Complete sequence and in vitro expression of a tissue-specific phosphatidylinositol-linked N-CAM isoform from skeletal muscle

Summary Neural cell adhesion molecules (N-CAMs) are a family of cell surface sialoglycoproteins encoded by a single copy gene. A full-length cDNA clone that encodes a nontransmembrane phosphatidylinositol (PI) linked N-CAM of A/..125X10 3 has been isolated from a human skeletal muscle cDNA library. The deduced protein sequence encodes a polypeptide of 761 amino acids and is highly homologous to the N-CAM isoform in brain of MrHOxXO 3 . The size difference between the 125xlO 3 /Wr skeletal muscle form and the 120x 10 3 Afr N-CAM form from brain is accounted for by the insertion of a block of 37 amino acids called MSD1, in the extracellular domain of the muscle form. Transient expression of the human cDNA in COS cells results in cell surface N-CAM expression via a putative covalent attachment to Pi-containing phospholipid. Linked in vitro transcription and translation experiments followed by immunoprecipitation with anti-NCAM antibodies demonstrate that the full-length clone of 761 amino acid coding potential produces a core polypeptide of AfrllOxlO 3 which is processed by microsomal membranes to yield a 122x 10 3 Mr species. Taken together, these results demonstrate that the cloned cDNA sequence encodes a lipid-linked, PIspeciHc phospholipase C releasable surface isoform of N-CAM with core glycopeptide molecular weight corresponding to the authentic muscle HSxlO 3 ^