Oligomeric species in glycerol-cycled bovine-brain microtubule protein. Analytical ultracentrifugal characterisation.

Abstract

1 Bovine brain microtubule protein, prepared by cycles of assembly and disassembly in the presence of glycerol, and possessing a full complement of the high-molecular-weight group of microtubule-associated proteins, has been examined by analytical ultracentrifugation at 13 °C. 2 Under various conditions of protein concentration, ionic composition and pH, three species are observed, namely the 6-S tubulin dimer, and two oligomeric forms with s°20,w values about 18 S and 30 S, which both show marked dependence of sedimentation coefficient upon protein concentration. 3 The 30-S form is favoured at high protein concentration and pH 7 and by the presence of 0.1 M NaC1. This material has hydrodynamic properties closely similar to the porcine microtubule protein prepared by cycling in the absence of glycerol. Similar properties are also found for glycerol-free bovine microtubule protein. 4 In electron microscopy the oligomeric species appear as clearly defined ring structures, viewed in horizontal projection; double concentric rings were not observed. 5 We conclude that the oligomeric species of bovine microtubule protein are principally determined by the content of microtubule-associated proteins, and that the high-molecular-weight proteins give rise to the 30-S oligomeric form. The nature of the oligomers is not necessarily affected by the inclusion of glycerol in the cyclisation procedure.