Activation of the MAPK signal cascade by the neural cell adhesion moleculeL1 requires L1 internalization

L1-mediated axon growth involves intracellular signaling, but the precise m echanisms involved are not yet clear, We report a role for the mitogen-acti vated protein kinase (MAPK) cascade in L1 signaling. L1 physically associat es with the MAPK cascade components Raf-l, ERK2, and the previously identif ied p90(rsk) in brain. In vitro, ERK2 can phosphorylate L1 at Ser(1204) and Ser(1248) of the L1 cytoplasmic domain. These two serines are conserved in the L1 family of cell adhesion molecules, also being found in neurofascin and NrCAM, The ability of ERK2 to phosphorylate L1 suggests that L1 signali ng could directly regulate L1 function by phosphorylation of the L1 cytopla smic domain. In L1-expressing 3T3 cells, L1 cross-linking can activate ERK2 , Remarkably, the activated ERK localizes with endocytosed vesicular L1 rat her than cell surface L1, indicating that L1 internalization and signaling are coupled. Inhibition of L1 internalization with dominant-negative dynami n prevents activation of ERK, These results show that L1-generated signals activate the MAPK cascade in a manner most Likely to be important in regula ting L1 intracellular trafficking.