A distinct signal peptidase for prolipoprotein in Escherichia coli

We have previously demonstrated the modification and processing of Escherichia coli prolipoprotein (Braun's) in vitro ( Tokunaga M, Tokunaga H, Wu HC: Proc Natl Acad Sci USA 79:2255, 1982). Using this in vitro assay of prolipoprotein signal peptidase and globomycin selection, we have isolated and partially characterized an E coli mutant which contained a higher level of prolipoprotein signal peptidase activity. In contrast, the procoat protein signal peptidase activity was not increased in this mutant as compared to the wild-type strain. Furthermore, E coli strains containing cloned procoat protein signal peptidase gene were found to contain elevated levels of procoat protein signal peptidase, but normal levels of prolipoprotein signal peptidase. These two signal peptidase activities were also found to exhibit different stabilities during storage at 4 degrees C. Thus biochemical, immunological, and genetic evidence clearly indicate that prolipoprotein signal peptidase is distinct from procoat protein signal peptidase in E coli.