Mechanism of self-association and filament capping by flagellar HAP2

HAP2 forms a capping structure, which binds very tightly to the distal end of flagellar filaments and still allows insertion of flagellin subunits below the cap by an unknown mechanism. Terminal regions of HAP2 from Salmonella typhimurium were found to be quickly degraded by various proteases, indicating that HAP2 also possesses disordered terminal regions like other axial proteins of bacterial flagellum. Removal of these portions by trypsin results in a fragment of 40 kDa (HP40), which lacks 42 NH2-terminal and 51 COOH-terminal residues. HAP2 in solution readily associates into a decameric structure without any significant population of intermediate oligomeric forms. The HP40 fragments, however, do not form decamers, while they can assemble into pentamers, as revealed by chemical cross-linking and analytical ultracentrifugation. Decameric HAP2 also dissociates into pentamers and smaller oligomers upon a heat induced conformational transition around 36 degreesC. While the highly mobile terminal regions are immobilized in decameric HAP2 complexes, they are still largely disordered in the pentameric state. These results demonstrate that the intersubunit interactions within the pentamers are mainly through the HP40 portions, whereas the terminal regions are responsible for association of pentamers into decameric complexes. Several observations indicate that HAP2 performs its capping function as a pentamer. We suggest that binding of the pentameric HAP2 cap to the filament is mediated by the highly flexible terminal regions. Indeed, HP40 fragments are unable to cap the end of filaments, while removal of about 30 residues from both terminal regions of HAP2 results in a highly reduced capping ability. A model is presented to explain the molecular mechanism of capping, in which conformational entropy in the disordered terminal regions moderates the otherwise too tight HAP2-filament interactions to allow insertion of flagellin subunits below the cap.