Regulation of L-selectin expression by membrane proximal proteolysis.

L-selectin is a lectin cell adhesion molecule expressed on the cell surfaces of lymphocytes, monocytes and granulocytes. Upon leukocyte activation or L-selectin cross-linking the transmembrane-bound L-selectin is rapidly shed from the cell surface. Based on these observations, it has been proposed that L-selectin is proteolytically cleaved from the cell surface. However a panel of common protease inhibitors have no effect on L-selectin proteolysis. To further define the mechanism of L-selectin down-regulation we have produced reagents to study proteolytic fragments of L-selectin. We have developed a trapping ELISA for the detection of soluble L-selectin. In addition we have produced a high affinity polyclonal antisera against the extracellular domain and against the cytoplasmic domain of L-selectin. Both antisera immunoprecipitate the intact form of L-selectin from metabolically labeled PHA lymphoblasts and peripheral blood neutrophils. We review here our progress in defining a 6 kD L-selectin transmembrane peptide (L-STMP) from PMA activated lymphoblasts and fMLP-activated neutrophils. Radiochemical sequencing data indicate that the cleavage site occurs between Lys321 and Ser322 in a short membrane-proximal region of the extracellular domain.