Three-Dimensional Structure ofBordetella pertussisFimbriae

Abstract

We describe the helical structure ofBordetella pertussisfimbriae of serotype 3/6 as determined to a resolution of ∼ 2.5 nm by three-dimensional reconstruction of negatively stained electron micrographs. The fimbria has a distinctly polar structure whose axial repeat of 13 nm contains five copies of thefim3gene product (22 kDa) in two complete turns. These subunits are connected by interactions along the fimbrial backbone which, unlike other classes of bacterial fimbriae, has no axial channel. Its outer diameter is ∼ 5.7 nm, and the most pronounced feature is a radially protruding domain that gives the fimbria its characteristic serrated appearance. Serotype 2 fimbriae, composed of thefim2subunit which is 60% homologous withfim3,have essentially the same quaternary structure. These observations are discussed in relation to fimbrial phase variation and structure-based classification of fimbriae/pili.