A Critical Region in the FlaA Flagellin Facilitates Filament Formation of theVibrio choleraeFlagellum
ABSTRACTVibrio choleraeis a Gram-negative bacterium with a monotrichous flagellum that causes the human disease cholera. Flagellum-mediated motility is an integral part of the bacterial life cycle inside the host and in the aquatic environment. TheV. choleraeflagellar filament is composed of five flagellin subunits (FlaA, FlaB, FlaC, FlaD, and FlaE); however, only FlaA is necessary and sufficient for filament synthesis.flaAis transcribed from a class III flagellar promoter, whereas the other four flagellins are transcribed from class IV promoters. However, expressingflaAfrom a class IV promoter still facilitated motility in a strain that was otherwise lacking all five flagellins (ΔflaA-E). Furthermore, FlaA fromV. parahaemolyticus(FlaAVP; 77% identity) supported motility of theV. choleraeΔflaA-Estrain, whereas FlaA fromV. vulnificus(FlaAVV; 75% identity) did not, indicating that FlaA amino acid sequence is responsible for its critical role in flagellar synthesis. Chimeric proteins composed of different domains of FlaAVCand FlaD or FlaAVVrevealed that the N-terminal D1domain (D1N) contains an important region required for FlaA function. Further analyses of chimeric FlaAVC-FlaD proteins identified a lysine residue present at position 145 of the other flagellins but absent from FlaAVCthat can prevent monofilament formation. Moreover, the D1Nregion of amino acids 87 to 153 of FlaAVVinserted into FlaAVCallows monofilament formation but not motility, apparently due to the lack of filament curvature. These results identify residues within the D1Ndomain that allow FlaAVCto fold into a functional filament structure and suggest that FlaAVCassists correct folding of the other flagellins.IMPORTANCEV. choleraecauses the severe diarrheal disease cholera. Its ability to swim is mediated by rotation of a polar flagellum, and this motility is integral to its ability to cause disease and persist in the environment. The current studies illuminate how one specific flagellin (FlaA) within a multiflagellin structure mediates formation of the flagellar filament, thus allowingV. choleraeto swim. This knowledge can lead to safer vaccines and potential therapeutics to inhibit cholera.