Overexpression of protease-deficient DegP(S210A) rescues the lethal phenotype of Escherichia coli OmpF assembly mutants in a degP background

Replacement of OmpF's conserved carboxy-terminal phenylalanine with dissimi lar amino acids severely impaired its assembly into stable trimers. In some instances, interactions of mutant proteins with the outer membrane were al so affected, as judged by their hypersensitivity phenotype, Synthesis of al l mutant OmpF proteins elevated the expression of periplasmic protease DegP , and synthesis of most of them made its presence obligatory for cell viabi lity. These results showed a critical role for DegP in the event of aberran t outer membrane protein assembly. The lethal phenotype of mutant OmpF prot eins in a degP null background was eliminated when a protease-deficient Deg P(S210A) protein was overproduced, Our data showed that this rescue from le thality and a subsequent increase in mutant protein levels in the envelope did not lead to the proper assembly of the mutant proteins in the outer mem brane. Rather, a detergent-soluble and thermolabile OmpF species resembling monomers accumulated in the mutants, and to a lesser extent in the parenta l strain, when DegP(S210A) was overproduced. Interestingly, this also led t o the localization of a significant amount of mutant polypeptides to the in ner membrane, where DegP(S210A) also fractionated, These results suggested that the DegP(S210A)-mediated rescue from toxicity involved preferential se questration of misfolded OmpF monomers from the normal assembly pathway.