Cooperative and divergent properties of bacterial actin isoforms in Spiroplasma swimming

The cytoskeleton, comprising intracellular filamentous structures composed of polymerized proteins, is crucial for the survival of both eukaryotes and prokaryotes. Although bacterial cytoskeletal proteins have diverged, they generally do not drive cellular motility. Spiroplasma, a genus of wall-less helical bacteria, swims by propagating a helicity-switching point (kink) along its cell axis. Unlike typical walled bacteria, whose motility depends on widespread motility machineries such as flagella and pili, Spiroplasma swimming is powered by the coordinated dynamics of five isoforms of bacterial actin MreB (SMreB1-5), which are grouped into three phylogenetic classes: SMreB1 and 4, SMreB2 and 5, and SMreB3. Despite the efforts to understand Spiroplasma swimming, its molecular mechanism remains unclear. In this review, we summarize how in vitro analyses of SMreBs have provided mechanistic insights into Spiroplasma swimming. While all SMreBs conserve the canonical actin fold, each SMreB class exhibits unique characteristics in its polymerized structures, ATPase activities, polymerization dynamics, and membrane binding. Studies of an essential SMreB subset for Spiroplasma swimming, i.e. SMreB1 and SMreB5, have revealed that SMreB1 binds to polymerized SMreB5 and disassembles it depending on the nucleotide state. These results challenge the previous model in which Spiroplasma swimming is driven by the coordinated extension and contraction of two distinct SMreB filaments. Finally, we discuss potential molecular mechanisms underlying Spiroplasma swimming and highlight key questions that must be answered to validate these models.