SummaryThe transporter associated with antigen processing (TAP) delivers peptides to the lumen of the endoplasmic reticuhm in an adenosine triphosphate (ATP) dependent fashion for presentation by major histocompatibility complex class I molecules. We show that the mouse TAP translocator (H-2 b haplotype) sdects peptides based on a minimal size of nine residues, and on the presence of a hydrophobic COOH-terminal amino add. The preponderance of COOH-terminal hydrophobic amino acids in peptides capable of binding to mouse class I molecules thus fits remarkably well with the specificity of the TAP translocator. In addition to transport in the lumenal direction, eit]ux of peptide in the cytosolic direction is observed in an ATP-and temperature-dependent manner. By maintaining a low peptide concentration at the site of class I assembly, this efl:hx mechanism may ensure that class I molecules are loaded preferentially with high affinity peptides.

Major histocompatibility complex (MHC) class I molecules combine with short peptides of defined length and sequence. Here we describe an approach that may be used in the analysis of peptide preference of different allelic MHC class I molecules, and in the determination of T cell epitopes. We produced synthetic "peptide libraries" of limited complexity by standard peptide chemistry. Using these peptide mixtures we show that H-2 Kb molecules can accommodate both 8- and 9-residue peptides, whereas Db molecules are unable to combine with peptides shorter than 9 amino acids present in these libraries. When these peptide mixtures are used to provide "fingerprints" of Db molecules and mutants thereof, both loss and gain of the ability to combine with certain peptides is observed. For the Kbm1 mutant a strong influence of amino acid substitutions in class I molecules on the peptides selected is observed. In these synthetic peptide mixtures, the presence of a specific T cell epitope, known to be represented once, can be detected. This approach may be extended to the identification of new T cell epitopes from larger peptide libraries.

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