Isolation of cDNA clones for the p33 invariant chain associated with HLA-DR antigens.

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Abstract
HLA-DR antigens are polymorphic cell surface glycoproteins involved in the control of the immune response in man. They consist of 2 subunits, the .alpha. and .beta. chains. An invariant glycoprotein of MW 33,000 [daltons] (DRp33) is associated intracellularly with HLA-DR antigens. A c[complementary]DNA clone for DRp33, called 33-10, was isolated. Because no amino acid sequence has yet been determined for DRp33 the identification of cDNA clone 33-10 was based on selection of mRNA by hybridization, subsequent translation in a rabbit reticulocyte lysate supplemented with microsomes and translation in microinjected Xenopus oocytes followed by immunoprecipitation with an anti-DR antiserum. The translation products assembled with DR .alpha. and .beta. chain in oocytes coinjected with all 3 mRNA. Assembly of DR .alpha. and .beta. chains was also observed in the absence of DRp33 mRNA. When compared with DRp33 immunoprecipitated from a human B-cell line [Burkitt''s lymphoma Raji cells], translation products of the hybrid-selected mRNA showed identical migration in 2-dimensional gel electrophoresis, identical apparent MW in the absence of N-linked glycosylation and a similar 2-dimensional peptide map. Transcription of the DRp33 gene into a mRNA 1400 nucleotides long was observed in B cells but was undetectable in T-cell lines and very low in liver. DRp33 appears to be coordinately expressed with DR .alpha. and .beta. chains. Hybridization to DNA of mouse-human somatic cell hybrids showed that DRp33 is encoded by a gene that is located outside the major histocompatibility complex.