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A protein that extends >0.4 µm across the cytoplasm might sound bizarre, but a member of a recently identified family of proteins — the spectraplakins — might do just that. The spectraplakins, which include Drosophila Short stop and mammalian BPAG1/dystonin, are cytoskeletal crosslinkers that interact with all three cytoskeletal filaments and could contain up to 9000 residues. In a Commentary on p. 4215, Nicholas Brown and co-workers discuss the roles of these proteins, which appear to be evolutionary precursors of the spectrin and plakin families. Each gene appears to encode a host of different splice forms, which contain various combinations of calponin-homology domains, plakin domains, GAS2 domains, plectin and spectrin repeats, and EF hands. Like spectrins and plakins, the spectraplakins link the plasma membrane with the cytoskeleton. They might also act as scaffolds that recruit signalling molecules to the cytoskeleton or generate cortical subdomains. Indeed, the fact that the largest predicted spectraplakin isoforms have actin- and microtubule-binding sites separated by 0.4 µm raises the intriguing prospect that an intracellular structure depends upon this spacing.
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