TAP association influences the conformation of nascent MHC class I molecules

The influence of TAP-MHC class I interactions on peptide binding to the cla ss I heavy chain is assessed during TAP-dependent assembly using K-b-specif ic Abs that recognize conformational changes induced by assembly with beta( 2)-microglobulin (beta(2)m) 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 Ab s, while only 20% of the K-b heavy chain molecules are isolated as K(b)beta (2)m complexes with the alpha-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 a nd binding of peptide to the K-b molecules within the RMA microsomes. In co ntrast, less than 5% of the K-b within TAP2-RMA-S microsomes associated wit h the remaining TAP1 subunit, However, greater than 60% of K-b heavy chain is isolated as K-10-56- and Y-3-reactive K(b)beta(2)m complexes. We propose that a TAP-MHC class I interaction serves to stabilize the MHC class I:bet a(2)m complex in an immature conformation (Y-3 and K-10-56 nonreactive) pri or to high affinity peptide binding? preventing the export of class I molec ules complexed with low affinity peptide ligands from the ER.