INTRODUCTION OF A GLYCOSYLATION SITE INTO A SECRETED PROTEIN PROVIDESEVIDENCE FOR AN ALTERNATIVE ANTIGEN-PROCESSING PATHWAY - TRANSPORT OFPRECURSORS OF MAJOR HISTOCOMPATABILITY COMPLEX CLASS I-RESTRICTED PEPTIDES FROM THE ENDOPLASMIC-RETICULUM TO THE CYTOSOL

We found that the presentation of a H-2K(d)-restricted determinant fro m influenza virus nucleoprotein (NP) to T cells is strictly dependent on expression of the transporter associated with antigen presentation (TAP), regardless of whether NP is expressed as a cytosolic or secrete d NP (SNP). Introducing an N-linked glycosylation site into the determ inant selectively reduced presentation of SNP. This indicates that gly cosylation does not interfere with TAP-transported peptides, and there fore that cytosolic peptides derived from SNP must have been exposed t o the glycosylation machinery of the endoplasmic reticulum (ER) before their existence in the cytosol. Based on these findings, we propose t hat TAP-dependent processing of at least some ER-targeted proteins ent ails the reimportation of protein from the secretory pathway to the cy tosol where the protein is processed via the classical pathway.