the signal sequence on the secretion of β-lactamase in Salmonella typhimurium

Mutations in the β-lactamase structural gene that alter the signal peptide were used to study secretion into the periplasm of Salmonella typhimurium. Processing and cellular location of mutant gene products were followed by pulse-chase and cell-fractionation experiments and by trypsin accessibility in intact and lysed spheroplasts. The precursor proteins examined never appear as a free species in the periplasm. Two of the signal-sequence mutants accumulate a precursor form that is trypsin-accessible in intact spheroplasts; the precursors synthesized by the remaining mutants resemble wild-type in that they remain trypsin-inaccessible. One of the latter mutants does produce mature protein, but at a very reduced rate. It thus appears that signal-sequence mutations can affect more than one step in the secretion process, and that processing of the signal peptide is not required for the protein to be translocated (at least partially) across the inner membrane.