Mutation of a putative MAP kinase consensus site regulates NCAM endocytosis and NCAM-dependent neurite outgrowth

The cytoplasmic domain of the neural cell adhesion molecule NCAM contains several putative serine/threonine phosphorylation sites whose functions are largely unknown. Human NCAM140 (NCAM140) possesses a potential MAP kinase phosphorylation site at threonine (T) 803. The aim of this study was to analyze a possible phosphorylation of NCAM140 by MAP kinases and to identify the functional role of T803. We found that NCAM140 is phosphorylated by the MAP kinase ERK2 in vitro. Exchange of T803 to aspartic acid (D) which mimics constitutive phosphorylation at the respective position resulted in increased endocytosis compared to NCAM140 in neuroblastoma cells and primary neurons. Consistently, NCAM140 endocytosis was inhibited by the MEK inhibitor U0126 in contrast to NCAM140-T803D or NCAM140-T803A endocytosis supporting a role of a potential ERK2 mediated phosphorylation at this site in endocytosis. Furthermore, cells expressing NCAM140-T803D developed significantly shorter neurites than NCAM140 expressing cells indicating that a potential phosphorylation of NCAM by ERK2 also regulates NCAM-dependent neurite outgrowth.