In vivo coassembly of a divergent β-tubulin subunit (cβ6) into microtubules of different function

Abstract

α- and β-Tubulin are encoded in vertebrate genomes by a family of ~6-7 functional genes whose polypeptide products differ in amino acid sequence. In the chicken, one β-tubulin isotype (cβ6) has previously been found to be expressed only in thrombocytes and erythroid cells, where it is assembled into a circumferential ring of marginal band microtubules. In light of its unique in vivo utilization and its divergent assembly properties in vitro, we used DNA transfecion to test whether this isotype could be assebled in vivo into microtubules of divergent functions. Using an antibody specific to cβ6, we have found that upon transfection this polypeptide is freely coassembled into an extensive array of interphase cytoplasmic microtubules and into astral and pole-to-chromosome or pole-to-pole microtubules during mitosis. Further, examination of developing chicken erythrocytes reveals that both β-tubulins that are expressed in these cells (cβ6 and cβ3) are found as co-polymers of the two isoforms. These results, in conjunction with efforts that have localized various other β-tubulin isotypes, demonstrate that to the resolution limit afforded by light microscopy in vivo microtubules in vertebrates are random copolymers of available isotypes. Although these findings are consistent with functional interchangeability of β-tubulin isotypes, we have also found that in vivo microtubules enriched in cβ3 polypeptides are more sensitive to cold depolymerization than those enriched in cβ6. This differential quantitative utilization of the two endogenous isotypes documents that some in vivo functional differences between isotypes do exist.