Biology subjects

MOTALEB, M. A.

Publications and source records attributed to MOTALEB, M. A..

6 recordsLinked to original sources

bioRxiv2026

Asymmetric flagellar filaments shape Borreliella burgdorferi morphology and motility

The Lyme disease spirochete Borreliella burgdorferi possesses bipolar periplasmic flagella that are essential for its distinctive flat-wave morphology, motility, and infectivity. However, how the molecular architecture of these flagella contributes to spirochetal morphology and motility remains poorly understood. Here, using in situ single-particle cryo-electron microscopy, we determine near-atomic structures of native periplasmic flagellar filaments and reveal two distinct architectures: half-sheathed and unsheathed. We show that the spirochete-specific protein FlaA forms an asymmetric sheath along the convex surface of the conserved FlaB filament core, with sheath assembly restricted to the proximal filament region near the flagellar motor. Atomic models reveal that FlaA anchors to the FlaB core through specific interactions with two surface-exposed loops, while genetic and motility analyses demonstrate distinct roles for these loops in maintaining cell morphology and motility. Structural comparisons reveal that the FlaB core possesses intrinsic curvature and supercoiling, which are substantially amplified by asymmetric FlaA assembly. These findings support a model in which localized FlaA sheath assembly reinforces the proximal filament and potentially modulates its mechanical properties. Together, our results uncover a previously unrecognized mechanism by which asymmetric sheath assembly enhances flagellar supercoiling, providing a molecular basis for understanding the distinctive morphology and motility of spirochetes.

microbiology↗

bioRxiv2026

A molecular clutch governs stator engagement and flagellar torque

Motile bacteria must continuously adjust their motility to navigate and colonize diverse environments, including mammalian hosts, yet the underlying mechanisms remain poorly understood. Here, using in situ single-particle cryo-electron microscopy, we determine near-atomic structures of the flagellar motor in the Lyme disease spirochete Borrelia burgdorferi and uncover a novel mechanism for controlling torque generation. We identify a distinct neutral state in which individual torque-generating MotA5B2 stator complexes remain incorporated in the motor but adopt an extended, tilted conformation disengaged from the rotor. Two conserved flagellar proteins, FliL and FlcA, form a transmembrane scaffold that retains 16 stator complexes, while FapA, a newly identified multidomain flagellar protein, binds the cytoplasmic base of the stator complex and stabilizes its rotor-disengaged conformation. Strikingly, rotor-engaged and rotor-disengaged stators coexist within individual motors, demonstrating that stator retention and torque engagement are separable states. Our findings establish a motor-retained neutral stator state and reveal a molecular-clutch mechanism that independently controls the engagement of individual stator units with the rotor, enabling rapid and precise regulation of flagellar torque and motility.

microbiology↗

bioRxiv2026

In situ cryo-EM reveals how a flagellar motor transmits high torque

Bacterial flagellar motors are diverse rotary machines that require robust mechanical coupling to transmit torque across components with distinct architectures, yet how such coupling is achieved remains poorly understood. Here, we determine near-atomic in situ structures of the periplasmic flagellar motor in the Lyme disease spirochete Borrelia burgdorferi, revealing how an expanded rotor is mechanically coupled to the conserved rod through symmetry breaking and local remodeling of the rotor rod interface. Surprisingly, pentameric FrbA forms a split washer-shaped brace around the rotor rod junction, providing a second layer of mechanical reinforcement. Deletion of frbA destabilizes distal flagellar structures and impairs motility, particularly under increased viscous load. FrbA homologues are broadly distributed across spirochetes, suggesting a conserved mechanism for reinforcing high-torque flagellar motors. Together, these findings reveal how symmetry breaking couples an expanded rotor to a conserved driveshaft, while a molecular split washer reinforces this interface to enable robust torque transmission and motility.

microbiology↗

bioRxiv2026

In-situ structure of the flagellar export apparatus in Borrelia burgdorferi

The transmembrane export apparatus is a conserved core of bacterial type III secretion systems, shared by the flagellum and injectisome. Powered by the proton-motive force (PMF), this machinery translocates protein substrates across the bacterial envelope to support motility and virulence. However, its assembly and operation within native membranes remain poorly understood. Here, using in-situ single-particle cryo-electron microscopy, we resolve near-atomic-resolution structures of the flagellar export apparatus within the periplasmic flagellar motor of the Lyme disease spirochete Borrelia burgdorferi. Structural and genetic analyses suggest that coordinated, stepwise assembly of the flagellar export apparatus and MS-ring drives local deformation of the cytoplasmic membrane through extensive protein-lipid-protein interactions, constructing a funnel-shaped conduit optimal for substrate translocation. In addition, a conserved hydrophilic pathway lies within core export protein FlhA, with key acidic residues essential for export activity and supporting PMF-driven substrate translocation. Together, these findings establish a mechanistic framework for export apparatus assembly and energy transduction, defining core principles of type III secretion.

microbiology↗

bioRxiv2023

Quantitative analyses of interactions between SpoVG and RNA/DNA

1.The Borrelia burgdorferi SpoVG protein has previously been found to be a DNA- and RNA-binding protein. To aid in the elucidation of ligand motifs, affinities for numerous RNAs, ssDNAs, and dsDNAs were measured and compared. The loci used in the study were spoVG, glpFKD, erpAB, bb0242, flaB, and ospAB, with particular focus on the untranslated 5 portion of the mRNAs. Performing binding and competition assays yielded that the 5 end of spoVG mRNA had the highest affinity while the lowest observed affinity was to the 5 end of flaB mRNA. Mutagenesis studies of spoVG RNA and ssDNA sequences suggested that the formation of SpoVG-nucleic acid complexes are not entirely dependent on either sequence or structure. Additionally, exchanging uracil for thymine in ssDNAs did not affect protein-nucleic acid complex formation.

microbiology↗

bioRxiv2021

Characterization of the flagellar collar reveals structural plasticity essential for spirochete motility

Spirochetes are a remarkable group of bacteria with distinct morphology and periplasmic flagella that enable motility in viscous environments, such as host connective tissues. The collar, a spirochete-specific complex of the periplasmic flagellum, is required for the unique spirochete motility, yet it has not been clear how the collar assembles and enables spirochetes to transit between complex host environments. Here, we characterize the collar complex in the Lyme disease spirochete Borrelia burgdorferi. We discover as well as delineate the distinct functions of two novel collar proteins, FlcB and FlcC, by combining subtractive bioinformatic, genetic, and cryo-electron tomography approaches. Our high-resolution in-situ structures reveal that the multi-protein collar has a remarkable structural plasticity essential not only for assembly of flagellar motors in the highly curved membrane of spirochetes but also for generation of the high torque necessary for spirochete motility.

microbiology↗