A natural tapasin isoform lacking exon 3 modifies peptide loading complex function

To assure efficient MHC class I (MHC-I) peptide loading, the peptide loading complex (PLC) recruits the peptide-receptive form of MHC-I, and in this process, tapasin (tpn) connects MHC-I with the peptide transporter TAP and forms a stable disulfide bond with ERp57. Here, we describe an alternatively spliced tpn transcript lacking exon 3, observed in cells infected with human cytomegalovirus. Recognition of exon 3 was regulated via G-runs, suggesting that members of the hnRNP (heterogeneous nuclear ribonucleoprotein)-family regulate expression of the Delta Exon3 variant of tpn. Exon 3 includes Cys-95, which is responsible for the disulfide bond formation with ERp57 and, consequently, interaction of the Delta Exon3 variant with ERp57 was strongly impaired. Although the Delta Exon3 variant specifically stabilized TAP expression but not MHC-I in tpn-deficient cells, in tpn-proficient cells, the Delta Exon3 tpn reduced cell surface expression of the tpn-dependent HLA-B*44:02 allele; the stability of the tpn-independent HLA-B*44:05 was not affected. Most importantly, detailed analysis of the PLC revealed a simultaneous binding of the Delta Exon3 variant and tpn to TAP, suggesting modification of PLC functions. Indeed, an altered MHC-I ligandome was observed in HeLa cells overexpressing the Delta Exon3 variant, highlighting the potential of the alternatively spliced tpn variant to impact CD8(+) T-cell responses.