Further characterization of the membrane anchor found on the tissue-specific class I molecule Qa2

Previous studies have determined that various Qa2 serologic determinants can be removed from the surface of spleen cells by treatment with a phospholipase C. Our studies have determined that the class I molecule Qa2, expressed on the surface of spleen cells and activated T cells, behaves as an integral membrane protein based on its ability to associate with detergent micelles. Studies utilizing two purified phospholipase C have revealed that although most (90 to 95%) of the Qa2 molecules expressed on the surface of resting spleen cells are released as intact 40-kDa polypeptides associated with β₂-microglobulin, activated T cells contain a major cell subpopulation expressing lipase-resistant Qa2 molecules. Flow cytometric analysis revealed that L3T4⁺-activated T cells expressed lipase-sensitive Qa2 molecules, whereas Lyt-2⁺ cells express lipase-resistant forms of the Qa2 molecule. The relationship between the secreted form of the QA2 molecule and the lipase-generated soluble Qa2 molecule was investigated. Based on SDS-PAGE analysis, the secreted Qa2 molecules has a M(r) of 39 kDa whereas the cell surface form released from either resting spleen or activated T cells by phosphatidylinositol-specific phospholipase C has a M(r) of ≃ 40 kDa. Furthermore, the secreted Qa2 molecule lacks an epitope, cross-reacting determinant, often present on lipase-solubilized cell surface molecules. Thus, based on serologic and biochemical criteria, the soluble Qa2 molecules generated by an exogenous phospholipase C and the secreted Qa2 molecule are structurally distinct.