The LEC-CAMs: an emerging family of cell-cell adhesion receptors based upon carbohydrate recognition.

Cell surface carbohydrates, because of their demonstrated and potential structural diversity, have long been considered as excellent candidates for determinants of cell-cell recognition. Recently, a gene family has been identified, which encodes a series of three adhesion proteins (pnHR, ELAM-1, and GMP-140), designated as the LEC-CAMs. Each receptor participates in highly specific cell-cell recognition events within the blood vascular compartment. The LEC-CAMs share a high degree of sequence homology and the same organization of protein motifs, which includes a calcium-type lectin domain at the extracellular amino-terminus of each. In the case of the pnHR (peripheral lymph node homing receptor), the lectin domain has been shown to be central to the adhesive function of the receptor, i.e., lymphocyte attachment to high endothelial venules (HEV) of lymph nodes. The cognate ligand for the pnHR on HEV is a sialylated glycoprotein. Sialic acid is required for the adhesive function of this ligand, and preliminary evidence suggests that this requirement may also apply to the ligand for GMP-140. It is not clear as yet whether sialic acid contributes directly to recognition determinants of these ligands or has a modulating effect on their function. Given the extreme diversity of sialyloligosaccharides, the former possibility is very attractive. The LEC-CAM family joins the three families of already identified cell-cell adhesion molecules (integrins, cadherins, and superimmunoglobulins). It remains to be seen whether additional examples of highly specific cell recognition events rely on as yet unidentified LEC-CAMs or related lectin-like receptors.