TYROSINE PHOSPHORYLATION AT A SITE HIGHLY CONSERVED IN THE L1 FAMILY OF CELL-ADHESION MOLECULES ABOLISHES ANKYRIN BINDING AND INCREASES LATERAL MOBILITY OF NEUROFASCIN

This paper presents evidence that a member of the L1 family of ankyrin -binding cell adhesion molecules is a substrate for protein tyrosine k inase(s) and phosphatase(s), identifies the highly conserved FIGQY tyr osine in the cytoplasmic domain as the principal site of phosphorylati on, and demonstrates that phosphorylation of the FIGQY tyrosine abolis hes ankyrin-binding activity. Neurofascin expressed in neuroblastoma c ells is subject to tyrosine phosphorylation after activation of tyrosi ne kinases by NGF or bFGF or inactivation of tyrosine phosphatases wit h vanadate or dephostatin. Furthermore, both neurofascin and the relat ed molecule Nr-CAM are tyrosine phosphorylated in a developmentally re gulated pattern in rat brain. The FIGQY sequence is present in the cyt oplasmic domains of all members of the L1 family of neural cell adhesi on molecules. Phosphorylation of the FIGQY tyrosine abolishes ankyrin binding, as determined by coimmunoprecipitation of endogenous ankyrin and in vitro ankyrin-binding assays. Measurements of fluorescence reco very after photobleaching demonstrate that phosphorylation of the FIGQ Y tyrosine also increases lateral mobility of neurofascin expressed in neuroblastoma cells to the same extent as removal of the cytoplasmic domain. Ankyrin binding, therefore, appears to regulate the dynamic be havior of neurofascin and is the target for regulation by tyrosine pho sphorylation in response to external signals. These findings suggest t hat tyrosine phosphorylation at the FIGQY site represents a highly con served mechanism, used by the entire class of L1-related cell adhesion molecules, for regulation of ankyrin-dependent connections to the spe ctrin skeleton.