Structure and expression of glycoproteins controlled by the Qa-1 ^a allele

Abstract

The alloantigen controlled by the Qa-1 ^a allele is a glycoprotein that exists in two forms. The first, an intracellular molecule of apparent M_r of 44 000 daltons, appears to be a kinetic precursor of the second, a cell-surface molecule with an apparent size of 47 000 daltons. The intracellular form of Qa-1 is distinct from that of the TL glycoprotein in two ways: (1) its polypeptide backbone is approximately 5000 daltons shorter, and (2) it possesses three sites of high-mannose carbohydrate attachment, while TL has only one. In the cell-surface form of Qa-1, all three carbohydrate chains are processed to structures that resist endoglycosidase H digestion, presumably complex-type oligosaccharides. Concomitant with these late carbohydrate-processing steps is the formation of stable complexes between Qa-1 and β ₂-microglobulin. The timing of this association provides a further contrast between Qa-1 and TL, which is associated with β₂-microglobulin shortly after its synthesis. The Qa-1 glycoproteins have been identified genetically by their synthesis in B6-TL⁺ (Qa-1 ^a/Tla ^a) splenocytes but not in splenocytes of congenic 136K1 and B6.K2 (Qa-I ^b/Tla ^b) mice, and by their absence from the products of BALB/c (Qa-I ^vb/Tla ^c) splenocytes. The cells synthesizing Qa-1 are at least as prevalent in Ig⁺ spleen-cell populations as in T-cell-enriched splenic Ig⁻ populations. Thus, active Qa-1 synthesis appears to take place at a high rate in normal splenic B cells without mitogenic stimulation.