Pairing of the nucleotide binding domains of the transporter associated with antigen processing

The transporter associated with antigen processing (TAP) comprises two stru cturally related subunits, TAP1 and TAPS, that form stable complexes in end oplasmic reticulum (ER) membranes. TAP complexes function in the translocat ion of peptides from the cytosol into the ER lumen for presentation by majo r histocompatibility complex class I molecules. Each TAP subunit contains a n N-terminal membrane-spanning region with multiple membrane-spanning segme nts, and a C-terminal, cytosolic nucleotide binding region. To study the na ture of the interactions occurring on the cytosolic face of TAP1/TAP2 compl exes, we investigated quaternary associations mediated by two C-terminal fr agments of human TAP1 (Tlc, residues 452-748 and T1ctr, residues 472-748) a nd two C-terminal fragments of human TAPS (T2c, residues 399-686 and T2ctr, residues 433-686), Each of these constructs contains the core nucleotide b inding region as well as a long or short N-terminal extension. We show stab le complex formation between T1c and T2c but not between T1ctr and T2ctr, T he mechanistic implications of Obese results are discussed. We also show th at each of the constructs except T1ctr interacts with wild type TAP1 and TA P2, indicating possibilities for homodimerization of TAP1 and TAP2, or of o ligomerization of TAP1/TAP2 heterodimers on membranes.