NOVEL PEPTIDE-BINDING PROTEINS AND PEPTIDE-TRANSPORT IN NORMAL AND TAP-DEFICIENT MICROSOMES

Most major histocompatibility complex (MHC) class I-binding peptides a re translocated by TAP heterodimers, but some enter the ER lumen by al ternative pathways. To further define mechanisms of peptide handling, we developed a system for the analysis of peptide-binding components i n the ER membrane and lumen using iodinated cross-linkable peptide der ivatives. Here we demonstrate that at least three proteins bind peptid es in the ER lumen. Peptide cross-linking to these lumenal proteins ca n be used as an alternative method to monitor peptide transport. TAP a nd one other protein bind peptides on the cytoplasmic face of the ER. The presence of multiple peptide-binding proteins necessitates caution in interpreting traditional peptide-binding and transport assays. Fin ally, we demonstrate sequence-specific peptide transport in TAP-defici ent cells transfected with only rat TAP1.