Peter W. Gunninga,*, Umesh Ghoshdastiderb,*, Shane Whitakera, David Poppb, Robert C. Robinsonb,c,1
a School of Medical Sciences, University of New South Wales, Sydney, NSW, 2052, Australia
b Institute of Molecular and Cell Biology, A*STAR (Agency for Science, Technology and Research), Biopolis, Singapore 138673
c Department of Biochemistry, National University of Singapore, 8 Medical Drive, Singapore 117597.
1 E-mail: email@example.com Tel: +65 6586 9832; Fax: +65 6779 1117
* These authors contributed equally to this work.
Published in J Cell Science on 18 March 2015.
The actin filament is astonishingly well conserved across a diverse set of eukaryotic species. It has essentially remained unchanged in the billion years that separate yeast, Arabidopsis and man. In contrast, bacterial actin-like proteins have diverged to the extreme, many of which are not readily identified from sequence-based homology searches. We use phylogenetic analyses that point to an evolutionary drive to diversify actin filament composition across kingdoms. Bacteria use a one-filament-one-function system to create distinct filament systems within a single cell. In contrast, eukaryotic actin is a universal force provider in a wide range of processes. In plants, there has been an expansion of the number of closely related actin genes, whereas in fungi and metazoa diversification in tropomyosins has increased the compositional variety in actin filament systems. Both mechanisms dictate the subset of actin-binding proteins that interact with each filament type, leading to specialization in function. In this Hypothesis, we thus propose that different mechanisms were selected in bacteria, plants and fungi/metazoa, which achieved actin filament compositional multiplicity leading to the expansion of their functional diversity.
Maximum likelihood (ML) phylogenetic tree of the actin-like bacterial proteins ParM, MreB, FtsA and eukaryotic actin. A) ParMs have diverged far more than the species in which they are found (B). In contrast, eukaryotic actins have hardly diverged in relation to their species. Horizontal branch lengths indicate the degree of divergence amongst the different proteins, and numbers indicate the branch support. The average numbers of substitutions per residue (SR) are 0.81, 1.16, and 4.10 for the MreBs, FtsAs and ParMs branches, respectively, and 0.08 between S. pombe (red hash) and human (blue hashes) cytoplasmic actins. Relatedness within FtsAs and MreBs, but often not for ParMs, generally follows the relatedness of the species (compare blue and red plus symbols, SRs for these proteins between two species of 0.10, 0.45 and 5.4, respectively), except where two MreBs are found in a single species (black asterisks, SR 1.05), which suggests lineage-specific gene duplication. There, a likely diversification in function will have led to greater sequence variation. ParMs from the same host species can be highly divergent (green asterisks, SR 4.55).
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