2 NOVEL ROUTES OF TRANSPORTER ASSOCIATED WITH ANTIGEN-PROCESSING (TAP)-INDEPENDENT MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-I ANTIGEN-PROCESSING

Jaw1 is an endoplasmic reticulum (ER) resident protein representative of a class of proteins post translationally inserted into membranes vi a a type II membrane anchor (cytosolic NH2 domain, lumenal COOH domain ) in a translocon-independent manner. We found that Jaw1 can efficient ly deliver a COOH-terminal antigenic peptide to class I molecules in t ransporter associated with antigen processing (TAP)-deficient cells or cells in which TAP is inactivated by the ICP47 protein. Peptide deliv ery mediated by Jaw1 to class I molecules was equal or better than tha t mediated by the adenovirus E3/19K glycoprotein signal sequence, and was sufficient to enable cytofluorographic detection of newly recruite d thermostabile class I molecules at the surface of TAP-deficient cell s. Deletion of the transmembrane region retargeted Jaw1 from the ER to the cytosol, and severely, although incompletely, abrogated its TAP-i ndependent peptide carrier activity. Use of different protease inhibit ors revealed the involvement of a nonproteasomal protease in the TAP-i ndependent activity of cytosolic Jaw1. These findings demonstrate two novel TAP-independent routes of antigen processing; one based on highl y efficient peptide liberation from the COOH terminus of membrane prot eins in the ER, the other on delivery of a cytosolic protein to the ER by an unknown route.