Biology subjects

Sindelar, C. V.

Publications and source records attributed to Sindelar, C. V..

5 recordsLinked to original sources

bioRxiv2026

Core-sheath coupling controls flagellar curvature and motility in Leptospira

Spirochete pathogens are among the most invasive bacteria known, causing syphilis, Lyme disease, and leptospirosis. Their tissue penetration depends on periplasmic flagellar filaments that, unlike other bacterial flagella, are encased in a spirochete-specific multi-protein sheath and deform the cell body into motile waves. How these filaments achieve the mechanical properties needed for invasive motility has remained unclear. Here we determine complete atomic structures of the Leptospira endoflagellar filament, revealing an elaborate sheath of 9 to 12 distinct asymmetrically arranged proteins. We show that the flagellin variant forming the filament core determines sheath composition, producing curvatures ranging from [~]3.5 {micro}m-1 to [~]5 {micro}m-1. The lower-curvature architecture, employed by pathogenic Leptospira interrogans, proves essential for motility in viscous environments and during infection. Thus, Leptospira achieves environment-specific motility through modular core-sheath coupling, linking atomic-scale structural plasticity to large-scale changes in swimming behaviour. Conservation of key sheath components suggests this mechanism may extend across spirochetes.

microbiology↗

bioRxiv2026

Asymmetric architecture and adaptation of Treponema flagella

Spirochetes exhibit a distinctive corkscrew-like motility driven by periplasmic flagella that wrap around the cell body in a supercoiled configuration, yet the structural basis of this unique propulsion remains poorly understood. Here we combine cryo-electron microscopy, cryo-electron tomography, and genetic and biochemical analyses to determine the assembly and adaptation principles of the supercoiled flagellar filament in Treponema denticola, a major periodontal pathogen. Near-atomic structures reveal a glycosylated FlaB flagellin core encased by a previously unrecognized asymmetric sheath. The major sheath protein FlaA forms the bulk of the sheath and mechanically couples to the core through defined interfaces required for efficient motility, whereas four minor sheath proteins (FlaA1, FlaA2, FlaAP1, and FlaAP2) assemble along the concave side of the filament to accommodate intrinsic curvature. Disruption of this asymmetric core-sheath organization compromises force transmission and impairs motility, establishing coordinated asymmetric assembly as a fundamental mechanism underlying spirochetal motility.

molecular biology↗

bioRxiv2025

High resolution structures of Myosin-IC reveal a unique actin-binding orientation, ADP release pathway, and power stroke trajectory

Myosin-IC (myo1c) is a class-I myosin that supports transport and remodeling of the plasma membrane and membrane-bound vesicles. Like other members of the myosin family, its biochemical kinetics are altered in response to changes in mechanical loads that resist the power stroke. However, myo1c is unique in that the primary force-sensitive kinetic transition is the isomerization that follows ATP binding, not ADP release as in other slow myosins. Myo1c also powers actin gliding along curved paths, propelling actin filaments in leftward circles. To understand the origins of this unique force-sensing and motile behavior, we solved actin-bound myo1c cryo-EM structures in the presence and absence of ADP. Our structures reveal that in contrast with other myosins, the myo1c lever arm swing is skewed, partly due to a different actin interface that reorients the motor domain on actin. The structures also reveal unique nucleotide-dependent behavior of both the nucleotide pocket as well as an element called the N-terminal extension. We incorporate these observations into a model that explains why force primarily regulates ATP binding in myo1c, rather than ADP release as in other myosins. Integrating our cryo-EM data with available crystallography structures allows the modeling of full-length myo1c during force generation, supplying insights into its role in membrane remodeling. These results highlight how relatively minor sequence differences in members of the myosin superfamily can significantly alter power stroke geometry and force-sensing properties, with important implications for biological function. Significance StatementMyosin-IC (myo1c) uses an ATP-driven power-stroke to support slow intracellular membrane and vesicle transport. We used cryo-electron microscopy to understand adaptations of myo1c to perform its unique roles. We discovered an altered interface between myo1c and actin compared with the closely related myo1b, which repositions the motor domain and alters the trajectory of its lever arm swing compared to other myosins. This explains why myo1c propels actin filaments in a leftward circular path. We also discovered a unique role in force sensing for a structural element called the N-terminal extension and built a full-length atomic model for the myo1c power-stroke. Our findings highlight how myosins can tune their power-stroke geometries and force-sensing properties to adapt to diverse cellular functions.

biophysics↗

bioRxiv2022

Structural basis of flagellar filament asymmetry and supercoil templating by Leptospira spirochete sheath proteins

Several Leptospira species are bacterial agents of leptospirosis, a neglected tropical disease responsible for ~1 million cases and 50,000 deaths each year worldwide. Leptospira, like other members of the Spirochaeta phylum, possess specially adapted flagella that remain confined within the periplasm. These appendages drive a unique, corkscrew-like swimming style that enables efficient motility and pathogenesis. However, the composition, function, and molecular architecture of spirochetal flagellar filaments remain poorly understood. We solved single-particle cryo-EM structures of isolated Leptospira flagellar filaments, comparing the wild-type form to two mutant forms with different missing components and abrogated motilities. The structures reveal a complex proteinaceous sheath surrounding a conserved core composed of the FlaB flagellin homolog. Sheath proteins were found to fall into two distinct categories, both of which are required for motility. Filament coiling proteins, FcpA and FcpB, exert force on the filament when they bind its surface, causing the filament to stretch. In contrast, we identify sheath components FlaAP (newly discovered in this study) and FlaA2 as template factors, which have little effect on filament shape by themselves, but partition the coiling proteins to one side of the filament. In this way, the two types of Leptospira sheath factors operate collectively on the flagellar filament to bend it from a relaxed form associated with cell immobility, to a motility-competent shape that is tightly supercoiled. Our structures also indicate that core-sheath interactions are largely mediated by carbohydrate moieties from flagellin core side chain O-glycosylations. The supercoiling mechanism presented here provides a benchmark for studies with other bacteria, for which near-atomic resolution structures of flagellar filament in native supercoiled forms, are still lacking.

biophysics↗

bioRxiv2019

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 the spirochete flagellum. One Sentence summaryThe corkscrew-like motility of Spirochete bacteria is enabled by a unique, asymmetrically constructed flagellum that wraps around the cell body within the periplasm.

biophysics↗