Length Control of the Flagellar Hook in a Temperature-Sensitive flgE Mutant of Salmonella enterica Serovar Typhimurium

The flagellar hook is a short, curved, extracellular structure located between the basal body and the filament. The hook is composed of the FlgE protein. In this study, we analyzed flagellum assembly in a temperature-sensitive flgE mutant of Salmonella enterica serovar Typhimurium. When the mutant cells were grown at 30°C, they produced flagella of a normal length (71% of the population) and short hooks without filaments (26%). At 37°C, 70% of the basal bodies lacked hooks, and intact flagella made up only 6% of the population. Mutant cells secreted monomeric FlgE in abundance at 37°C, suggesting that the mutant FlgE protein might be defective in polymerization at higher temperatures. The average length of the hooks in intact filaments was 55 nm, whereas after acid treatment, it was 45 nm. SDS-PAGE analysis of the hook-basal body showed that HAP1 was missing in the mutant but not in the wild type. We concluded that hook length in the mutant is controlled in the same way as in the wild type, but the hook appeared short after acid treatment due to the lack of HAP1. We also learned that the true length of the hook is possibly 45 nm, not 55 nm, as has been believed.