FILAMENTOUS HEMAGGLUTININ OF BORDETELLA-PERTUSSIS - A BACTERIAL ADHESIN FORMED AS A 50-NM MONOMERIC RIGID-ROD BASED ON A 19-RESIDUE REPEAT MOTIF RICH IN BETA-STRANDS AND BETA-TURNS

The filamentous hemagglutinin (FHA) of Bordetella pertussis is an adhe sin that binds the bacteria to cells of the respiratory epithelium in whooping-cough infections. Mature FNA is a 220 kDa secretory protein t hat is highly immunogenic and has been included in acellular vaccines. We have investigated its structure by combining electron microscopy a nd circular dichroism spectroscopy (CD) with computational analysis of its amino acid sequence. The FHA molecule is 50 nm in length and has the shape of a horseshoe nail: it has a globular head that appears to consist of two domains; a 35 nm-long shaft that averages 4 nm in width , but tapers slightly from the head end; and a small, flexible, tail. Mass measurements by scanning transmission electron microscopy establi sh that FHA is a monomer. Its sequence contains two regions of tandem 19-residue pseudo-repeats: the first, of 38 cycles, starts at residue 344; the second, of 13 cycles, starts at residue 1440. The repeat moti fs are predicted to consist of short beta-strands separated by beta-tu rns, and secondary structure measurements by CD support this predictio n. We propose a hairpin model for FHA in which the head is composed of the terminal domains; the shaft consists mainly of the repeat regions conformed as amphipathic, hyper-elongated beta-sheets, with their hyd rophobic faces apposed and the tail is composed of the intervening seq uence. Further support for the model was obtained by immuno-labeling e lectron microscopy. The 19-residue repeats of FHA have features in com mon with the leucine-rich repeats (LRRs) that are present in many euka ryotic proteins, including some adhesion factors. The model is also co mpared with the two other classes of filamentous proteins that are ric h in beta-structure, i.e. viral adhesins and two beta-helical secretor y proteins. Our proposed structure implies how the functionally import ant adhesion sites and epitopes of PHB are distributed: its tripeptide (RGD) integrin-binding site is assigned to the tail; the putative hem agglutination site forms part of the head. and two classes of immunodo minant epitopes are assigned to opposite ends of the molecule. Possibl e mechanisms are discussed for two modes of FHA-mediated adhesion.