Decoding the swimming secrets of Spiroplasma: a unified framework for motility in a wall-less bacterium

Spiroplasma , a wall-less helical bacterium, exhibits a unique kink-based swimming mechanism that fundamentally differs from flagella- or pilus-driven motility. This review synthesizes two decades of evidence from cryo-electron microscopy, genetics, biochemistry, and synthetic reconstitution to propose a unified framework that couples structure, function, and regulation. The helical geometry of the cell is primarily defined by the self-assembling Fibril protein. The key to understanding motility, however, lies in the functional remodeling of the bacterial actin homolog MreB. In the absence of a cell wall, the five MreB paralogs (MreB1–MreB5) have undergone precise division of labor. This review categorizes them into three functional modules: a power module (MreB2 and MreB5) that drives dynamic remodeling via ATP hydrolysis; a regulatory module (MreB1 and MreB4) that controls the polymerization-depolymerization cycle of the power module and couples the cytoskeleton to the membrane; and an anchoring module (MreB3), which utilizes its N-terminal amphipathic helix to stabilize the motility apparatus on the membrane. This functional specialization enables tight coupling between ATP hydrolysis and the propagation of kinks along the fibril scaffold. While ATP hydrolysis provides the direct mechanical energy for movement, the transmembrane potential creates a permissive electrochemical environment essential for motor function. This integrated model, which links helical morphology, kink-based propulsion, and modular molecular machinery, advances our understanding of motility diversity in wall-less bacteria and provides a conceptual basis for developing strategies to target this unique cytoskeletal system.