We investigated the supramolecular structure of the SHIGELLA: type III secretion machinery including its major components. Our results indicated that the machinery was composed of needle and basal parts with respective lengths of 45.4 +/- 3.3 and 31.6 +/- 0.3 nm, and contained MxiD, MxiG, MxiJ and MxiH. spa47, encoding a putative F(1)-type ATPase, was required for the secretion of effector proteins via the type III system and was involved in the formation of the needle. The spa47 mutant produced a defective, needle-less type III structure, which contained MxiD, MxiG and MxiJ but not MxiH. The mxiH mutant produced a defective type III structure lacking the needle and failed to secrete effector proteins. Upon overexpression of MxiH in the mxiH mutant, the bacteria produced type III structures with protruding dramatically long needles, and showed a remarkable increase in invasiveness. Our results suggest that MxiH is the major needle component of the type III machinery and is essential for delivery of the effector proteins, and that the level of MxiH affects the length of the needle.

Eukaryotic flagella beat rhythmically. Dynein is a protein that powers flagellar motion, and oscillation may be inherent to this protein. Here we determine whether oscillation is a property of dynein arms themselves or whether oscillation requires an intact axoneme, which is the central core of the flagellum and consists of a regular array of microtubules. Using optical trapping nanometry, we measured the force generated by a few dynein arms on an isolated doublet microtubule. When the dynein arms on the doublet microtubule contact a singlet microtubule and are activated by photolysis of caged ATP8, they generate a peak force of approximately 6pN and move the singlet microtubule over the doublet microtubule in a processive manner. The force and displacement oscillate with a peak-to-peak force and amplitude of approximately 2 pN and approximately 30 nm, respectively. The geometry of the interaction indicates that very few (possibly one) dynein arms are needed to generate the oscillation. The maximum frequency of the oscillation at 0.75 mM ATP is approximately 70 Hz; this frequency decreases as the ATP concentration decreases. A similar oscillatory force is also generated by inner dynein arms alone on doublet microtubules that are depleted of outer dynein arms. The oscillation of the dynein arm may be a basic mechanism underlying flagellar beating.

Axonemal dyneins are force-generating ATPases that produce movement of eukaryotic cilia and flagella. Several studies indicate that inner-arm dyneins mainly produce bending moments in flagella and that these motors have inherent oscillations in force and motility. Processive motors such as kinesins have high duty ratios of attached to total ATPase cycle (attached plus detached) times compared to sliding motors such as myosin. Here we provide evidence that subspecies-c, a single-headed axonemal inner-arm dynein, is processive but has a low duty ratio. Ultrastructurally it is similar to other dyneins, with a single globular head, long stem and a slender stalk that attaches to microtubules. In vitro studies of microtubules sliding over surfaces coated with subspecies-c at low densities (measured by single-molecule fluorescence) show that a single molecule is sufficient to move a microtubule more than 1 microm at 0.7 microm s(-1). When many motors interact the velocity is 5.1 microm s(-1), fitting a duty ratio of 0.14. Using optical trap nanometry, we show that beads carrying a single subspecies-c motor move processively along the microtubules in 8-nm steps but slip backwards under high loads. These results indicate that dynein subspecies-c functions in a very different way from conventional motor proteins, and has properties that could produce self-oscillation in vivo.

High-speed atomic force microscopy was employed to observe structural changes in actin filaments induced by cofilin binding. Consistent with previous electron and fluorescence microscopic studies, cofilin formed clusters along actin filaments, where the filaments were 2-nm thicker and the helical pitch was ∼25% shorter, compared to control filaments. Interestingly, the shortened helical pitch was propagated to the neighboring bare zone on the pointed-end side of the cluster, while the pitch on the barbed-end side was similar to the control. Thus, cofilin clusters induce distinctively asymmetric conformational changes in filaments. Consistent with the idea that cofilin favors actin structures with a shorter helical pitch, cofilin clusters grew unidirectionally toward the pointed-end of the filament. Severing was often observed near the boundaries between bare zones and clusters, but not necessarily at the boundaries.DOI: http://dx.doi.org/10.7554/eLife.04806.001

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