Axonal branching increases when L1 does not bind to ezrin family members.

The cell adhesion molecule (CAM) L1 is important for the growth of axons during neuronal developmentā€”mutations in L1 cause birth defects and neurological disorders. Although its extracellular domain mediates cell-to-cell adhesion, the cytoplasmic domain of L1 determines the stickiness of this CAM by regulating its rate of internalization, according to new results from Schaefer et al. on page 1223.

The new report shows that phosphorylation of the cytoplasmic domain makes L1 more adhesive by preventing its endocytosis. The residue Tyr1176, which is specific to the neuronal isoform of L1, was phosphorylated in vitro by the nonreceptor tyrosine kinase p60src. In vivo, phosphorylation of L1 prevented its endocytosis by inhibiting its interaction with the clathrin- associated AP-2 complex. Homophilic binding to L1 on a neighboring cell caused its dephosphorylation, which should promote endocytosis....

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