Bacterial Actins and Their Interactors

Prospects for the Mechanism of Spiroplasma Swimming

Spiroplasma are helical bacteria that lack a peptidoglycan layer. They are widespread globally as parasites of arthropods and plants. Their infectious processes and survival are most likely supported by their unique swimming system, which is unrelated to well-known bacterial motility systems such as flagella and pili. Spiroplasma swims by switching the left- and right-handed helical cell body alternately from the cell front. 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 called the "ribbon" has been focused to elucidate the mechanisms for the cell helicity formation and swimming. The ribbon is composed of Spiroplasma-specific fibril protein and a bacterial actin, MreB. Here, we 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.

Actin: Structure, Function, Dynamics, and Interactions with Bacterial Toxins

Actin is one of the most abundant proteins in any eukaryotic cell and an indispensable component of the cytoskeleton. In mammalian organisms, six highly conserved actin isoforms can be distinguished, which differ by only a few amino acids. In non-muscle cells, actin polymerizes into actin filaments that form actin structures essential for cell shape stabilization, and participates in a number of motile activities like intracellular vesicle transport, cytokinesis, and also cell locomotion. Here, we describe the structure of monomeric and polymeric actin, the polymerization kinetics, and its regulation by actin-binding proteins. Probably due to its conserved nature and abundance, actin and its regulating factors have emerged as prefered targets of bacterial toxins and effectors, which subvert the host actin cytoskeleton to serve bacterial needs.