Association of immunoproteasomes with the endoplasmic reticulum

Proteasomes ale complex multisubunit proteases which play a critical role i n intracellular proteolysis. Immunoproteasomes, which contain three gamma - interferon-inducible subunits, are a subset of proteasomes which have a spe cialized function in antigen processing for presentation by the MHC class I pathway. Two of the gamma -interferon inducible subunits, LMP2 and LMP7, a re encoded within the MHC class II region adjacent to the two TAP (transpor ter associated with antigen presentation) genes. We have investigated the l ocalization of immunoproteasomes using monoclonal antibodies to LMP2 and LM P7. Immunoproteasomes were strongly enriched around the endoplasmic reticul um as judged by double-immunofluorescence experiments with anti-calreticuli n antibodies, but were also present in the nucleus and throughout the cytos ol. In contrast, proteasome subunit C2, which is present in all proteasomes , was found to be evenly distributed throughout the cytoplasm and in the nu cleus, as was the delta subunit, which is replaced by LMP2 in immunoproteas omes. gamma -Interferon increased the level of immunoproteasomes, but had n o effect on their distribution. Our results provide the first direct eviden ce that immunoproteasomes are strongly enriched at the endoplasmic reticulu m, where they may be located close to the TAP transporter to provide effici ent transport of peptides into the lumen of the endoplasmic recticulum for association with MHC class I molecules.