cDNA cloning of the CEPUS, a secreted type of neural glycoprotein belonging to the immunoglobulin-like opioid binding cell adhesion molecule (OBCAM) subfamily

GP55 is a family of glycoproteins distributed predominantly in the nervous system, and its previously characterized members, including the GP55A (EMBL Y08170) and E19S (EMBL Y08171) reveal a typical glycosyl phosphatidyl inos itol (GPI)anchored pattern for membrane proteins. CEPUS identified in this study appeared to represent the third member of GP55, This 3.2 kb long comp lete cDNA clone from the chicken brain exhibited 3 Ig-like domains. The ope n reading frame of CEPUS contains 313 amino acids, which can encode a 31.7 kDa core protein (pI 5.75) for the mature form. The signal peptide cleavage site was predicted at Gln(25). The structural features of the CEPUS cDNA s equence represented a soluble counterpart to the recently identified cerebe llar Purkinje cell specific antigen, CEPU-1, The sequence difference betwee n CEPU-1 and CEPUS was only found in the C-terminus in which the CEPUS lack ed the GPI-anchored binding site. It displays significant sequence homology to GP55-related molecules, including OBCAM, GP55A, E19S/LAMP, neurotrimin, and CEPU-1, which are all membrane attached types. The absence of the hydr ophobic tail sequence in CEPUS may, therefore, suggest that CEPUS would rep resent the first identified secreted member in this group of genes. We defi ned that this molecule forms the opioid-binding cell adhesion molecule (OBC AM) subfamily in the molecular phylogeny, Structurally, these molecules rep resent acidic proteins (pI 5.47-6.09). Sis cysteins, as well as 5 Asn-linke d potential glycosylation sites were evolutionary-conserved, suggesting tha t this OBCAM subfamily resembles immunoglobulin-like and highly glycosylate d molecules. The presence of CEPUS would probably suggest to us that the sp atial/local expression of the CEPU gene may provide a favorable route for m igrating CEPU-positive population of neurons to generate a neuron-specific guidance in developing neurons in vivo.