MOLECULAR-CLONING AND SEQUENCING OF THE MUCIN SUBUNIT OF A HETERODIMERIC, BIFUNCTIONAL CELL-SURFACE GLYCOPROTEIN COMPLEX OF ASCITES RAT MAMMARY ADENOCARCINOMA CELLS

Ascites sublines of the highly metastatic 13762 rat mammary adenocarci noma contain abundant amounts of a heterodimeric cell surface glycopro tein complex composed of a mucin subunit ASGP-1 (ascites sialoglycopro tein-1) and a transmembrane subunit (ASGP-2). Previous studies showed that the complex is synthesized from a single polypeptide encoded by a 9 kb transcript. The sequence of the transmembrane subunit was obtain ed from a 5-kilobase (kb) cDNA isolated from a plasmid library (Sheng, Z., Wu, K., Carraway, K. L., and Fregien, N. (1992) J. Biol. Chem. 26 7, 16341-16346). Completion of the sequence of this cDNA revealed the C-terminal domain of ASGP-1, which is rich in serine and threonine but contains no typical mucin type repeats. The remainder of the sequence of ASGP-1 and the 9-kb transcript was obtained by two 5'-RACE (rapid amplification of cDNA ends) steps and primer extension analysis. These results revealed that the 5' half of the 9-kb transcript contains a s hort B'-noncoding region and encodes a signal sequence, a short nonrep eat region, and a repeat domain containing 11 repeats. Nine of these r epeats are found in tandem, but the two end repeats are separated from the others by short unique sequences. The repeats vary from 117-124 a mino acids and are 70-90% identical to a consensus sequence. Overall, the sequence predicts that ASGP-1 contains 2172 amino acids (M(r) 224, 190), 43% of which are serine and threonine. We propose that the compl ex of this mucin and its transmembrane subunit, which contains growth factor-modulating activity, may play an important role in tumor progre ssion.