Recent biochemical characterization of the T23-encoded Qa-1 molecule revealed an additional higher molecular mass species of 50 kD coprecipitated with the 48-kD Qa-1 molecule in H-2b and H-2d mouse strains. We now demonstrate that the 50-kD protein coprecipitated with Qa-1 is the class I-a antigen Ld in all H-2Ld-positive mouse strains examined. Furthers analyses of a panel of recombinants revealed that the 50-kD protein coprecipitated with Qa-1 in H-2b haplotype mouse strains is encoded or controlled by a gene centromeric to major histocompatibility complex class II I-E beta. We have designated this gene and corresponding protein product as Qsm, Qa-1 structure modifier. Both Ld and Qsm can interact with Qa-1 to form cell surface-expressed heterodimers in vivo. These Qa-1 heterodimers are not expressed in H-2k haplotype cells. The Qa-1/Ld and Qa-1/Qsm heterodimers are associated by noncovalent interactions and occur only between fully processed proteins. In addition, we show that the Qsm-encoded protein can form heterodimers with Ld as well, and that the Ld molecules participating in these interactions with Qa-1 and Qsm may be devoid of beta 2-microglobulin and/or peptide. These data represent the first demonstration that class I molecules can be expressed as heterodimers (Qa-1/Ld) on the cell surface, and map a gene (Qsm) that may potentially encode a novel class I molecule, or another protein, that associates with both Qa-1 and Ld. These interactions may enable increased levels of Qa-1 to reach the cell surface and may subsequently influence T cell recognition of Qa-1 and/or Ld molecules.

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