Novel adapter protein AP162 connects a sialyl-Le(x)-positive mucin with an apoptotic signal transduction pathway.

Glycoproteins modified with a sialyl-Le(x)-moiety are important sensors for extracellular signals regulating cellular recognition, adhesion and migration. The transduction pathways and signals mediated by these glycoproteins within the cell are largely unknown. In search of novel glycoproteins modified with sialyl-Le(x)-moiety, we screened a human colonic cDNA expression library with a rabbit antiserum produced against sialyl-Le(x)-positive mucins. The antiserum detected a new protein, named B2, which was cloned and characterised in detail. The analysis of the B2 gene revealed a 5.7 kb RNA transcript detectable in all investigated tissues and a complete coding sequence of 2778 bp. The B2 protein exhibited two putative PH (pleckstrin homology) domains and a leucine zipper motif but no homology to any known proteins. Monospecific antibodies against the B2-protein precipitated from the solubilised membrane fraction of the colon carcinoma cell line LS 174T a protein with an apparent Mr = 162 kDa and, additionally, a mucin-like glycoprotein with an apparent Mr = 220 kDa. Protein fractionation on a CsCl gradient, Western blots and sandwich ELISA showed that the 220 kDa mucin carries the sialyl-Le(x) moiety and is tightly bound to the 162 kDa protein. The expression of the recombinant B2-protein enhanced staurosporine-induced apoptosis in epithelial cancer cell lines. These data indicate that B2 is a novel, ubiquitously expressed protein with a putative adapter function. The protein has been named AP162 (adapter protein 162). In colon carcinoma cells B2-protein is tightly associated with a sialyl-Le(x)-positive mucin and has a potential for involvement in sialyl-Le(x)-mediated transduction of apoptotic signals.