Late-gestation mouse submandibular salivary glands comprise a network of large and small ducts that terminate in lumen-containing, presumptive acini expressing low-molecular-weight mucin. Our studies find that: (1) embryonic mucin is an alternatively spliced Muc10 gene product, 220 amino acids in size (~25 kDa), rich in potential O-glycosylation sites, and variably glycosylated (~40 and 68 kDa); (2) consensus secondary-structure is consistent with a molecule that is anchored to the plasma membrane, directly or indirectly (via a glycolipid), and has a protein core that serves as a scaffold for carbohydrate presentation; (3) embryonic mucin and L-selectin are colocalized to the plasma membrane region of epithelial cells of presumptive acini; (4) embryonic, but not adult, mucin is able to bind L-selectin and does so endogenously. Since the primary role of L-selectin is to mediate cell adhesion and its ligands are mucin-like glycoproteins, the embryonic low-molecular-weight mucin has been termed MucCAM (GenBank AF247816.1). Of particular evolutionary interest is the relation between MucCAM and the salivary glue proteins of Drosophila (fruitfly). In the larva of Drosophila, the major function of the salivary glands is the production of a mucoprotein glue that serves to attach the pupal case to the substrate. Of note, there is a 60% similarity between Drosophila salivary gland protein lpg-1 and mouse MucCAM, and they likely belong to the same protein family.

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Salivary Gland Mucin
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