A novel microtubule protein in the marginal band of human blood platelets

A characterization is reported of the major cytoskeletal protein, called IEF (isoelectric focusing)-51K, of marginal band microtubule cells from human blood platelets (Kenney, D.M. and Linck, R.W. (1985) J. Cell Sci. 78, 1-22). IEF-51K is a unique biochemical species which is distinguishable from platelet and mammalian neuronal α-tubulin and β-tubulin by 1) its faster mobility on discontinuous sodium dodecyl sulfate electrophoresis corresponding to an apparent M(r) 51,000; 2) its more alkaline relative isoelectric point at pH 5.7 compared with that of α- and β-tubulin at pH 5.3 and 5.5, respectively; 3) lack of coincidence in peptide maps prepared with chymotrypsin or Staphylococcus aureus V8 protease; and 4) lack of immunochemical cross-reactivity of polyclonal anti-IEF-51K with α- and β-tubulin and of monoclonal anti-α-tubulin and anti-β-tubulin with IEF-51K. In contrast to its chemical uniqueness, IEF-51K is tubulin-like in some of its properties. IEF-51K is localized in the marginal band of intact platelets by immunofluorescence; it undergoes cycles of microtubule disassembly/reassembly both in vitro and in vivo. Furthermore, IEF-51K was not extracted from isolated Taxol-stabilized marginal band microtubules by elevated NaCl concentrations (to 0.45 M), conditions that do not disrupt the polymeric structure of α- and β-tubulin. These results indicate that IEF-51K together with α-tubulin and β-tubulin are the major structural polypeptides of platelet marginal band microtubules. The unusual subunit composition of the platelet marginal band microtubule may be related to specialization(s) of microtubule structure and function in the marginal band coil of platelets.