Phosphorylation of a neuronal-specific beta-tubulin isotype.

Adult rats were intracraneally injected with [32P] phosphate and brain microtubules isolated. The electrophoretically purified, in vivo phospholabeled, beta-tubulin was digested with the V8-protease and the labeled peptide purified by reversed-phase liquid chromatography. Its amino acid sequence corresponds to the COOH-terminal sequence of a minor neuronal beta 3-tubulin isoform from chicken and human. The phosphorylation site was at serine 444. A synthetic peptide with sequence EMYEDDEEESESQGPK, corresponding to that of the COOH terminus of beta 3-tubulin, was efficiently phosphorylated in vitro by casein kinase II at the same serine 444. The functional meaning of tubulin phosphorylation is still unclear. However, the modification of the protein takes place after microtubule assembly, and phosphorylated tubulin is mainly present in the assembled microtubule protein fraction.