In search of the primordial actin filament.

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 all filament structures determined to date. In contrast, association between strands involves different surfaces of the protofilaments to generate the diversity in multistrand filament architectures. This maintenance of contacts within a strand suggests that the primordial actin filament was single-stranded. In PNAS, Braun et al. (2) determine the structure of the crenactin filament from the Archaea Pyrobaculum calidifontis. Crenactin forms a single-stranded filament, and thus is a candidate presentday record of the lost ancestor of eukaryotic actin. Actin sequences that have recently been identified in Archaea are more closely related to eukaryotic actin than to prokaryotic actins (3, 4). Indeed, the closest homologs are found in Lokiarchaeota, which also possess homologs of the actin-severing protein gelsolin, suggesting a potential regulated actin filament system (5). Crenactin (crenarchaeal actin) from P. calidifontis is more distant than the Lokiarchaeota sequences (Fig. 1) and forms a cell shape-determining cytoskeleton. Previous structural studies had revealed that crenactin forms a protomer structure that is highly similar to actin, and a single-stranded “protofilament” was observed in the crystal packing (6, 7). Low-resolution electron micrographs (EMs) of crenactin filaments also indicated a structure that is consistent with either singleor double-stranded filaments (6). Now, Braun et al. (2) definitively show in an 18-Å cryo-EM reconstruction that crenactin can form a single-stranded filament in vitro. In determining whether crenactin represents a record of the ancestral eukaryotic actin filament, its binding partners and functions must be considered. Crenactin appears to interact with a unique set of proteins, the arcadins (3, 4). Crenactin’s role as a cell shape-determining cytoskeletal element has some parallels with the bacterial actin homologs MreB and FtsA, and the tubulin homolog FtsZ. Thus, crenactin may be a dedicated cell shape-determining actin. Evidence from bacteria suggests that when actin filaments have a single vital function, their sequences and filament arrangements have evolved to become optimized for that function (1). Consequently, one possible reason why crenactin forms a single-stranded filament may be that this conformation is optimal for its role in cell shape determination,