Subsets of sialylated, sulfated mucins of diverse origins are recognized by L-selectin. Lack of evidence for unique oligosaccharide sequences mediating binding

Previous studies have shown that the mucin-type polypeptides GlyCAM-1, CD34, and MAdCAM-1 can function as ligands for L-selectin only when they are synthesized by the specialized high-endothelial venules (HEV) of lymph modes. Since sialylation, sulfation, and possibly fucosylation are required for generating recognition, we reasoned that other mucins known to have such components might also bind L-selectin. We show here that soluble mucins secreted by human colon carcinoma cells, as well as those derived from human bronchial mucus can bind to human L-selectin in a calcium-dependent manner. As with Gly-CAM-1 synthesized by lymph node HEV, alpha 2-3 linked sialic acids and sulfation seem to play a critical role in generating this L-selectin binding. In each case, only a subset of the mucin molecules is recognized by L-selectin. Binding is not destroyed by boiling, suggesting that recognition may be based primarily upon carbohydrate structures. Despite this, O-linked oligosaccharide chains released from these ligands by beta-elimination do not show any detectable binding to L-selectin. Following protease treatment of the ligands, binding persists in a subset of the resulting fragments, indicating that specific recognition is determined by certain regions of the original mucins. However, O-linked oligosaccharides released from the subset of non-binding mucin fragments do not show very different size and charge profiles compared to those that do bind. Furthermore, studies with polylactosamine-degrading endoglycosidases suggest that the core structures involved in generating binding can vary among the different ligands. Taken together, these data indicate that a single unique oligosaccharide structure may not be responsible for high-affinity binding. Rather, diverse mucins with sialylated, sulfated, fucosylated lactosamine-type O-linked oligosaccharides can generate high-affinity L-selectin ligands, but only when they present these chains in unique spacing and/or clustered combinations, presumably dictated by the polypeptide backbone.