The Nature of the Actin Molecule
The actin molecule is one of nature’s most remarkable objects: its very ubiquity throughout eukaryotic biology and the tenacity of its structure in the face of aeons of evolutionary adaptation bespeak a fundamental functional characteristic that invites our curiosity. Actin stands at the crossroads where eukaryotes first diverged from our unicellular prokaryotic ancestors, a point when cells learned to replicate and package their genomic material and to divide into daughter cells. In the structural timescape, it was time to abandon rigid cell-walls and simple flagellar-based motility, and to invest the cytoplasm with the vast new organising potential afforded by the actin molecule. The essential utility of the actin molecule for the cyto-architect is that it can spontaneously assemble into long flexible filaments, from which bundles, cables and mesh-works can be constructed. Through such extended structures forces can be transmitted, sieves and boundaries assembled, and cell membranes buttressed. Add to this the capacity to alter in a controlled manner the pattern of interconnections and the cell acquires the means to move, change shape and perform mechanical work. It is our view that, once the actin molecule emerged at the dawn of the eukaryotes as a universal mechanochemical coupling unit, its structure remained virtually unaltered throughout evolution. Instead, other actin binding proteins evolved capable of modifying and controlling the behaviour of actin to meet new functional requirements.
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