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 35000-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-180000 and Mr-140000 proteins but not the Mr-120000 chain, which copurify 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-120000 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.