Occurrence of two β-tubulin isoforms with different polymerizing abilities in L5178Y cells

Abstract

Mouse lymphoma L5178Y cells express at least two isoforms of β-tubulin, designated M β I, and M β II, as revealed by isoelectrofocusing, whereas two independently isolated normal T-cell clones, 3D10 and K23, express only M β I. M β II-tubulin is more acidic (pI, 5.10) than M β I-tubulin (pI, 5.15). L5178Y cells were disrupted under the microtubule-stabilizing conditions, followed by centrifugation to separate fractions containing polymerized and unpolymerized tubulin. We found that a proportion of M β II to total β-tubulins is larger in the fraction containing unpolymerized tubulin than in that containing polymerized tubulin. In addition, when tubulin was purified from extracts of L5178Y cells by repeated cycles of polymerization-depolymerization, the M β II-tubulin isoform was gradually lost during the successive purification steps. The low recovery of M β II-tubulin was observed, irrespective of the presence or absence of MAPs, and even in the presence of an excess amount of essentially polymerizable porcine brain tubulin. These results indicate that M β II-tubulin is less able to polymerize than is M β I-tubulin, both in vivo and in vitro.