Synthesis of outer membrane proteins in cpxA cpxB mutants of Escherichia coli K-12

Abstract

Two major proteins, the murein lipoprotein and the OmpF matrix porin, are deficient in the outer membrane of cpxA cpxB mutants of Escherichia coli K-12. The authors present evidence that the cpx mutations prevent or retard the translocation of these proteins to the outer membrane. The mutations had no effect on the rate of lipoprotein synthesis. Mutant cells labeled for 5 min with radioactive arginine accumulated as much lipoprotein as otherwise isogenic cpxA+ cpxB+ cells. This lipoprotein accumulated as such; no material synthesized in mutant cells and reactive with antilipoprotein antibodies had the electrophoretic mobility of prolipoprotein. Hence, the initial stages of prolipoprotein insertion into the inner membrane leading to its cleavage to lipoprotein appeared normal. However, after a long labeling interval, mutant cells were deficient in free lipoprotein and lacked lipoprotein covalently bound to peptidoglycan, suggesting that little if any of the lipoprotein synthesized in mutant cells reaches the outer membrane. Immunoreactive OmpF protein could also be detected in extracts of mutant cells labeled for 5 min, but the amount that accumulated was severalfold less in mutant cells than in cpxA+ cpxB+ cells. Analysis of β-galactosidase synthesis from ompF-lacZ fusion genes showed this difference to be the result of a reduced rate of ompF transcription in mutant cells. Even so, little or none of the OmpF protein synthesized in mutant cells was incorporated into the outer membrane.