The Role of the FliK Molecular Ruler in Hook-length Control in Salmonella enterica

摘要

A molecular ruler, FliK, controls the length of the flagellar hook. FliK measures hook length and catalyzes the secretion-substrate specificity switch from rod-hook substrate specificity to late substrate secretion, which includes the filament subunits. Here, we show normal hook-length control and filament assembly in the complete absence of the C-ring thus refuting the previous “cup” model for hook-length control. Mutants of C-ring components, which are reported to produce short hooks, show a reduced rate of hook-basal body assembly thereby allowing for a premature secretion-substrate specificity switch. Unlike fliK null mutants, hook-length control in an autocleavage-defective mutant of flhB, the protein responsible for the switch to late-substrate secretion, is completely abolished. FliK deletion variants that retain the ability to measure hook length are secreted thus demonstrating that FliK directly measures rod-hook length during the secretion process. Finally, we present a unifying model accounting for all published data on hook-length control in which FliK acts as a molecular ruler that takes measurements of rod-hook length while being intermittently secreted during the assembly process of the hook-basal body complex.