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 turns.

The filamentous hemagglutinin (FHA) of Bordetella pertussis is an adhesin that binds the bacteria to cells of the respiratory epithelium in whooping-cough infections. Mature FHA is a 220 kDa secretory protein that is highly immunogenic and has been included in acellular vaccines. We have investigated its structure by combining electron microscopy and 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 establish 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 motifs are predicted to consist of short beta-strands separated by beta-turns, and secondary structure measurements by CD support this prediction. 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 hydrophobic faces apposed; and the tail is composed of the intervening sequence. Further support for the model was obtained by immuno-labeling electron microscopy. The 19-residue repeats of FHA have features in common with the leucine-rich repeats (LRRs) that are present in many eukaryotic proteins, including some adhesion factors. The model is also compared with the two other classes of filamentous proteins that are rich in beta-structure, i.e. viral adhesins and two beta-helical secretory proteins. Our proposed structure implies how the functionally important adhesion sites and epitopes of FHA are distributed: its tripeptide (RGD) integrin-binding site is assigned to the tail; the putative hemagglutination site forms part of the head; and two classes of immunodominant epitopes are assigned to opposite ends of the molecule. Possible mechanisms are discussed for two modes of FHA-mediated adhesion.

百日咳博德特氏菌的丝状血凝素（FHA）是一种黏附素，在百日咳感染中能将细菌结合到呼吸道上皮细胞上。成熟的FHA是一种220 kDa的分泌蛋白，具有高度免疫原性，并已被纳入无细胞疫苗中。我们通过将电子显微镜和圆二色光谱（CD）与对其氨基酸序列的计算分析相结合的方法研究了它的结构。FHA分子长50 nm，形状像马蹄钉：它有一个球状头部，似乎由两个结构域组成；一个35 nm长的杆部，平均宽度为4 nm，但从头部一端略微变细；还有一个小的、灵活的尾部。通过扫描透射电子显微镜进行的质量测量确定FHA是一个单体。其序列包含两个串联的19个残基的假重复区域：第一个有38个循环，从第344位残基开始；第二个有13个循环，从第1440位残基开始。重复基序预计由被β -转角隔开的短β -链组成，通过CD进行的二级结构测量支持这一预测。我们提出了一个FHA的发夹模型，其中头部由末端结构域组成；杆部主要由构象为两亲性、超长β -折叠的重复区域组成，其疏水面相对；尾部由间隔序列组成。通过免疫标记电子显微镜进一步支持了该模型。FHA的19个残基重复序列与许多真核蛋白质（包括一些黏附因子）中存在的富含亮氨酸重复序列（LRRs）有共同特征。该模型还与另外两类富含β -结构的丝状蛋白，即病毒黏附素和两种β -螺旋分泌蛋白进行了比较。我们提出的结构暗示了FHA的功能重要的黏附位点和表位是如何分布的：其三肽（RGD）整合素结合位点被确定在尾部；假定的血凝位点构成头部的一部分；两类免疫优势表位被确定在分子的相对两端。讨论了FHA介导的两种黏附模式的可能机制。