Heat shock induction by a misassembled cytoplasmic membrane protein complex in Escherichia coli

We analysed the effects of the overproduction of parts or all of a multisubunit ATP-binding cassette (ABC) transporter, the MalFGK2 complex, involved in the uptake of maltose and maltodextrins in Escherichia coli . We found that production of the MalF protein alone was inducing the phtrA promoter, which is under the control of a recently discovered sigma factor, σ²⁴, involved in the response to extracytoplasmic stresses. The production level, stability and localization of MalF were not altered when produced without its partners, suggesting that the protein was correctly inserted in the membrane. Our results indicate that a large periplasmic loop located between the third and fourth transmembrane segment of MalF, the L3 loop, is responsible for phtrA induction: (i) deleted MalF proteins with no L3 loop or with a L3 loop lacking 120 amino acids do not induce the phtrA promoter; (ii) the export to the periplasm of the L3 loop alone or fused to MalE induces the phtrA promoter. Moreover, the proteolytic sensitivity of MalF is different when it is produced alone and when MalF and MalG are produced together, suggesting a change in the conformation and/or accessibility of MalF. These results suggest that some inner membrane proteins can be sensed outside the cytoplasm by a quality control apparatus or by the export machinery. Moreover, the observation of the phtrA induction by MalF could be a useful new tool for studying the insertion and assembly of the MalFGK2 complex.