How do exported proteins and antibiotics bypass the periplasm in Gram-negative bacterial cells?

The next task will obviously be to understand how TolC interacts with the periplasmic components of the system, such as HlyD and AcrA, whose amino-terminal ends are anchored to the inner membrane via either a lipid extension or a lipophilic sequence. Although a crystal structure is not available for any HlyD or AcrA homologs, the hydrodynamic properties of AcrA, modified to remove the attached lipids, suggest that it is a very asymmetric protein, with a predicted length of nearly 200 A if modeled as a prolate ellipsoid.11xAcrA from Escherichia coli is a highly asymmetric protein capable of spanning the periplasm. Zgurskaya, H.I. and Nikaido, H. J. Mol. Biol. 1999; 285: 409–420Crossref | PubMed | Scopus (147)See all References11 Furthermore, proteins of this family always contain interrupted coiled-coil structures,10.xAlignment and structure prediction of divergent protein families: periplasmic and outer membrane proteins of bacterial efflux pumps. Johnson, J.M. and Church, G.M. J. Mol. Biol. 1999; 287: 695–715Crossref | PubMed | Scopus (126)See all References, 12.xThe Gram-negative cell envelope ‘springs’ to life: coiled-coil trans-envelope proteins. Pimenta, A. et al. Mol. Microbiol. 1996; 19: 643–645Crossref | PubMedSee all References flanked by a pair of lipoyl arm domain sequences.10xAlignment and structure prediction of divergent protein families: periplasmic and outer membrane proteins of bacterial efflux pumps. Johnson, J.M. and Church, G.M. J. Mol. Biol. 1999; 287: 695–715Crossref | PubMed | Scopus (126)See all References10 These observations suggest at least two models.10.xAlignment and structure prediction of divergent protein families: periplasmic and outer membrane proteins of bacterial efflux pumps. Johnson, J.M. and Church, G.M. J. Mol. Biol. 1999; 287: 695–715Crossref | PubMed | Scopus (126)See all References, 12.xThe Gram-negative cell envelope ‘springs’ to life: coiled-coil trans-envelope proteins. Pimenta, A. et al. Mol. Microbiol. 1996; 19: 643–645Crossref | PubMedSee all References In one model (Fig. 1Fig. 1a), these long proteins fold back on themselves using the interrupted coiled coils and the lipoyl domains, thus bringing the outer and inner membranes closer (if the distal end of the proteins is attached to the outer membranes). Perhaps an extreme manifestation of this property of AcrA was shown by our observation that, at pH 5.0 and at high Mg2+ concentrations, AcrA produces hemifusion events between reconstituted membrane vesicles.13xBypassing the periplasm: reconstitution of the AcrAB multidrug efflux pump of Escherichia coli. Zgurskaya, H.I. and Nikaido, H. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 7190–7195Crossref | PubMed | Scopus (228)See all References13 In the other model (Fig. 1Fig. 1b), the periplasmic linker proteins such as AcrA and HlyD interact with TolC (or more specifically the α-barrel of TolC), leading to the opening of the end of the TolC channel, or the stabilization or expansion of the α-barrel channel. The trimeric state of these periplasmic proteins and the length of the TolC α-barrel are consistent with this model. Interestingly, the inner-membrane-anchored component of a type II protein export system, XcpP, also contains a coiled-coil domain.14xStructure–function analysis of XcpP, a component involved in general secretory pathway-dependent protein secretion in Pseudomonas aeruginosa. Bleves, S. et al. J. Bacteriol. 1999; 181: 4012–4019PubMedSee all References14 Like any good work in biology, this new study of TolC structure8xCrystal structure of the bacterial membrane protein TolC central to multidrug efflux and protein export. Koronakis, V. et al. Nature. 2000; 405: 914–919Crossref | PubMed | Scopus (687)See all References8 will help us to devise additional experiments, in this case experiments with which we can further our understanding of the mechanism of these interesting translocation assemblies.