Structure and function of the Haemophilus influenzae autotransporters

Abstract

Autotransporters are a large class of proteins that are found in the outer membrane of Gram-negative bacteria and are almost universally implicated in virulence. These proteins consist of a C-terminal β-domain that is embedded in the outer membrane and an N-terminal domain that is exposed on the bacterial surface and is endowed with effector function. In this article, we review and compare the structural and functional characteristics of the Haemophilus influenzae IgA1 protease and Hap monomeric autotransporters and the H. influenzae Hia and Hsf trimeric autotransporters. All of these proteins play a role in colonization of the upper respiratory tract and in the pathogenesis of H. influenzae disease.

Keywords: Haemophilus influenzae; adhesin; autotransporter; serine protease.

Figure 1

Domain arrangement of Haemophilus influenzae autotransporters. Autotransporters contain three general domains: a signal peptide (green), a passenger domain (blue), and an outer membrane β-barrel domain (red). Within the passenger domains, binding domains are represented in pale blue, and protease domains are represented with diagonal lines. Asterisks (*) indicate regions of autoproteolysis, and SAAT refers to self-associating autotransporter.

Figure 2

Haemophilus influenzae autotransporter structures. (A) Crystal structure of IgA1 protease passenger domain (IgA_p) with an N-terminal globular protease domain and a larger Domain 2 (D2) compared to Hap_s. (B) Crystal structure of Hap passenger domain (Hap_s) with an N-terminal globular protease domain and a C-terminal SAAT β-helical domain. (C) Hap dimer highlighting Hap–Hap interactions via the SAAT domain.

Figure 3

Domain arrangements of Hia and Hsf trimeric autotransporters. The Hia passenger domain is characterized by a repetitive architecture consisting of multiple domain types. Hsf consists of a similar but extended domain arrangement compared to Hia. Domain arrangements were obtained from the daTAA server (http://toolkit.tuebingen.mpg.de/dataa).

Figure 4

Haemophilus influenzae autotransporter proteins. (A) Summary of key features of H. influenzae autotransporter proteins. (B) Schematic of the role of H. influenzae autotransporter proteins in the pathogenesis of disease. (a) Hap and Hia facilitate H. influenzae attachment to epithelial cells. Hap also promotes adherence to the ECM on the cell layer and at areas of damaged epithelium (*). Hap autoproteolysis is inhibited by SLPI, resulting in the retention of Hap_s on the bacterial surface. IgA1 protease is released from the bacterial surface and cleaves human IgA1. (b) Once bacteria have adhered, Hap–Hap interactions lead to microcolony formation. (c) Microcolony formation leads to the establishment of a biofilm that is resistant to host immune factors. (d) In the absence of SLPI, Hap autoproteolytic activity allows some bacteria to disperse, presumably to new sites of infection. (e) Hap-mediated attachment promotes bacterial invasion into epithelial cells, potentially providing a protected niche, and allowing bacterial evasion of local immune mechanisms.