YaeT-independent multimerization and outer membrane association of secretin PulD

Previous studies demonstrated that targeting of the dodecameric secretin PulD to the Escherichia coli outer membrane is strictly dependent on the chaperone-like pilotin PulS. Here, we report that PulD multimerization and membrane association in strains producing PulS were unaffected when the levels of the essential outer membrane assembly factor YaeT(Omp85) were reduced by controlled expression of a paraBAD-yaeT transcriptional fusion. This behaviour contrasted markedly to that of the trimeric porin LamB, which remained monomeric under these conditions. Furthermore, resistance to extraction by the detergent Sarkosyl and by urea, and susceptibility to trypsin digestion all suggested that PulD localized to the outer membrane in YaeT-depleted cells. We conclude that, unlike classical beta-barrel outer membrane proteins such as LamB, multimerization of PulD is largely YaeT-independent.