Signal Peptide Digestion in Escherichia coli

Abstract

Upon incubation of the envelope fraction of E. coli a precursor of the major outer membrane lipoprotein that accumulates in the cytoplasmic membrane of the globomycin-treated cell is processed to the mature form. When this precursor-containing envelope fraction was incubated in the presence of protease inhibitors such as antipain, leupeptin, chymostatin and elastatinal, a new peptide appeared on a polyacrylamide gel at the position where the signal peptide was expected to appear. This was the signal peptide of the lipoprotein. Its appearance was in proportion to the appearance of the lipoprotein and disappearance of the precursor. When the cleavage of the signal peptide from the precursor was inhibited by globomycin, the peptide did not appear on the gel. The results of labeling of the peptide with [3H]leucine, [35S]methionine and [3H]arginine were consistent with the amino acid composition of the signal peptide. The signal peptide thus accumulated in the envelope fraction was hydrolyzed by an enzyme named signal peptide peptidase when the envelope fraction was washed to remove the the inhibitors. The hydrolysis was inhibited by readdition of these inhibitors. The signal peptide peptidase hydrolyzed the signal peptide only after its cleavage from the lipoprotein precursor.