Stabilization of flagellar filaments by HAP2 capping

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Abstract
In vivo growth of bacterial flagellar filaments by self-assembly of flagellin is promoted by a capping structure composed of a pentameric assembly of hook associated protein 2 (HAP2). Isolated native filaments with intact HAP2 cap exhibited higher melting temperature (ΔTm=4 °C) and significantly increased resistance against heat-induced depolymerization than non-capped ones. Reconstituted filaments were also stabilized by HAP2 binding, but the obtained filament–HAP2 complexes were less stable than native assemblies. Their fast depolymerization at elevated temperatures and sensitivity to proteolysis indicated that native-like filament–HAP2 complexes are rarely obtained by in vitro reconstitution. A procedure was developed to isolate perfectly capped native filaments to facilitate high-resolution structural analysis