Beta‐helix model for the filamentous haemagglutinin adhesin of Bordetella pertussis and related bacterial secretory proteins

Abstract

SummaryBordetella pertussis establishes infection by attaching to epithelial cells of the respiratory tract. One of its adhesins is filamentous haemagglutinin (FHA), a 500-Å -long secreted protein that is rich in b-structure and contains two regions, R1 and R2, of tandem 19-residue repeats. Two models have been proposed in which the central shaft is (i) a hairpin made up of a pairing of two long antiparallel b-sheets; or (ii) a b-helix in which the polypeptide chain is coiled to form three long parallel b-sheets. We have analysed a truncated variant of FHA by electron microscopy (negative staining, shadowing and scanning transmission electron microscopy of unstained specimens): these observations support the latter model. Further support comes from detailed sequence analysis and molecular modelling studies. We applied a profile search method to the sequences adjacent to and between R1 and R2 and found additional 'covert' copies of the same motifs that may be recognized in overt form in the R1 and R2 sequence repeats. Their total number is sufficient to support the tenet of the b-helix model that the shaft domain -a 350 Å rod -should consist of a continuous run of these motifs, apart from loop inserts. The N-terminus, which does not contain such repeats, was found to be weakly homologous to cyclodextrin transferase, a protein of known immunoglobulin-like structure. Drawing on crystal structures of known b-helical proteins, we developed structural models of the coil motifs putatively formed by the R1 and R2 repeats. Finally, we applied the same profile search method to the sequence database and found several other proteins -all large secreted proteins of bacterial provenance -that have similar repeats and probably also similar structures.

The crystal structure of filamentous hemagglutinin secretion domain and its implications for the two-partner secretion pathway

“…Previous structural predictions suggested that the TPS domain might adopt an Ig-like fold (26). The Fha30 structure reveals that these predictions are incorrect and our results have important biological implications.…”

“…So far, Ͼ100 TpsA proteins, including many adhesins and virulence factors, have been identified based on the presence of a conserved, N-proximal TPS domain, and the list will continue to grow with the ongoing efforts of microbial genome sequencing. Many of them present amphipatic repeated motifs with ␤-helical propensities (26). They are thus expected to share a similar architecture with Fha30 and to adopt righthanded ␤-helical structures, despite their limited overall sequence similarities, mainly confined to the TPS domain.…”

A model for Ure2p prion filaments and other amyloids: The parallel superpleated β-structure

“…Among known structures, the only exceptions to the ''minimal hydrophobic content'' rule are some ␤-helical proteins (44)(45)(46). These proteins have repetitive three-dimensional structures (coils of the helix), which are occasionally reflected in detectable sequence repeats (''overt'' repeats) but usually are not detectable as such (''covert'' repeats) (47). The difficulty of detecting such repeats in sequences is compounded by the fact that they may vary in length through the insertion of loops at turn sites.…”

Structural basis of toxicity and immunity in contact-dependent growth inhibition (CDI) systems

“…CdiA exoproteins are very large (250-650 kDa) and composed of an N-terminal transport domain followed by a variable number of hemagglutinin repeats (1). The hemagglutinin-repeat region is predicted to form an extended β-helical filament capable of projecting several hundred angstroms from the inhibitor cell surface (3). The current model of CDI postulates that CdiA binds to receptors on the surface of susceptible bacteria, initiating delivery of a CdiA-derived toxin into the target cell (Fig.…”