The role of the FliK molecular ruler in hook‐length control in Salmonella enterica

Abstract

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.

Structure of a type III secretion needle at 7-Å resolution provides insights into its assembly and signaling mechanisms

“…Two models have been proposed to explain how the rulers work (30)(31)(32)(33). In one model, polymer elongation is controlled by several ruler molecules passing stochastically through the growing structure, with subunits and rulers traveling alternatively in the channel (30,31,33). In the other model, one ruler molecule positioned inside the channel is stretched gradually during polymer assembly, where the ruler coexists with polymer proteins translocated through the channel (32).…”

“…In the absence of their rulers, extra-long needles and hooks are produced. Two models have been proposed to explain how the rulers work (30)(31)(32)(33). In one model, polymer elongation is controlled by several ruler molecules passing stochastically through the growing structure, with subunits and rulers traveling alternatively in the channel (30,31,33).…”

Protein Export According to Schedule: Architecture, Assembly, and Regulation of Type III Secretion Systems from Plant- and Animal-Pathogenic Bacteria

“…Since these FliK derivatives were initially not detected in the culture supernatant, FliK was proposed to act as an internal ruler (502). However, secretion of FliK derivatives with deletions in the central protein region was shown in a later study by the use of a more sensitive FliK-specific antibody (159).…”

“…As mentioned above, this interaction is probably favored at a hook length of approximately 55 nm (159,381,383,392). Prior to hook completion, binding of FliK C to FlhB C is suppressed by an additional regulatory protein, termed RflH/Flk or Fluke (to clearly distinguish it from FliK), which prevents the premature secretion of filament components and is anchored in the IM (7,265,296,385).…”

“…It is still unclear whether the inner diameter of the hook, which is smaller than 2 nm, allows the simultaneous passage of FliK and the hook protein FlgE (498). It was therefore proposed that FliK acts as a more flexible ruler molecule that is constantly secreted through the growing hook structure and switches the substrate specificity when the hook has reached its final length (159) (Fig. 8C).…”