MOLECULAR-CLONING OF DEVELOPMENTALLY-REGULATED NEONATAL RAT SUBMANDIBULAR-GLAND PROTEINS

At birth, the rat submandibular gland (SMG) contains two transient sec retory cell types that produce several characteristic salivary protein s. Proteins SMG-A, B 1, and B2 (23.5, 26 and 27.5 kDa) are products of the neonatal type III cells, but not the adult acinar cells. Protein C (89 kDa), a major product of the neonatal type I cells, is either ab sent or present at greatly diminished levels in the secretory cells of the adult gland. The decrease in biosynthesis of these neonatal saliv ary proteins occurs concomitantly with the increase in levels of chara cteristic adult SMG products. In order to understand these development ally regulated changes in SMG salivary protein gene expression, we hav e initiated the molecular cloning and characterization of neonatal sub mandibular gland proteins from a 5-d-old rat submandibular gland cDNA library. Clones encoding SMG-A were isolated by homology to the mouse parotid secretory protein (PSP). SMG-A was shown to be derived from a salivary protein multigene family that also includes PSP. Cloning and characterization of additional neonatal rat submandibular gland protei ns was initiated by screening the 5-d-old rat submandibular gland cDNA library with first strand cDNA prepared from 1-d-old rat submandibula r glands. Clones corresponding to a highly abundant 3 kb transcript pr esent in the neonatal rat SMG. but not in adult submandibular, subling ual, or parotid gland have been identified. The size, abundance, and o rgan specificity of this transcript suggest that it may encode protein C. One clone derived from an unknown transcript that is developmental ly regulated in the neonatal SMG and is present in the adult parotid, submandibular, and sublingual glands was also identified.