Calcium-binding proteins in a vorticellid contractile organelle

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Abstract
The proteins of the contractile spasmoneme of Zoothamnium have been examined for comparison with other motile systems. Though capable of calcium-induced contraction, glycerinated preparations of the spasmoneme contain neither actin nor tubulin at levels that can be detected in polyacrylamide gels. Sixty per cent of the protein in sodium dodecyl sulphate gels migrates in a band at a molecular weight of approximately 20000, consisting largely of 2 similar protein species which are here given the name of spasmins. The amino acid composition of 2 spasmin fractions has been determined by a fluorimetric method. They are rich in Asx, Glx and serine, but have few aromatic amino acids and no cystine or methionine. In calcium-buffered poly-acrylamide gels, it was observed that a reduction in the electrophoretic mobility of the spasmins was induced specifically by calcium (but not magnesium) at the same low concentiations as induce contraction. This indicates that the spasmins are calcium-binding proteins which may be involved directly in the calcium-induced contraction of the spasmoneme.