FOLDING-BASED SUPPRESSION OF EXTRACYTOPLASMIC TOXICITY CONFERRED BY PROCESSING-DEFECTIVE LAMB

We have utilized processing-defective derivatives of the outer membran e maltoporin, LamB, to study protein trafficking functions in the cell envelope of Escherichia coli. Our model proteins contain amino acid s ubstitutions in the consensus site for cleavage by signal peptidase, i hs a result, the signal sequence is cleaved with reduced efficiency, e ffectively tethering the precursor protein to the inner membrane. Thes e mutant porins are toxic when secreted to the cell envelope, Furtherm ore, strains producing these proteins exhibit altered outer membrane p ermeability, suggesting that the toxicity stems from some perturbation of the cell envelope (J. H. Carlson and T. J. Silhavy, J. Bacteriol. 175:3327-3334, 1993). We have characterized a multicopy suppressor of the processing-defective porins that appears to act by a novel mechani sm. Using fractionation experiments and conformation-specific antibodi es, me found that the presence of this multicopy suppressor allowed th e processing-defective LamB precursors to be folded and localized to t he outer membrane. Analysis of the suppressor plasmid revealed that th ese effects are mediated by the presence of a truncated derivative of the polytopic inner membrane protein, TetA. The suppression mediated b y TetA' is independent of the CpxA/CpxR regulon and the sigma(E) regul on, both of which are involved in regulating protein trafficking funct ions in the cell envelope.