Fig. 12.

Schematic representation of two possible mechanisms underlying antagonistic effects of TAG-1 and F3 on neurite outgrowth. F3 transfectants inhibit outgrowth of granule cell neurites. TAG-1 coexpressed together with F3 in double transfectants prevents the inhibitory effect of F3. A- COMPETITION, TAG-1 and F3 compete for the same receptor on the responding neuron. Two factors argue in favor of this hypothesis: (1) microspheres conjugated with F3 and TAG-1 bind the same proximal domains of granule cell growth cones; (2) both TAG-1 and F3 exhibit in vitro binding activities toward L1/Ng-CAM and Bravo/Nr-CAM, which are possible candidates as neuronal receptors. B- CLUSTERING OF RECEPTORS AND LIGANDS, Binding of TAG-1 and F3 to their neuronal receptors leads to the clustering of a multimolecular complex. This hypothesis is supported by clustering experiments achieved with anti-TAG-1 antibodies in double-transfected CHO cells resulting incis association of F3 and TAG-1. The signal conveyed to neurons via this complex differs from the signal delivered individually by F3 molecules.