Signal peptide digestion in Escherichia coli. Effect of protease inhibitors on hydrolysis of the cleaved signal peptide of the major outer-membrane lipoprotein.

Upon incubation of the envelope fraction of Escherichia 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 [Hussain, M., Ichihara, S., and Mizushima, S. (1980) J. Biol. Chem. 255, 3707-3712; (1982) J. Biol. Chem. 257, 5177-5182]. 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 proved to be the signal peptide of the lipoprotein from the following facts: (a) its appearance is in proportion to the appearance of the lipoprotein and disappearance of the precursor; (b) when the cleavage of the signal peptide from the precursor was inhibited by globomycin, the peptide did not appear on the gel; and (c) 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 inhibitors. The hydrolysis was inhibited by re-addition of these inhibitors. The signal peptide peptidase hydrolyzed the signal peptide only after its cleavage from the lipoprotein precursor.