Flagellar Hook/Needle Length Control and Secretion Control in Type III Secretion Systems

Aizawa, Shin‐Ichi

doi:10.1007/82_2019_169

Cited by 1 publication

(1 citation statement)

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“…Accordingly, our results support a model in which FliK functions as a hook growth terminator, which prevents the hook from growing beyond the length required for its universal joint function (34). In this model, both hook and FliK subunits are secreted during hook assembly.…”

Section: Discussionsupporting

confidence: 84%

Controlling minimal and maximal hook-length of the bacterial flagellum

Guse

Rohde

Erhardt

2020

Preprint

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22Hook-length control is a central checkpoint during assembly of the bacterial flagellum. During 23 hook growth, a 405 amino acids (aa) protein, FliK, is intermittently secreted and thought to 24 function as a molecular measuring tape that, in Salmonella, controls hook-length to 55 nm ± 6 25 nm. The underlying mechanism involves interactions of both the α-helical, N-terminal 26 domain of FliK (FliKN) with the hook and hook cap, and of its C-terminal domain with a 27 component of the export apparatus. However, various deletion mutants of FliKN display 28 uncontrolled hook-length, which is not consistent with a ruler mechanism. Here, we carried 29 out an extensive deletion analysis of FliKN to investigate its contribution in the hook-length 30 control mechanism. We identified FliKN mutants deleted for up to 80 aa that retained wildtype 31 motility. However, the short FliK variants did not produce shorter hook-lengths as expected 32 from a physical ruler. Rather, the minimal length of the hook depends on the level of hook 33 protein production and secretion. Our results thus support a model in which FliK functions as 34 a hook growth terminator protein that limits the maximal length of the hook, and not as a 35 molecular ruler that physically measures hook-length. 37Many bacteria use a complex nanomachine, called the flagellum, to propel themselves through 38 liquid environments. Flagella consist of three main structures, (i) a membrane-spanning basal 39 body complex, which is connected by (ii) a flexible hook to (iii) the long rigid filament (1). 40The hook adapts a curved structure composed of approximately 120 subunits of the same 41 protein, FlgE, reaching a length of 55 nm (± 6 nm) in Salmonella enterica (2-4). We recently 42 showed that the physiological hook-length of 55 nm is optimal for the formation of the 43 flagellar filament bundle in Salmonella, as too short hooks might be too stiff and too long ones 44 might buckle because of a too high flexibility, thereby disrupting the filament bundle also in 45 the absence of chemotactic stimuli (5). 46Hook-length control has been intensely studied over the years as unraveling the precise 47 underlying mechanism is complex. Even more so, considering that the control of hook-length 48 is directly connected with substrate specificity switching of the entire flagellar protein 49 secretion machinery (6). A type-III secretion system (T3SS) secretes most building blocks of 50 the flagellum. The primary export gate is made of FliP, FliQ and FliR, which form an unique 51 helical pore complex; and FlhA and FlhB, which make up the substrate docking platform and 52 are involved in energy transduction (7-11). Substrate specificity switching of the T3SS 53 describes the ability of this machinery to switch from recognizing early substrates, such as the 54 rod and hook subunits, to accepting and secreting late substrates, including the hook-filament 55 junction proteins, as well as the filament subunits (6, 9). This enables the cell to coordinate 56 gene expression with the spat...

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Section: Discussionsupporting

confidence: 84%