Identification of the Tapasin gene in the chicken major histocompatibilitycomplex

The Tapasin molecule plays a role in the assembly of major histocompatibili ty complex (Mhc) class I molecules in the endoplasmic reticulum, by mediati ng the interaction of class I-beta(2)-microglobulin dimers with TAP. We rep ort here the identification of the Tapasin gene in the chicken Mhc (B compl ex). This gene is located at the centromeric end of the complex, between th e class II B-LBI and B-LBII genes. Like its human counterpart it comprises 8 exons, but features a significantly reduced intron size as compared to th e human gene. Chicken Tapasin codes for a transmembrane protein with a prob able endoplasmic reticulum retention signal. Exons IV and V, and possibly e xon III, code for separate domains that are related to the immunoglobulin ( Ig) superfamily (this relationship was so far unrecognized for human Tapasi n domain IV which has lost its two cysteines). Two different cDNAs correspo nding to the Tapasin gene were isolated, possibly related to alternative sp licing events; the Ig-like domain encoded by exon IV is missing in one of t he cDNAs, suggesting either that this domain is not necessary for the prote in to perform its function, or that the two alternatively spliced cDNAs are translated into two functionally different forms of the protein.