Sialomucin complex (rat Muc4) transmembrane subunit binds the differentiation marker peanut lectin in the normal rat mammary gland

Sialomucin complex (SMC, rat Muc4) is a heterodimeric glycoprotein composed of two subunits, the mucin component ASGP-1 and the transmembrane subunit ASGP-2. SMC/Muc4 is highly expressed on the surface of 13762 rat mammary ad enocarcinoma cells at approximately 100 times the level found in the lactat ing mammary gland. Immunocytochemical staining of SMC/Muc4 in the developin g rat mammary gland is localized to the apical membrane of the ductal epith elium. This staining pattern is similar to that for peanut lectin, a differ entiation marker, which binds to cells expressing the disaccharide Thomsen- Friedenreich or TF antigen. Blotting of glycoproteins expressing the TF ant igen from mammary tissues with peanut lectin detects a protein matching the migration of ASCP-2. Analysis of immunoprecipitated SMC/Muc4 by peanut lec tin blotting shows that the TF antigen is abundantly present on the ASGP-2 subunit, hence the similarity of staining pattern with SMC/Muc4 anti sera a nd peroxidase-conjugated lectin in mammary tissues. The TF antigen is also present on ASGP-2 of SMC/Muc4 produced by confluent cultures of Rama 37 rat mammary epithelial stem cells after their induction to an alveolar-like ph enotype with prolactin. These results indicate that the TF antigen is prese nt on the ASCP-2 transmembrane subunit of SMC/Muc4 from phenotypically norm al tissues and cells, in contrast Co malignant cells whose peanut lectin-bi nding disaccharide is located on ASGP-1. (C) 2001 Wiley-Liss, Inc.