Trimming of antigenic peptides in an early secretory compartment.
Major histocompatibility complex (MHC) class I molecules bind peptides of 8-10 residues in the endoplasmic reticulum (ER) and convey them to the cell surface for inspection by CD8-expressing T cells (TCD8+). Antigenic peptides are predominantly derived from a cytosolic pool of polypeptides. The proteolytic generation of peptides from polypeptides clearly begins in the cytosol, but it is uncertain whether the final proteolytic steps occur before or after peptides are transported into the ER by the MHC-encoded peptide transporter (TAP). To study the trimming of antigenic peptides in the secretory pathway in the absence of cytosolic processing, we used an NH2-terminal signal sequence to target to the ER of TAP-deficient cells, "tandem" peptides consisting of two defined TCD8+ determinants arranged from head to tail. We find that in contrast to cytosolic proteases in TAP-expressing cells, which are able to liberate antigenic peptides from either end of a tandem peptide, proteases (probably aminopeptidases) present in an early secretory compartment preferentially liberate the COOH-terminal determinant. These findings demonstrate that proteolytic activities associated with antigen processing are not limited to the cytosol, but that they also exist in an early secretory compartment. Such secretory aminopeptidases may function to trim TAP-transported peptides to the optimal size for binding to class I molecules.
Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, Bethesda, Maryland 20892.
H L
HL
Snyder
J W
JW
Yewdell
J R
JR
Bennink
D000071181:ATP Binding Cassette Transporter, Subfamily B, Member 2; D018528:ATP-Binding Cassette Transporters / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D000956:Antigens, Viral / Q000378:metabolism*; D001665:Binding Sites; D002460:Cell Line; D010450:Endopeptidases / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000276:immunology; D015395:Histocompatibility Antigens Class I / Q000378:metabolism*; D006801:Humans; D009980:Influenza A virus / Q000276:immunology; D007739:L Cells; D008285:Major Histocompatibility Complex; D051379:Mice; D008807:Mice, Inbred BALB C; D008810:Mice, Inbred C57BL; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000276:immunology / Q000378:metabolism; D011485:Protein Binding; D011499:Protein Processing, Post-Translational; D013602:T-Lymphocytes, Cytotoxic / Q000378:metabolism*; D014407:Tumor Cells, Cultured; D014721:Vesicular stomatitis Indiana virus / Q000276:immunology
D000071181:ATP Binding Cassette Transporter, Subfamily B, Member 2; D018528:ATP-Binding Cassette Transporters; D000956:Antigens, Viral; D015395:Histocompatibility Antigens Class I; D010446:Peptide Fragments; C073119:TAP1 protein, human; C486171:Tap1 protein, mouse; D010450:Endopeptidases