Syk-dependent phosphorylation of microtubules in activated B-lymphocytes

Syk is a protein-tyrosine kinase that is essential for B-lymphocyte develop ment and B-cell signaling. Syk phosphorylates tubulin on tyrosine both in v itro and in intact lymphocytes. Here we show that alpha-tubulin present wit hin the cytoskeletal microtubule network was phosphorylated in a Syk-depend ent manner following the activation of B-cells by engagement of the B-cell antigen receptor or by treatment with the phosphotyrosine phosphatase inhib itor, pervanadate, Immunofluorescence staining of microtubule cytoskeletons and western blotting studies with antibodies to phosphotyrosine confirmed the phosphorylation of polymerized tubulin in Syk-expressing, but not Syk-d eficient, cells, At low concentrations of pervanadate, centrosomes appeared to be preferentially tyrosine-phosphorylated. Tubulin phosphorylated to a high stoichiometry on tyrosine assembled into microtubules in vitro, and pr eassembled microtubules were also phosphorylated by Syk kinase in vitro, Th us, Syk has the capacity to interact with microtubule networks within the B -lymphocyte and catalyzes the phosphorylation of the alpha-tubulin subunit, Syk-dependent phosphorylation of microtubules may affect the ability of th e microtubule cytoskeleton to serve as a platform upon which signaling comp lexes are assembled.