Analysis of a flagellar filament cap mutant reveals that HtrA serine protease degrades unfolded flagellin protein in the periplasm of Borrelia burgdorferi. Issue 6 (26th April 2019)

• Record Type: Journal Article
• Authors: Zhang, Kai; Qin, Zhuan; Chang, Yunjie; Liu, Jun; Malkowski, Michael G.; Shipa, Saimtun; Li, Li; Qiu, Weigang; Zhang, Jing‐Ren; Li, Chunhao
• Is Part Of: Molecular microbiology. Volume 111:Issue 6(2019)
• Page Start: 1652
• Page End: 1670
• Publication Date: 2019-04-26
• Subjects: Molecular microbiology -- Periodicals; 572.829
• DOI: 10.1111/mmi.14243
• Languages: English
• ISSNs: 0950-382X

Abstract

Summary: Unlike external flagellated bacteria, spirochetes have periplasmic flagella (PF). Very little is known about how PF are assembled within the periplasm of spirochaetal cells. Herein, we report that FliD (BB0149), a flagellar cap protein (also named hook‐associated protein 2), controls flagellin stability and flagellar filament assembly in the Lyme disease spirochete Borrelia burgdorferi . Deletion of fliD leads to non‐motile mutant cells that are unable to assemble flagellar filaments and pentagon‐shaped caps (10 nm in diameter, 12 nm in length). Interestingly, FlaB, a major flagellin protein of B. burgdorferi, is degraded in the fliD mutant but not in other flagella‐deficient mutants (i.e., in the hook, rod, or MS‐ring). Biochemical and genetic studies reveal that HtrA, a serine protease of B. burgdorferi, controls FlaB turnover. Specifically, HtrA degrades unfolded but not polymerized FlaB, and deletion of htrA increases the level of FlaB in the fliD mutant. Collectively, we propose that the flagellar cap protein FliD promotes flagellin polymerization and filament growth in the periplasm. Deletion of fliD abolishes this process, which leads to leakage of unfolded FlaB proteins into the periplasm where they are degraded by HtrA, a protease that prevents accumulation of toxic products in the periplasm. Abstract : The flagellar cap protein FliD forms a pentagon‐shaped structure (10 nm in diameter, 12 nm in length), promoting flagellin (FlaB) polymerization and flagellar filament formation in the periplasm of Borrelia burgdorferi . Deletion of fliD leads to hypersecretion of FlaB into the periplasm in which unfolded FlaBs are degraded by a serine protease HtrA.