An infrequent molecular ruler controls flagellar hook length inSalmonella enterica

Abstract

The bacterial flagellum consists of a long external filament connected to a membrane-embedded basal body at the cell surface by a short curved structure called the hook. In Salmonella enterica, the hook extends 55 nm from the cell surface. FliK, a secreted molecular ruler, controls hook length. Upon hook completion, FliK induces a secretionspecificity switch to filament-type substrate secretion. Here, we demonstrate that an infrequent ruler mechanism determines hook length. FliK is intermittently secreted during hook polymerization. The probability of the specificity switch is an increasing function of hook length. By uncoupling hook polymerization from FliK expression, we illustrate that FliK secretion immediately triggers the specificity switch in hooks greater than the physiological length. The experimental data display excellent agreement with a mathematical model of the infrequent ruler hypothesis. Merodiploid bacteria expressing simultaneously short and long ruler variants displayed hook-length control by the short ruler, further supporting the infrequent ruler model. Finally, the velocity of FliK secretion determines the probability of a productive FliK interaction with the secretion apparatus to change secretion substrate specificity.

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

“…According to the infrequent molecular ruler model, a high secretion rate of FliK increases the probability of an interaction between FliK and FlhB C . Since this interaction induces the substrate specificity switch, the switch might occur earlier in strains that oversecrete FliK, thus leading to shorter hooks (162). In agreement with this model, overexpression of FliK was previously shown to result in shorter hooks (399).…”

“…This so-called infrequent ruler model, which predicts temporal mea-surements of the hook length by intermittently secreted FliK molecules, was recently supported by experiments in S. enterica in which fliK expression and hook polymerization were uncoupled. It was shown that the substrate specificity switch occurred immediately when fliK expression was induced in a strain with elongated hook structures (162). Furthermore, the simultaneous production of a short and a long FliK derivative resulted in short hooks corresponding to the short FliK molecule, in agreement with the infrequent ruler model (162).…”

“…It was shown that the substrate specificity switch occurred immediately when fliK expression was induced in a strain with elongated hook structures (162). Furthermore, the simultaneous production of a short and a long FliK derivative resulted in short hooks corresponding to the short FliK molecule, in agreement with the infrequent ruler model (162). Thus, since the substrate specificity switch depends on the interaction between the C-terminal regions of FliK and FlhB, the first FliK molecule that travels a hook with an appropriate length will signal the switch to the secretion of filament proteins.…”

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).…”