A comparative study of high molecular weight proteins in various types of muscle across the animal kingdom.

A wide range of phyla have been surveyed by SDS-PAGE for the new large proteins of the myofibril. Connectin (or titin) appears to be widely distributed. It is seen as a band of constant intensity and mobility in vertebrate striated muscle, but is absent from smooth muscle. It appears in more variable amounts, in a form of constant but greater mobility in many invertebrates: worms, molluscs (adductor but not gastropod feet), insects, a myriapod, and even in human blood platelets. Nebulin shares the same distribution in vertebrate muscles except for its notable absence in all heart muscle examined. It too is found in many invertebrates, not always with titin. It has been found in a worm, molluscs (adductor and gastropod feet), insects, crustaceans and an echinoderm. The mobility of nebulin varies within the vertebrates and more so between invertebrates (where, as with titin, it is greater). The isoforms of filamin in skeletal, cardiac, and smooth muscles of vertebrates are recorded. C-protein in rabbit muscles has four isoforms: white, alpha-red (X-protein), beta-red, and cardiac.