Evolution of spectrin function in cytoskeletal and membrane networks

Spectrin is a cytoskeletal protein thought to have descended from an α-actinin-like ancestor. It emerged during evolution of animals to promote integration of cells into tissues by assembling signalling and cell adhesion complexes, by enhancing the mechanical stability of membranes and by promoting assembly of specialized membrane domains. Spectrin functions as an (αβ_[H])₂ tetramer that cross-links transmembrane proteins, membrane lipids and the actin cytoskeleton, either directly or via adaptor proteins such as ankyrin and 4.1. In the present paper, I review recent findings on the origins and adaptations in this system. (i) The genome of the choanoflagellate Monosiga brevicollis encodes α-, β- and β_Heavy-spectrin, indicating that spectrins evolved in the immediate unicellular precursors of animals. (ii) Ankyrin and 4.1 are not encoded in that genome, indicating that spectrin gained function during subsequent animal evolution. (iii) Protein 4.1 gained a spectrin-binding activity in the evolution of vertebrates. (iv) Interaction of chicken or mammal β-spectrin with PtdInsP₂ can be regulated by differential mRNA splicing, which can eliminate the PH (pleckstrin homology) domain in βI- or βII-spectrins; in the case of mammalian βII-spectrin, the alternative C-terminal region encodes a phosphorylation site that regulates interaction with α-spectrin. (v) In mammalian evolution, the single pre-existing α-spectrin gene was duplicated, and one of the resulting pair (αI) neo-functionalized for rapid make-and-break of tetramers. I hypothesize that the elasticity of mammalian non-nucleated erythrocytes depends on the dynamic rearrangement of spectrin dimers/tetramers under the shearing forces experienced in circulation.