An asymmetric sheath controls flagellar supercoiling and motility in the leptospira spirochete.

Spirochete bacteria, including important pathogens, exhibit a distinctive means of swimming via undulations of the entire cell. Motility is powered by the rotation of supercoiled 'endoflagella' that wrap around the cell body, confined within the periplasmic space. To investigate the structural basis of flagellar supercoiling, which is critical for motility, we determined the structure of native flagellar filaments from the spirochete Leptospira by integrating high-resolution cryo-electron tomography and X-ray crystallography. We show that these filaments are coated by a highly asymmetric, multi-component sheath layer, contrasting with flagellin-only homopolymers previously observed in exoflagellated bacteria. Distinct sheath proteins localize to the filament inner and outer curvatures to define the supercoiling geometry, explaining a key functional attribute of this spirochete flagellum.

The kinesin-5 tail domain directly modulates the mechanochemical cycle of the motor domain for anti-parallel microtubule sliding.

Kinesin-5 motors organize mitotic spindles by sliding apart microtubules. They are homotetramers with dimeric motor and tail domains at both ends of a bipolar minifilament. Here, we describe a regulatory mechanism involving direct binding between tail and motor domains and its fundamental role in microtubule sliding. Kinesin-5 tails decrease microtubule-stimulated ATP-hydrolysis by specifically engaging motor domains in the nucleotide-free or ADP states. Cryo-EM reveals that tail binding stabilizes an open motor domain ATP-active site. Full-length motors undergo slow motility and cluster together along microtubules, while tail-deleted motors exhibit rapid motility without clustering. The tail is critical for motors to zipper together two microtubules by generating substantial sliding forces. The tail is essential for mitotic spindle localization, which becomes severely reduced in tail-deleted motors. Our studies suggest a revised microtubule-sliding model, in which kinesin-5 tails stabilize motor domains in the microtubule-bound state by slowing ATP-binding, resulting in high-force production at both homotetramer ends.

Circular oligomerization is an intrinsic property of synaptotagmin.

Previously, we showed that synaptotagmin1 (Syt1) forms Ca2+-sensitive ring-like oligomers on membranes containing acidic lipids and proposed a potential role in regulating neurotransmitter release (Zanetti et al., 2016). Here, we report that Syt1 assembles into similar ring-like oligomers in solution when triggered by naturally occurring polyphosphates (PIP2 and ATP) and magnesium ions (Mg2+). These soluble Syt1 rings were observed by electron microscopy and independently demonstrated and quantified using fluorescence correlation spectroscopy. Oligomerization is triggered when polyphosphates bind to the polylysine patch in C2B domain and is stabilized by Mg2+, which neutralizes the Ca2+-binding aspartic acids that likely contribute to the C2B interface in the oligomer. Overall, our data show that ring-like polymerization is an intrinsic property of Syt1 with reasonable affinity that can be triggered by the vesicle docking C2B-PIP2 interaction and raise the possibility that Syt1 rings could pre-form on the synaptic vesicle to facilitate docking.