Protease IV, a cytoplasmic membrane protein of Escherichia coli, has signal peptide peptidase activity

During export of the outer membrane lipoprotein across the cytoplasmic membrane, the signal peptide of the lipoprotein undergoes two successive proteolytic attacks, cleavage of the signal peptide by signal peptidase and digestion of the cleaved signal peptide by an enzyme called signal peptide peptidase(s) (Hussain, M., Ichihara, S., and Mizushima, S. (1982) J. Biol. Chem. 257, 5177-5182; Hussain, M., Ozawa, Y., Ichihara, S., and Mizushima, S. (1982) Eur. J. Biochem. 129, 233-239). Here we report that protease IV, a cytoplasmic membrane protease, exhibits the signal peptide peptidase activity. The signal peptide peptidase activity was cofractionated with protease IV throughout the entire process of purification of the latter enzyme. Only the signal peptide was digested by the peptidase among membrane proteins. Both the signal peptide peptidase activity and the protease IV activity were inhibited to similar degrees by antipain, leupeptin, chymostatin, and elastatinal that are known to inhibit the signal peptide peptidase activity in the cell envelope. From these results we conclude that protease IV is the signal peptide peptidase that is responsible for signal peptide digestion in the cytoplasmic membrane. The peptidase attacked the signal peptide only after its release from the precursor protein.