Phosphorylation of microtubule-associated proteins MAP2a,b and MAP2c at Ser136 by proline-directed kinases in vivo and in vitro.

The microtubule-associated protein 2 (MAP2) and its juvenile splicing variant MAP2c contain a phosphorylation site at Ser136 which is part of a Ser-Pro motif. This site lies within the N-terminal region common to MAP2b and MAP2c. It has been mapped by site-directed mutagenesis of recombinant MAP2c and by a monoclonal antibody AP18 whose epitope contains the phosphorylated Ser136. In vitro this site is phosphorylated by proline-directed kinases such as MAP kinase, GSK-3, or members of the cdk family, but not by other kinases such as PKA, PKC, or CaMK-II. MAP2a,b or MAP2c isolated from brain is found to be endogenously phosphorylated at Ser136. After microinjection into several cell lines dephosphorylated MAP2 isoforms or recombinant MAP2c become also phosphorylated at Ser136 in vivo. Injection of MAP2a,b or MAP2c into living cells causes reorganization of microtubules, including bundle formation. This effect is independent of the phosphorylation at Ser136. The specificity of the phosphorylation reaction provides a tool for analyzing the role and posttranslational processing of MAP2 in nerve cell development.