Structural implications of the location and stability to proteolytic enzymes of immunodominant determinants of the human leukocyte common molecule

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Abstract
In this report, we use 8 different mouse monoclonal antibodies to define 3 immunodominant determinants of the human leukocyte common (LC) antigen, and by defining the relative location and the stability to proteolytic enzymes of these determinants we make some structural predictions about the LC family of molecules. One lineage‐restricted determinant is expressed predominantly on B lymphocytes and is located on the two higher (210 and 195) kDa bands of LC, with possibly some expression on the 180‐kDa band. The two other determinants, one of which is a complex of 3 partially overlapping sites, are expressed on all leukocytes and found on all four (210, 195, 180, 160 kDa) LC bands. Pronase and trypsin treatment of peripheral blood lymphocytes and of Daudi cells gave complete cleavage of the lineage‐restricted determinant from the cells, but the two common determinants were, surprisingly, left intact on the cell surface. By contrast, pronase and trypsin treatment of pure, detergent‐solubilized LC resulted in rapid degradation of the common determinants, while the restricted determinant was sensitive to pronase but not to trypsin. Purification of the LC molecule from pronase‐treated Daudi cells yielded a 130‐kDa lentil lectin‐binding protein, while undegraded LC from Daudi cells has a molecular mass of 210 kDa. Our results demonstrate that both common and restricted determinants are at least partly protein in nature and that the restricted determinant is external to and therefore on the presumed amino terminal side of the common determinants. Moreover, because LC has been reported to have a large (approximately 100 kDa) intracellular component, our data suggest the presence of a relatively small (approximately 25 kDA) membrane proximal domain containing both of the common LC determinants and at least one N‐linked oligosaccharide chain. This domain is potentially susceptible to proteolytic cleavage, but it is interesting that on the intact cell it is both protected from proteolytic degradation and unable to be cleaved from the cell surface.