THE NEURAL CELL-ADHESION MOLECULE EXPRESSES A TYROSINE-INDEPENDENT BASOLATERAL SORTING SIGNAL

Transmembrane isoforms of the neural cell adhesion molecule, N-CAM (N- CAM-140 and N-CAM-180), are vectorially targeted from the trans-Golgi network to the basolateral domain upon expression in transfected Madin -Darby canine kidney cells (Powell, S. K., Cunningham, B. A. Edelman, G. M., and Rodriguez-Boulan, E. (1991) Nature 353, 76-77), To localize basolateral targeting information, mutant forms of N-CAM-140 were con structed and their surface distribution analyzed in Madin-Darby canine kidney cells. N-CAM-140 deleted of its cytoplasmic domain shows a non -polar steady state distribution, resulting from delivery from the tra ns-Golgi network to both the apical and basolateral surfaces. This res ult suggests that entrance into the basolateral pathway may occur with out cytoplasmic signals, implying that apical targeting from the trans -Golgi network is not a default mechanism but, rather, requires positi ve sorting information. Subsequent construction and analysis of a nest ed set of C-terminal deletion mutants identified a region of 40 amino acids (amino acids 749-788) lacking tyrosine residues required for bas olateral targeting. Addition of these 40 amino acids is sufficient to restore basolateral targeting to both the non-polar cytoplasmic deleti on mutant of N-CAM as well as to the apically expressed cytoplasmic de letion mutant of the p75 low affinity neurotrophin receptor (p75(NTR)) , indicating that this tyrosine-fi ee sequence is capable of functioni ng independently as a basolateral sorting signal. Deletion of both cyt oplasmic and transmembrane domains resulted in apical secretion of N-C AM, demonstrating that the ectodomain of this molecule carries recessi ve apical sorting information.