Discrete proteolytic intermediates in the MHC class I antigen processing pathway and MHC I-dependent peptide trimming in the ER

The antigen processing pathway generates the peptides displayed by MHC I mo lecules on the cell surface. Whether these peptides are generated in the cy tosol or from longer intermediates transported into the ER is unclear, beca use peptides other than those bound to MHC I have been difficult to find. U sing a novel assay, we show that N-terminally extended antigenic analogs we re associated with high-molecular weight material in the cytosol and were t ransported by TAP. In the ER, a nonapeptide was predominant that was conver ted to the final octapeptide only in presence of the appropriate MHC I mole cule. The existence of extended peptides and their MHC I-dependent trimming suggest a mechanism for efficiently satisfying the distinct sequence prefe rences of polymorphic MHC I molecules.