Trends in Microbiology
The great escape: structure and function of the autotransporter proteins
Section snippets
Mechanisms of secretion in gram-negative bacteria
Several secretion systems in Gram-negative bacteria have been studied extensively. The so-called type I secretion systems are exemplified by the HlyA hemolysin of Escherichia coli. HlyA is secreted through the inner and outer membrane via an oligomeric complex composed of a large ATP-binding cassette (ABC) transporter (HlyB), an accessory factor (HlyD) and an additional outer membrane factor (TolC)[2].
The type II secretion system is a two-step process and has been best studied for the
The autotransporter secretion mechanism
The autotransporter secretion system, as distinct from the mechanisms detailed above, was first described for the IgA1 proteases of Neisseria gonorrhoeae by Pohlner et al.1, 6, who elegantly elucidated the fundamental mechanism. (The term `type IV secretion' has been proposed for the autotransporter proteins[4]. We favor the use of this term; however, autotransporter is a more descriptive designation and will be used throughout this review.) During secretion, the Iga polyprotein precursor is
Function and phylogeny of the autotransporters
The passenger domains of autotransporters are widely divergent, yet phylogenic relationships can be inferred. Conservation of the serine protease motif (consensus GDSGSP) at similar positions in the E. coli proteins Pet (Ref. [32]), Tsh (Ref. [30]), EspP (Ref. [8]) and EspC (Ref. [33]) and the Shigella proteins ShMu (Ref. [34]) and SepA (Ref. [29]) strongly suggests that these proteins are related serine proteases. In fact, these proteins comprise a subfamily of the autotransporters, for which
Potential biotechnological applications
One compelling feature of autotransporter proteins is their promise as tools for the analysis of macromolecular interactions. Possible applications include the expression of (1) antigenic determinants for vaccine development, (2) peptide libraries for epitope mapping or antibody specificity tests and (3) receptors or ligands for binding assays or simple protein purification.
Autotransporter molecules would theoretically allow nearly any protein antigen to be expressed from a Gram-negative
Conclusions
Notwithstanding the controversies regarding the specifics of structure and function, there is an overall unifying structure for the autotransporter molecules, which is intrinsically associated with their mode of biogenesis. The defining but still hypothetical characteristics are that the β-domain forms a β-barrel structure within the outer membrane and that the passenger protein is translocated to the cell surface via the autogenous pore. Further experimental analyses are required to clarify
Acknowledgements
Work in the laboratory of J.P.N. is supported by Public Health Service grant AI-33096. We thank James B. Kaper and Harry L.T. Mobley for helpful review of the manuscript.
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