Three SpoA-domain proteins interact in the creation of the flagellar type III secretion system in Helicobacter pylori

• Published in: The Journal of biological chemistry Vol. 293; no. 36; pp. 13961 - 13973
• Main Authors: Lam, Kwok Ho, Xue, Chaolun, Sun, Kailei, Zhang, Huawei, Lam, Wendy Wai Ling, Zhu, Zeyu, Ng, Juliana Tsz Yan, Sause, William E., Lertsethtakarn, Paphavee, Lau, Kwok Fai, Ottemann, Karen M., Au, Shannon Wing Ngor
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 07-09-2018; American Society for Biochemistry and Molecular Biology
• Subjects: Bacterial Proteins - metabolism; Crystallography, X-Ray; Flagella - chemistry; Flagella - ultrastructure; Helicobacter pylori - chemistry; Membrane Proteins; Multiprotein Complexes - chemistry; Protein Domains; Protein Interaction Domains and Motifs; Protein Multimerization; Protein Structure and Folding; Type III Secretion Systems - chemistry; flagellar motor switch complex; virulence factor; protein assembly; SpoA domain; bacterial motility; protein structure; Helicobacter pylori; protein export; flagellum; Fli protein; type III secretion system; FliY; protein-protein interaction; molecular motor
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.RA118.002263

Bacterial flagella are rotary nanomachines that contribute to bacterial fitness in many settings, including host colonization. The flagellar motor relies on the multiprotein flagellar motor-switch complex to govern flagellum formation and rotational direction. Different bacteria exhibit great diversity in their flagellar motors. One such variation is exemplified by the motor-switch apparatus of the gastric pathogen Helicobacter pylori, which carries an extra switch protein, FliY, along with the more typical FliG, FliM, and FliN proteins. All switch proteins are needed for normal flagellation and motility in H. pylori, but the molecular mechanism of their assembly is unknown. To fill this gap, we examined the interactions among these proteins. We found that the C-terminal SpoA domain of FliY (FliYC) is critical to flagellation and forms heterodimeric complexes with the FliN and FliM SpoA domains, which are β-sheet domains of type III secretion system proteins. Surprisingly, unlike in other flagellar switch systems, neither FliY nor FliN self-associated. The crystal structure of the FliYC–FliNC complex revealed a saddle-shaped structure homologous to the FliN–FliN dimer of Thermotoga maritima, consistent with a FliY–FliN heterodimer forming the functional unit. Analysis of the FliYC–FliNC interface indicated that oppositely charged residues specific to each protein drive heterodimer formation. Moreover, both FliYC–FliMC and FliYC–FliNC associated with the flagellar regulatory protein FliH, explaining their important roles in flagellation. We conclude that H. pylori uses a FliY–FliN heterodimer instead of a homodimer and creates a switch complex with SpoA domains derived from three distinct proteins.