Structure and Function of L-selectin

The selectins are a newly described family of carbohydrate-binding adhesion molecules involved in the regulation of leukocyte traffic. Selectins are composed of an N-terminal C-type lectin domain, a single EGF domain, a variable number of short consensus repeat (SCR) domains, a transmembrane region and a cytoplasmic tail. L-selectin (LAM-1/LECAM-1/LECCAM-1) is the only selectin expressed on leukocytes, and mediates a number of leukocyte-endothelial interactions, including the binding of lymphocytes to HEV of peripheral lymph node high endothelial venules (HEV), neutrophil rolling, and leukocyte attachment to cytokine-treated endothelium in vitro. Stable transfectants expressing a series of chimeric selectins and deletion mutants were functionally analyzed in order to determine the molecular basis of adhesion mediated by L-selectin. The specificity of adhesion was found to reside entirely within the lectin domain, suggesting that this domain is the only domain of the protein to interact with the carbohydrate ligand. These results make previous observations that certain mAbs which block function map to each of the extracellular domains difficult to interpret. In addition, deletion of the cytoplasmic tail of L-selectin abolished adhesion, without affecting ligand recognition. Thus, each domain of the selectins has an important, but distinct, role in cell adhesion.