Analysis of D2^d: a D-region class I gene

The mouse major histocompatibility complex is composed of several genes arranged into the K, D, Qa, and Tla regions. The D region of the BALB/c mouse includes genes D2^d, D3^d, and D4^d, in addition to H-2D^dand H-2L^d. We have determined the DNA sequence of the D2^dgene and compared it with the known sequences of several class I genes. The exon/intron structure of the D2^dgene is similar to other class I genes. It also contains similar 5′ regulatory elements. A frameshift occurs in exon seven, resulting in a gene product with a truncated cytoplasmic tail. To examine the surface expression of the D2^d molecule, we generated an exon-shuffled construct containing the promoter and exons 1-3, encoding the signal peptide, α1, and α2 external domains of the D2^dgene linked to exons 4-8, encoding the α3, transmembrane and cytoplasmic domains, of the H-2D^dgene. The construct was transfected into mouse L cells, and a protein was detected at the cell surface by a monoclonal antibody (mAb) specific for the α3 domain of H-2D^d, as well as by other class I-specific mAbs. Although D2^d is expressed at low levels, it may be a functional class I gene that most probably evolved from a Qa region gene.