Nucleotide binding of the C-terminal domains of the major histocompatibility complex-encoded transporter expressed in Drosophila melanogaster cells

The C-terminal domains of the mouse transporter associated with antigen processing (TAP) were expressed as soluble proteins in Drosophila melanogaster cells and labeled by [α-³²P]8-azido-ATP after UV-irradiation. The relative potencies of the nucleotides in preventing azido-ATP labeling were in the order of ATP > GTP > CTP > ITP > UTP for both the TAP1 and TAP2 C-terminal domains, suggesting ATP to be the natural substrate of the transporter. Our data provide the first evidence that the individual C-terminal domain of either TAP1 or TAP2 can be expressed as a functional ATP-binding protein.