AbstractN‐terminal sequencing of the 55‐ and 50‐kDa polypeptides affinity purified on a phosphotyrosine monoclonal antibody column from activated Jurkat T cells identified α and β tubulin. Two‐dimensional gel analysis indicated that α tubulin was directly phosphorylated on tyrosine. β Tubulin was not detectably tyrosine phosphorylated but was precipitated by anti‐phosphotyrosine (PTyr) antibody by virtue of its association with the α subunit as a heterodimer. Phosphotyrosyl α tubulin was not incorporated into intact microtubules and was all in the unpolymerized soluble fraction. These results suggest that tyrosine phosphorylation of α tubulin may inhibit the ability of this subunit to polymerize into microtubules. Stimulation of Jurkat T cells via T cell receptor increased the amount of tubulin precipitated by the anti‐PTyr antibody. These data raise the possibility that the polymerization of tubulin heterodimers may be regulated by phosphorylation on tyrosine during
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