TAP association influences the conformation of nascent MHC class I molecules

The influence of TAP-MHC class I interactions on peptide binding to the class I heavy chain is assessed during TAP-dependent assembly using Kb-specific Abs that recognize conformational changes induced by assembly with beta2-microglobulin (beta2m) and by peptide binding. A significant portion (45%) of Kb molecules in TAP+, RMA-derived microsomes are associated with the TAP complex as measured by coimmunoisolation of Kb using anti-TAP1 Abs, while only 20% of the Kb heavy chain molecules are isolated as Kbbeta2m complexes with the alpha-Kb-specific Abs, Y-3 or K-10-56. The amount of Kb isolated with Y-3 and K-10-56 increases in proportion to transport and binding of peptide to the Kb molecules within the RMA microsomes. In contrast, less than 5% of the Kb within TAP2-RMA-S microsomes associated with the remaining TAP1 subunit. However, greater than 60% of Kb heavy chain is isolated as K-10-56- and Y-3-reactive Kbbeta2m complexes. We propose that a TAP-MHC class I interaction serves to stabilize the MHC class I:beta2m complex in an immature conformation (Y-3 and K-10-56 nonreactive) prior to high affinity peptide binding, preventing the export of class I molecules complexed with low affinity peptide ligands from the ER.