The structures of two rat brain-specific 1B236 mRNAs, alternative splice products from a single gene regulated differently during postnatal brain development, were deduced from full-length cDNA clones. The 626-and 582-amino acid-long encoded proteins are indistinguishable from two forms of myelin-associated glycoprotein, a cell adhesion molecule involved in axonal-glial and glial-glial interactions in postnatal brain development, particularly in myelination. The two proteins share a single membrane-spanning domain and a glycosylated N terminus but differ in the structures of their C termini. The N terminus consists of five domains related in sequence to each other and to immunoglobulin-like molecules, especially the neural cell adhesion molecule N-CAM, suggesting a common structure for cell adhesion molecules. Rat brain protein 1B236 was originally defined by nucleotide sequence analysis of randomly selected cDNA clones of mRNAs expressed in adult rat brain but not detectable in liver or kidney (1,2). Antisera to synthetic peptides corresponding to nonoverlapping regions of the partial 1B236 sequence detected a postnatally expressed 100-kDa rat brain protein containing "30% N-linked carbohydrate and proteolytic fragments derived from its C-terminal regions (2-5). During early postnatal development, 1B236 is expressed predominantly by oligodendrocytes in myelinating fiber tracts. Subsequently, there is gradual elaboration of the adult pattern in which 1B236 mRNA is detected predominantly in subsets of neurons in grey matter regions and the 1B236 protein is restricted to specific neuronal cell bodies and fibers (refs. 2, 4, and 6; G. A. Higgins, H. Schmale, F.E.B., M. C. Wilson, R.J.M., unpublished data).We have now analyzed 1B236 expression more completely and report here the structures of two differently regulated, alternatively spliced 1B236 mRNAs that encode proteins with alternative C-terminal tails. The shared N-terminal region consists of five domains that are related in sequence to each other and to proteins of the immunoglobulin super family (8), especially the neural cell adhesion molecule N-CAM (9). We show that the 1B236 protein is indistinguishable from myelin-associated glycoprotein (MAG), a nervous system-specific glycoprotein of 100 kDa that contains -30% carbohydrate (10). MAG has been implicated in the interactions between myelinating cells and axons (10) and the formation and maintenance of the periaxonal space (11). In the peripheral nervous system, MAG appears to take over Schwann cell-axon and Schwann cell-Schwann cell interactions initiated by N-CAM and L1/neuron-glia cell adhesion molecule (12). Thus two cell adhesion molecules that act at successive stages of neural development also share significant structural properties. EXPERIMENTSPrimary Structure of the 1B236 mRNA: Alternative Splicing Produces Two Forms. We described a 1500-nucleotide (nt) partial cDNA clone of the 2500-nt 1B236 mRNA (2). Two additional clones (p1B236-18 and plB236-20) with apparently full-length inserts were...