Mutation at amino acid position 133 of H-2Dd prevents beta 2m association and immune recognition but not surface expression.

The histocompatibility loss mutation H-2dm6 was derived from a mouse treated with the chemical mutagen ethylnitrosourea, and previous mapping studies implicated Dd as the affected locus. Southern blot analyses of DNA from H-2dm6 cells did not detect major deletions in the Ddm6 gene, suggesting that H-2dm6 was different from the previously characterized D region mutants H-2dm1 and H-2dm2. RNA blot analysis identified Ddm6 transcripts of appropriate size and a Ddm6 protein was immunoprecipitated from biosynthetically labeled H-2dm6 cells. Interestingly, the Ddm6 protein showed no beta 2m association and was only precipitated by a mAb to the alpha 3 domain. Furthermore, oligosaccharide maturation and low levels of surface expression of Ddm6 molecules were detected. However, the surface Ddm6 was nonfunctional as a target Ag in in vitro cytotoxicity assays, consistent with its original in vivo detection as a loss mutation. Sequence analyses of Ddm6 cDNA identified a single nucleotide base difference from wild-type, resulting in the substitution of a Trp to Arg at position 133. The significance of this substitution is discussed in the context of other class I expression variants.