9.B.460.  The Spiroplasma Cytoskeleton Underlying Motility (SCUM) Family 

The Spiroplasma cell is a helical dynamic membranal tube (diameter approximately 0.2 microm). A flat cytoskeletal ribbon of parallel fibrils is attached to the inside of the cellular tube. Both tube and cytoskeleton are mutually coiled into a dynamic helix driven by differential length changes of the fibrils, which function as linear motors. The cytoskeletal ribbon follows the shortest (inner) helical line on the inner surface of the cellular tube. The structural unit of the contractile cytoskeleton is a approximately 50-Angstrom-wide filament comprised of pairs of the 59-kD fib gene product. The monomers are arranged in pairs with opposite polarities allowing for a approximately 100-Angstrom-long axial repeat. The functional unit of the contractile cytoskeletal ribbon is a fibril comprised of an aligned pair of filaments. Neighboring repeats form a tetrameric ring with a lateral repeat of approximately 100 Å. The axial length of the rings may shorten by approximately 40%, driving the changes in the fibril lengths and, consequently, helical dynamics. Local length changes result in helical symmetry breaking and nonreciprocating cell movements allowing for net directional displacement. Flexing allows for changes in swimming direction (Trachtenberg 2004). 

Spiroplasma swims by switching the left- and right-handed helical cell body alternately from the cell front (Sasajima and Miyata 2021). The kinks generated by the helicity shift travel down along the cell axis and rotate the cell body posterior to the kink position like a screw, pushing the water backward and propelling the cell body forward. An internal structure, the "ribbon", mediates the mechanisms for cell helicity formation and swimming. The ribbon is composed of Spiroplasma-specific fibril protein and a bacterial actin, MreB. Sasajima and Miyata 2021 propose a model for helicity-switching swimming focusing on the ribbon, in which MreBs generate a force like a bimetallic strip based on ATP energy and switch the handedness of helical fibril filaments. Cooperative changes of these filaments cause helicity to shift down the cell axis. Interestingly, unlike other motility systems, the fibril protein and Spiroplasma MreBs can be traced back to their ancestors. The fibril protein has evolved from methylthioadenosine/S-adenosylhomocysteine (MTA/SAH) nucleosidase, which is essential for growth, and MreBs, which function as a scaffold for peptidoglycan synthesis in walled bacteria (Sasajima and Miyata 2021).  The spiroplasma MreB protein is homologous t9 actin-like proteins in TC Family 9.B.157. 

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