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 constructed 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 trans-Golgi network to both the apical and basolateral surfaces. This result suggests that entrance into the basolateral pathway may occur without cytoplasmic signals, implying that apical targeting from the trans-Golgi network is not a default mechanism but, rather, requires positive sorting information. Subsequent construction and analysis of a nested set of C-terminal deletion mutants identified a region of 40 amino acids (amino acids 749-788) lacking tyrosine residues required for basolateral targeting. Addition of these 40 amino acids is sufficient to restore basolateral targeting to both the non-polar cytoplasmic deletion mutant of N-CAM as well as to the apically expressed cytoplasmic deletion mutant of the p75 low affinity neurotrophin receptor (p75(NTR)), indicating that this tyrosine-free sequence is capable of functioning independently as a basolateral sorting signal. Deletion of both cytoplasmic and transmembrane domains resulted in apical secretion of N-CAM, demonstrating that the ectodomain of this molecule carries recessive apical sorting information.
Le Gall
A H
AH
Cornell University Medical College, Dyson Vision Research Institute, Department of Ophthalmology, New York, New York 10021, USA.
Powell
S K
SK
Yeaman
C A
CA
Rodriguez-Boulan
E
E
D000595:Amino Acid Sequence; D000818:Animals; D001485:Basement Membrane / Q000378:metabolism; D015816:Cell Adhesion Molecules, Neuronal / Q000235:genetics*; D002460:Cell Line; D003593:Cytoplasm / Q000378:metabolism; D004285:Dogs; D008969:Molecular Sequence Data; D021382:Protein Sorting Signals / Q000235:genetics*; D014443:Tyrosine / Q000378:metabolism*
D015816:Cell Adhesion Molecules, Neuronal; D021382:Protein Sorting Signals; D014443:Tyrosine