ABSTRACT
Mucin glycoproteins are a major constituent of salivary secretions and play a primary role in the protection of the oral cavity. Rat submandibular glands (RSMG) synthesize and secrete a low molecular weight (114 kDa) mucin glycoprotein. We have isolated, partially sequenced, and characterized the gene which encodes the RSMG apomucin. The gene is encoded by three exons of 106 nt, 69 nt, and 991 nt, separated by introns of 921 nt and 12.5 kb. CAAT and TATA elements are present, at -68 and -26, respectively, in the 5' flanking sequence of the RSMG apomucin gene. The tandem repeat domain present in exon III consists of ten tandem repeats of 39 nt encoding the consensus sequence PTTDSTTPAPTTK. Sequence comparison and organization of the nucleic acid sequence encoding the tandem repeats of two alleles for this gene suggests that the apomucin gene has undergone recombinational events during its evolution. No significant sequence similarity was found with other mucin genes, or with other known salivary gland-specific genes. The gene was localized to rat chromosome 14 using somatic cell hybrids that segregate rat chromosomes. Since this, to our knowledge, represents the first RSMG mucin gene cloned, we have designated this gene Mucsmg.
Subject(s)
Gastric Mucins/chemistry , Submandibular Gland/chemistry , Amino Acid Sequence , Animals , Bacteriophage lambda/genetics , Base Sequence , Chromosome Mapping , Cloning, Molecular , DNA Primers , DNA, Complementary/chemistry , Gene Library , Molecular Sequence Data , Rats , Repetitive Sequences, Nucleic Acid , Sequence Alignment , Sequence AnalysisABSTRACT
Overlapping cDNA clones which encode the protein core of a rat submandibular gland mucin-glycoprotein have been isolated and characterized. Sequence analysis revealed a translated region of 966 nucleotides encoding a protein of 322 amino acid residues. The translational start site begins with a putative signal sequence comprising the initial 22 N-terminal residues. The predicted secreted portion of the apomucin revealed three distinct domains: an N-terminal domain which is enriched in glutamine (14%), proline (13%), and tyrosine (10%); a central region which consisted of eleven, 39-base pair tandem repeats with the consensus sequence PTTDSTTPAPTTK; and a C-terminal domain which is enriched in threonine and serine residues (47%) which are not part of a repeat motif. The expression of apomucin transcript appears restricted to the rat submandibular and sublingual glands. Southern blot analysis of rat genomic DNAs suggested a low copy number (1, 2) for this apomucin gene and a limited polymorphism in the number of tandem repeats. Collectively, our sequence and expression data indicate that the cloned rat submandibular gland apomucin is distinct from any of the other salivary (bovine, porcine, or human) or rat apomucins reported thus far.
