Four exons encode a 93-base-pair insert in three neural cell adhesion molecule mRNAs specific for chicken heart and skeletal muscle.

The neural cell adhesion molecule (N-CAM) is detected in chicken brain as three polypeptides of 180 kDa, 140 kDa, and 120 kDa that arise from a single gene by alternative splicing. Heart tissue, however, contains components of 150 kDa, 140 kDa, and 130 kDa; neither the differences in molecular mass among these components nor the difference between neural and cardiac N-CAM could be accounted for by variations in glycosylation alone. A cDNA clone isolated from an embryonic chicken heart library, [lambda N101B, 1.8 kilobases (kb)] contained a 93-base-pair (bp) insert not found in neural N-CAM cDNAs. In the N-CAM gene this sequence mapped within a large region between exons 12 and 13 and was derived from four exons (12A-D) of 15, 33, 42, and 3 bp. Exons 12C and 12D together coded for 15 amino acids very similar to the second half of the muscle-specific insert (MSD1) found in N-CAM cDNA from human muscle cell cultures [Dickson, G., Gower, H. J., Barton, C. H., Prentice, H. M., Elsom, V. L., Moore, S. E., Cox, R. D., Quinn, C., Putt, W. & Walsh, F. S. (1987) Cell 50, 1119-1130]; the sequences of 12A and 12B, however, were much less similar to the corresponding region of the MSD1 sequence. Two oligonucleotides, one specific to exons 12A plus 12B and one specific to exon 12C both recognized mRNA species of 6.4 kb, 4.3 kb, and 3.0 kb in chicken cardiac and skeletal muscle and no mRNA species in smooth muscle or brain. The 3' end of clone lambda N101B contained a sequence coding for a potential phosphatidylinositol linkage signal as does the smallest form of brain N-CAM. In heart cell membranes only the 130-kDa N-CAM polypeptide was released by phospholipase C, suggesting that this form of N-CAM is encoded by clone lambda N101B. The other heart N-CAM species (150 kDa and 140 kDa) may be transmembrane forms that include the 12A-D (and possibly other) inserts. Tissue-specific forms of N-CAM can thus be formed by alternative use of multiple small exons that may alter the conformation of the extracellular region of the molecule. Differential use or switching of these small exons in conjunction with the differential expression of larger exons specifying regions associated with the cell membrane and cytoplasmic domains may signal key events in embryogenesis and histogenesis.