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 K^b- specific Abs that recognize conformational changes induced by assembly with β₂-microglobulin (β₂m) and by peptide binding. A significant portion (45%) of K^b molecules in TAP⁺, RMA-derived microsomes are associated with the TAP complex as measured by coimmunoisolation of K^b using anti-TAP1 Abs, while only 20% of the K^b heavy chain molecules are isolated as K^bβ₂m complexes with the α-K^b-specific Abs, Y-3 or K-10-56. The amount of K^b isolated with Y-3 and K-10-56 increases in proportion to transport and binding of peptide to the K^b molecules within the RMA microsomes. In contrast, less than 5% of the K^b within TAP2-RMA-S microsomes associated with the remaining TAP1 subunit. However, greater than 60% of K^b heavy chain is isolated as K-10-56- and Y-3-reactive K^bβ₂m complexes. We propose that a TAP-MHC class I interaction serves to stabilize the MHC class I:β₂m 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.