Studies on the transmembrane disposition of the neural cell adhesion molecule N-CAM. A monoclonal antibody recognizing a cytoplasmic domain and evidence for the presence of phosphoserine residues.

The N-CAMs are a group of surface glycoproteins involved in adhesive interactions of neurones. Related molecules of the mouse nervous system, identified in our laboratory, have been called BSP-2 and shown to act as ligands in adhesion of neuroblastoma cells. Results presented in this report show that they are immunochemically identical with N-CAM. A monoclonal anti-(N-CAM) antibody, that recognized a determinant accessible only after permeabilization of intact cells, was used to define the mode of association of the N-CAMs with the plasma membrane. This antibody bound a 35 000-Mr fragment in lysates of trypsin-treated neuroblastoma cells. It is concluded that the antibody reacts with a transmembrane or cytoplasmic domain of the molecules. The same antibody recognized the Mr-180 000 and Mr-140 000 proteins but not the Mr-120 000 chain, which co-purify from adult mouse brain. The latter polypeptide was detected in the cytosol and could be partially released from brain membranes by osmotic shock. Part or all of the Mr-120 000 protein may thus lack a transmembrane segment. Our conclusion that the N-CAM forms of higher Mr are transmembrane proteins was further corroborated by our finding that they contain phosphoserine residues, which can be labeled with (32P)phosphate in intact neuroblastoma cells.