A developmentally regulated plasmalemmal antigen present in synaptosomes but not in growth cones

Monoclonal antibody 2L4 was generated against rat synaptosomes but does not cross-react with nerve growth cones. Expression of the 2L4 antigen is developmentally regulated in a manner that is, in part, the opposite of the expression of the 5B4-CAM antigen, a marker of neuronal outgrowth belonging to the N-CAM family. While 5B4-CAM appears and increases during sprouting and then decreases to reach low levels in the adult, the 2L4 antigen appears only late in development, when neuronal outgrowth ceases, at or around the time of synaptogenesis. Once expressed, the antigen is found on the entire plasmalemmal surface of the neuron, but seems to be enriched at synaptic endings, at least of some neuron types. Biochemical analyses involving blotting of non-denaturing gels and immunoaffinity chromatography identify the antigen as a pair of polypeptides with similar, basic isoelectric points. These polypeptides form a somewhat diffuse, probably glycosylated band at 67 kDa and may be part of a hetero-oligomeric complex. The localization, biochemical, and developmental results suggest that the 2L4 antigen is a plasmalemmal marker of maturing and/or mature neurons whose expression may be triggered by synaptogenesis.