SHEATH PROTEINS - SYNTHESIS, SECRETION, DEGRADATION AND FATE IN FORMING ENAMEL

We investigated expression of ameloblastin and sheathlin, recently clo ned enamel matrix proteins from the rat and pig, in forming enamel imm unocytochemically and immunochemically, using region-specific antibodi es. The results obtained from the rat and pig were essentially the sam e. Antibodies which recognize the N-terminal region stained the secret ory machinery of the secretory ameloblast and the entire thickness of the enamel matrix, especially the peripheral region of the enamel rod. Immunostained protein bands were observed near 65 or 70 kDa and below 20 kDa. C-terminal-specific antibodies stained the secretory machiner y of the ameloblast and the immature enamel adjacent to the secretion sites. Immunostained protein bands were found ranging from 25 to 70 kD a. Antibodies which recognize a region in the protein just prior to th e C-terminal region stained the cis-side of the Golgi apparatus but no t the enamel matrix. Immunostained protein bands were observed of abou t 55 kDa. These results suggest that post-translational and post-secre tory modifications of ameloblastin and sheathlin are similar to each o ther, and further showed that their cleaved N-terminal polypeptides co ncentrate in the prism sheath. We propose that sheathlin and ameloblas tin share the same role in amelogenesis and should be classified as sh eath proteins.