TAP (TRANSPORTER ASSOCIATED WITH ANTIGEN PROCESSING)-INDEPENDENT PRESENTATION OF ENDOGENOUSLY SYNTHESIZED PEPTIDES IS ENHANCED BY ENDOPLASMIC-RETICULUM INSERTION SEQUENCES LOCATED AT THE AMINO-TERMINUS BUT NOTCARBOXYL-TERMINUS OF THE PEPTIDE

Under most circumstances, cell surface MHC class I molecules display p eptides derived from a cytosolic pool of proteins. The efficient prese ntation of such peptides requires the functioning of two MHC gene prod ucts [TAP1 and TAP2 (transporter-associated with Ag processing 1 and 2 )] that form a complex that facilitates transmembrane movement of pept ides from the cytosol to the endoplasmic reticulum, the site of peptid e association with class I molecules. It has been previously shown tha t peptides can be presented in a TAP-independent manner in association with HLA A2.1 or H-2 K(d) if they are expressed COOH-terminal to an e ndoplasmic reticulum insertion/signal sequence derived from the adenov irus E3/19K glycoprotein (Anderson et al., 1991. J. Exp. Med. 174: 489 ; Eisenlohr et al., 1992. Cell 71: 963). We show that: 1) the E3/1 9K signal sequence greatly enhances the presentation of each of four addi tional peptides tested in association with H-2 K(b) or K(k), 2) the E3 /19K signal sequence can be substituted by a signal sequence derived f rom beta-IFN, and 3) the E3/19K signal sequence does not function when located at the COOH terminus of antigenic peptides. These findings in dicate that first, many peptides require TAP for efficient presentatio n to T cells, second, expression of peptides COOH-terminal to signal s equences is a generally applicable method of bypassing the TAP-depende nce of peptide presentation and third, the leader sequence does not ac t to bypass TAP simply by increasing the hydrophobic nature of peptide s.