There can be no doubt now that many types of cell that neither contract nor contain organized myofibrils nevertheless contain muscle-like proteins. A very long list could be made of the sources, ranging from vertebrate brain and spermatazoa to ameba and green algae, which have been shown to contain actin-like filaments or to yield actomyosin-like extracts. In the best studied of these, such as blood platelets (Bettex-Galland and Lüscher, 1965; Adelstein et al., 1971), the similarities to the muscle system are impressive and include proteins which purify together with muscle actin or myosin, have the same molecular weights, solubilities and enzymatic properties, and which cross-react with their muscle analogs.
No takes yet. Share an insight, caveat, or question.
Douglas Bray (1973) studied this question.