One paradox of evolution is the actin filament, which is an obligate right-handed, double-stranded helical filament in eukaryotes, yet forms diverse architectures in prokaryotes (1) (Fig. 1). Uncovering the origin of this asymmetrical distribution in filament morphologies is fundamental to understanding the emergence of the domains of cellular life. The variation in actin amino acid sequences magnifies the diversity in filament structures (Fig. 1). Eukaryotic actins are far more highly related than their parent genomes, whereas prokaryotic actins, particularly plasmid-based actins, are uncommonly diverged and are often difficult to identify from sequence-based homology searches (Fig. 1). Despite this variance, two features are preserved between prokaryotic and eukaryotic actins. The first common feature is the ATP-binding site, which operates as an ATP-hydrolysis and phosphate-release controlled conformational switch that is activated by polymerization. The ATP switch acts as a timing mechanism to coordinate depolymerization, conferring on actins the ability to polymerize dynamically and, subsequently, to depolymerize. The second feature is the conservation of in-strand protomer contacts, particularly the association between subdomains 3 and 4, which have been observed in …
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