Characterization of porcine TAP genes: alternative splicing of TAP1

The transporter associated with antigen processing (TAP) is a heterodimer composed of TAP1 and TAP2 subunits that belong to the ATP-binding cassette family of transporters. TAP translocates small peptides (usually 8- to 12-amino-acid-long) from the cytosol to the endoplasmic reticulum for subsequent loading onto the major histocompatibility complex (MHC) class I molecules. The translocated peptides are required for the stable cell surface expression of MHC class I molecules. Virus-encoded proteins, which inhibit TAP activity, include ICP47 from herpes simplex virus and US6 from human cytomegalovirus. We have previously shown that ICP47 downregulated porcine MHC class I [swine leukocyte Ag class I (SLA I)] cell-surface expression in the pig epithelial cell line PK(15). Here we show that SLA I cell-surface expression in the pig epithelial cell line LLC-PK1 is relatively unaffected by expression of ICP47. Anticipating that this might be due to differences in the primary structure of TAP1 or TAP2 expressed by these two cell lines, cDNAs from PK(15) and LLC-PK1 encoding the complete open reading frames of porcine TAP1 and TAP2 were cloned and sequenced. Porcine TAP1 and TAP2 exhibited 80% amino acid identity with their human orthologs. Two splice variants of TAP1 were found. In LLC-PK1 cells, an alternatively spliced TAP1 transcript was detected, which was predicted to encode a protein with nine fewer amino acids. While the deleted amino acids may be in close proximity to the putative peptide/ICP47-binding site, we were unable to demonstrate that this imparted an apparent resistance to the effects of ICP47 on SLA I surface expression.