Molecular architecture of the TAP-associated MHC class I peptide-loading complex

Tapasin organizes the peptide-loading complex (PLC) by recruiting peptide-receptive MHC class I (MHC-1) and accessory chaperones to the N-terminal regions of the TAP subunits TAP and TAP2. Despite numerous studies have shown that the formation of the PLC is essential to facilitate proper MHC-1 loading, the molecular architecture of this complex is still highly controversial. We studied the stoichiometry of the PLC by blue native-PAGE in combination with Ab-shift assays and found that TAP/tapasin complexes exist at steady state as a mixture of two distinct oligomers of 350 and 450 kDa. Only the higher m.w. complex contains MHC-I and disulfide-linked tapasin/ER60 conjugates. Moreover, we show for the first time to our knowledge that the fully assembled PLC comprises two tapasin, two ER60, but only one complex of MHC-1 and calreticulin. Based hereon we postulate that the TAP subunits alternate in the recruitment and loading of a single MHC-I.