Self-Oligomerizing Structure of the Flagellar Cap Protein FliD and Its Implication in Filament Assembly.
FliD is a self-oligomerizing structural protein that caps the growing end of the bacterial flagellar filament. FliD also plays a key role in the flagellar system by continuously adding a new flagellin protein to the tip of the filament. To structurally characterize FliD oligomerization and to provide a FliD-mediated flagellin polymerization mechanism, we have determined the crystal structures of FliD proteins from Escherichia coli and Salmonella enterica serovar Typhimurium (ecFliD and stFliD, respectively). ecFliD consists of three domains (D1, D2, and D3) and forms a hexamer plate of the D2 and D3 domains that resembles a six-pointed star with legs consisting of the D1 domain. In contrast, the D2 and D3 domains of stFliD assemble into a pentamer as a five-pointed star plate. Despite their distinct oligomeric states, ecFliD and stFliD engage a common molecular surface for oligomerization. FliD also features interdomain and intersubunit flexibility, suggesting that FliD reorganizes its domains and adjacent subunits depending on the FliD binding partner. The similarity of the FliD shape to flagellin and the structural dynamics of FliD led us to propose a FliD-catalyzed filament elongation mechanism. In this model, FliD occupies a position in place of a nascent flagellin until the flagellin reaches the growing end of the filament, and then, FliD moves aside to repeat the positional replacement.
Division of Biomedical Convergence, College of Biomedical Science, Kangwon National University, Chuncheon 24341, Republic of Korea.
Wan Seok
WS
Song
Division of Biomedical Convergence, College of Biomedical Science, Kangwon National University, Chuncheon 24341, Republic of Korea.
So Yeon
SY
Cho
Division of Biomedical Convergence, College of Biomedical Science, Kangwon National University, Chuncheon 24341, Republic of Korea.
Ho Jeong
HJ
Hong
Division of Biomedical Convergence, College of Biomedical Science, Kangwon National University, Chuncheon 24341, Republic of Korea.
Sun Cheol
SC
Park
Division of Biomedical Convergence, College of Biomedical Science, Kangwon National University, Chuncheon 24341, Republic of Korea; Institute of Bioscience and Biotechnology, Kangwon National University, Chuncheon 24341, Republic of Korea. Electronic address: sungil@kangwon.ac.kr.
Sung-Il
SI
Yoon
D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000737:chemistry*; D008954:Models, Biological; D008958:Models, Molecular; D011487:Protein Conformation; D055503:Protein Multimerization; D012486:Salmonella typhimurium / Q000737:chemistry*
D001426:Bacterial Proteins; C076733:FlaD protein, Bacteria
FliD; crystal structure; flagellum; oligomerization