Biosynthesis of glycophospholipid bound and secreted murine class I Qa-2 polypeptides

Murine T cells synthesize and express a cell-surface glycophospholipid anchored 40 kDa and a secreted water-soluble 39 kDa Qa-2 polypeptide. We have examined the biosynthetic pathways which lead to the production of the membrane-bound and water-soluble isoforms of the Qa-2 molecule. Using the detergent TX-114, both detergent (membrane)-bound and soluble Qa-2 polypeptides can be identified in cell lysates and can be distinguished by charge and molecular weight. Two membrane-bound forms, a 40-kDa Endo H resistant cell-surface form and a 38 kDa-Endo H sensitive form can be identified, both of which can be biosynthetically labeled with 3H-ethanolamine and can be converted to water soluble forms by digestion with a phosphatidylinositol specific phospholipase C. In addition, several water soluble polypeptides at 39, 37, 35 kDa, and a minor species at 33 kDa were identified, none of which radiolabel with 3H-ethanolamine. While the 39-kDa polypeptide was Endo H resistant, the other isoforms were sensitive to Endo H digestion. Pulse chase experiments and molecular weights of the deglycosylated core polypeptides suggest a precursor to product relationship between the intracellular water-soluble species and the mature 39-kDa secreted Qa-2 molecule. This relationship is supported by the observation that murine L cells transfected with the Qa-2 encoding class I gene Q7 fail to express membrane-bound Qa-2 molecules yet synthesize both intracellular water-soluble and secreted Qa-2 molecules. These findings argue for a pathway in which secreted soluble Qa-2 molecules are derived from intracellular precursors.