Conserved Features in the Structure, Mechanism, and Biogenesis of the Inverse Autotransporter Protein Family

Abstract

The bacterial cell surface proteins intimin and invasin are virulence factors that share a common domain structure and bind selectively to host cell receptors in the course of bacterial pathogenesis. The β-barrel domains of intimin and invasin show significant sequence and structural similarities. Conversely, a variety of proteins with sometimes limited sequence similarity have also been annotated as “intimin-like” and “invasin” in genome datasets, while other recent work on apparently unrelated virulence-associated proteins ultimately revealed similarities to intimin and invasin. Here we characterize the sequence and structural relationships across this complex protein family. Surprisingly, intimins and invasins represent a very small minority of the sequence diversity in what has been previously the “intimin/invasin protein family”. Analysis of the assembly pathway for expression of the classic intimin, EaeA, and a characteristic example of the most prevalent members of the group, FdeC, revealed a dependence on the translocation and assembly module as a common feature for both these proteins. While the majority of the sequences in the grouping are most similar to FdeC, a further and widespread group is two-partner secretion systems that use the β-barrel domain as the delivery device for secretion of a variety of virulence factors. This comprehensive analysis supports the adoption of the “inverse autotransporter protein family” as the most accurate nomenclature for the family and, in turn, has important consequences for our overall understanding of the Type V secretion systems of bacterial pathogens.

A tamB homolog is involved in maintenance of cell envelope integrity and stress resistance of Deinococcus radiodurans

“…Generally, secretory proteins are synthesized in the cytoplasm and transported through the cytoplasmic membrane by the Sec translocon system289, after which the signal peptide is cleaved by signal peptidase, and further transportation of the mature protein is achieved by outer membrane transportation systems including the β-barrel assembly machinery (BAM), the two-partner secretion system (TPSS) or the translocation and assembly module (TAM)1. The TAM consists of two components, TamA and TamB, that form a recently identified protein complex10 that is crucial for the assembly of outer membrane proteins1011121314, as well as the virulence and colonization of pathogenic bacteria, but is not essential for viability in organisms studied thus far101415. TamA belongs to the Omp85 protein family11015 originally identified in Neisseria meningitidis 2.…”

An investigation into the Omp85 protein BamK in hypervirulent Klebsiella pneumoniae, and its role in outer membrane biogenesis

“…The assembly of trimeric porins can be measured with a pulse‐chase assay based on semi‐native PAGE to separate the assembled trimers from the monomeric forms of unassembled protein (Heinz et al, , Stubenrauch et al, ). To monitor the rate of assembly of the trimeric porin, translation of [ 35 S]‐labeled PhoE precursor was induced in each strain and the assembly of the porin trimer was determined by semi‐native PAGE and densitometry.…”

“…LPS display on the cell surface depends on the activity of the LptDE complex, which is assembled by the BAM complex. The type I and type III fimbrial adhesins are extended through usher proteins (Klemm & Schembri, , Wilksch et al, , Khater et al, ), which are themselves dependent on the BAM complex and the TAM for efficient assembly (Stubenrauch et al, , Stubenrauch et al, , Heinz et al, ). While the Wza secretion pore for capsular polysaccharide is not assembled by the BAM complex (Dunstan et al, ), the capsule has a key attachment factor Wzi that is a β‐barrel protein (Bushell et al, ).…”

Reductive evolution in outer membrane protein biogenesis has not compromised cell surface complexity in Helicobacter pylori

“…This is a feature of broader β‐barrel protein groups such as the Type V secretion system which contains autotransporters (Celik et al., ), trimeric autotransporters (Qin, Wang, & Lei, ), and inverse‐autotransporters (Heinz et al., ).…”

“…The topography whereby an extracellular passenger domain is displayed via a transmembrane β‐barrel assembled into the cell surface is the defining feature of a Type 5 secretion system. Currently there are at least six subtypes (Bleves et al., ; Fan, Chauhan, Udatha, Leo, & Linke, ; Heinz et al., ; Leo, Oberhettinger, Schutz, & Linke, ) distinguished by the β‐barrel domain used as the protein translocation device: autotransporters (Type Va), two‐partner secretion systems (Type Vb), trimeric autotransporters (Type Vc), patatins (Type Vd), inverse‐autotransporters (Type Ve) and two‐partner inverse‐autotransporters (Type Vf). The Hp_OMP families of Hop and Hor proteins would constitute the first subtype of the Type 5 secretion system to be exclusive to a single genus of bacteria.…”