Umesh Ghoshdastider1, Shimin Jiang1, David Popp1 and Robert C. Robinson1,2
1 Institute of Molecular and Cell Biology, A*STAR (Agency for Science, Technology and Research), Biopolis, Singapore 138673.
2 Department of Biochemistry, Yong Loo Lin School of Medicine, National University of Singapore, Singapore 117597.
Published online in PNAS, on 15th July, 2015
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 dynamically polymerize and subsequently depolymerize. The second feature is the conservation of in-strand protomer contacts, in particular 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 multi-strand 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 present-day record of the lost ancestor of eukaryotic actin.
1 Gunning PW, Ghoshdastider U, Whitaker S, Popp D, Robinson RC (2015) The evolution of compositionally and functionally distinct actin filaments. J Cell Sci 128(11):2009-2019.
2 Braun T, Orlova A, Valegård K, Ann-Christin Lindås A-C, Schröder GF, Egelman EH (2015). Archaeal actin from a hyperthermophile forms a single-stranded filament. PNAS published ahead of print June 29, 2015,
Fig. 1 Relatedness of actins. The structures of actin protofilaments are shown below a maximum-likelihood phylogenetic tree of the actin protein sequences. The structures are aligned via the central protomer, which is shown as a ribbon (slate blue) with the nucleotide in (red). The subdomains are numbered for actin. Subsequent protomers are shown as surfaces (alternating between fawn and slate blue). For dynactin, Arp1 is in slate blue or fawn, actin is in black, and Arp11 is in red. The number of protofilaments that comprise each filament is indicated in blue. BLAST protein sequence similarities to human skeletal actin are given in red. BLAST searches with the human skeletal actin sequence align less than half of each prokaryotic actin sequence.
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