Abstract
Tubulin, the microtubule subunit protein, has been isolated from a soluble extract of pig platelets, by an in vitro polymerisation process. Several physicochemical properties of platelet tubulin have been investigated and compared with those of mammalian brain tubulins. The molecular weight of the tubulin monomeric subunits was found to be 55 000 by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, and the platelet protein co-migrated with rat, rabbit, guinea-pig, mouse and calf brain tubulins. The [ 3H]colchicine-binding dimer form of platelet tubulin sedimented in a 5% to 20% linear sucrose gradient with a sedimentation coefficient of 5.7 S, compared with that of 5.9 S for rat brain tubulin. The amino acid compositions of platelet and mammalian brain tubulins were found to be very similar, with glutamate and asparate as the predominant residues. Moreover, the one and two dimensional fingerprints of 125I-labelled tryptic peptides of platelet and brain tubulins showed considerable similarity. Platelet tubulin could be separated into two non-identical (α and β) subunits by Tris/glycine discontinuous alkaline sodium dodecyl sulphate-polyacrylamide gel electrophoresis and the β-tubulin had two or three 125I labelled peptides not seen in the α-tubulin. Two, or perhaps three, high-molecular-weight proteins were always present in the platelet tubulin samples, prepared by in vitro polymerisation and these had molecular weights greater than 200 000 and were probably analogous to the high-molecular weight proteins reported to be present in brain microtubule preparations. Platelet tubulin with its associated high molecular weight proteins will assemble in vitro into tubular structures which in electron micrographs resemble brain microtubules and the microtubules seen in whole platelets. It is believed that this is one of the first mammalian microtubule systems other than that of nervous tissue which has been isolated and studied in detail at the subunit level.
Published Version
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