Peptide traffic across the ER membrane: TAPs and other conduits

Transport of antigenic peptides across the membrane of the endoplasmic reticulum (ER) is an essential step in the association of peptide and MHC class I molecules for antigen presentation. The genes for the peptide transporter subunits were cloned several years ago, but only recently has it been possible to demonstrate their activity in vitro, distinct from an alternative peptide import route, and to characterize their substrate specificity. In addition, this work has led to the discovery of a novel peptide export activity from the mammalian ER.