The cell surface proteoglycan of mouse mammary epithelial cells. The extracellular domain contains N terminus and a peptide sequence present in a conditioned medium proteoglycan

The cell surface proteoglycan of mouse mammary epithelial (NMuMG) cells behaves as a receptor for interstitial matrix materials and consists of a membrane-associated domain and an extracellular domain (ectodomain). The ectodomain can be released intact from the cell surface by mild trypsin treatment and appears to be shed from the cells into the culture medium by cleavage from the membrane-associated domain. We have examined the chemical relationship between the trypsin-released proteoglycan and shed proteoglycan to assess their relationship to each other and to the cell surface. Purification and amino acid sequencing of the ectodomain released by mild trypsin treatment resulted in no clear signal until the protein was cleaved by CNBr treatment, suggesting that its N terminus is blocked and oriented extracellularly. The amino acid sequence identified in the trypsin-released ectodomain is present near the N terminus of the shed proteoglycan purified from conditioned medium, indicating that both forms possess closely related (if not identical) core proteins. The sequence reveals a pentapeptide identical to one near the C terminus of the rat hepatic lectin (RHL-1, rat asialoglycoprotein receptor). The medium proteoglycan, which migrates as a smear on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (between 93 and 200 kDa), is heterogeneous due to varying amounts of glycosaminoglycan and substituted O-linked oligosaccharide present on an approximately 46-kDa polypeptide.