This paper presents the first structural analysis of the cytoplasmic domain of neurofascin, which is highly conserved among the L1CAM family of cell adhesion molecules, and describes sequence requirements for neurofascin-ankyrin interactions in living cells. The cytoplasmic domain of neurofascin dimerizes in solution, has an asymmetric shape, and exhibits a reversible temperature-dependent beta-structure. Residues Ser56-Tyr81 are necessary for ankyrin binding but do not contribute to either dimerization or formation of structure. Transfected neurofascin recruits GFP-tagged 270-kDa ankyrinG to the plasma membrane of human embryo kidney 293 cells. Deletion mutants demonstrate that the sequence Ser56-Tyr81 contains the major ankyrin-recruiting activity of neurofascin. Mutations of the FIGQY tyrosine (Y81H/A/E) greatly impair neurofascin-ankyrin interactions. Mutation of human L1 at the equivalent tyrosine (Y1229H) is responsible for certain cases of mental retardation (Van Camp, G., Fransen, E., Vits, L., Raes, G., and Willems, P. J. (1996) Hum. Mutat. 8, 391). Mutations F77A and E73Q greatly impair ankyrin binding activity, whereas mutation D74N and a triple mutation of D57N/D58N/D62N result in less loss of ankyrin binding activity. These results provide evidence for a highly specific interaction between ankyrin and neurofascin and suggest that ankyrin association with L1 is required for L1 function in humans.