Equimolar heterodimers in microtubules.

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Abstract
Two equimolar .beta. chains can be resolved from sea urchin [Tripneustes gratilla and Strongylocentrotus droebachiensis] sperm flagellar and scallop [Aequipecten irradians] gill ciliary tubulins, as well as from certian brain tubulins [beef, chick, skate], using the Triton X-100-acid-urea polyacrylamide gel system commonly used for histone analysis. The .beta. chains are identified as such from their mobility on urea-free SDS PAGE [sodium dodecyl sulfate polyacrylamide gel electrophoresis], from amino acid composition, and from tryptic peptide distribution. Scallop .beta. chains have almost identical amino acid profiles but they differ by one tryptic peptide. Optimal conditions for .beta. chain resolution are very species-dependent, with some closely related species showing either maximal or no .beta. chain separation. In addition, beef brain tubulin on Triton X-100-acid-urea electrophoresis and scallop gill ciliary tubulin upon isoelectric focusing in the presence of SDS show 2 approximately equimolar .alpha. chains. These data, indicating equimolar amounts of 2 potentially different tubulin heterodimers from a variety of microtubule types, support a model for microtubule structure wherein protofilaments consist of alternating heterodimers of 2 kinds, generating a 16-nm (2-dimer) axial repeat.

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