Identification of the phosphorylated β-tubulin isotype in differentiated neuroblastoma cells

The tubulin molecule consists of an α- and a β-subunit, each of which exists in several isotypic forms. It has been previously shown that one of the isotypes of neuroblastoma β-tubulin is phosphorylated at a serine residue in vivo [(1985) J. Cell Biol. 100, 764-774]. Here we identify the phosphorylated isotype as β₂ (type III). Moreover, the large size of the phosphorylated tryptic peptide and sequence comparisons of vertebrate β-tubulins suggest that one of the two serines in positions 444 and 446 is the phosphorylated residue. Our results raise the possibility that β₂-tubulin differs functionally from the other β-tubulin isotypes.