The proteasome regulator PA28 > / I can enhance antigen presentation without affecting 20S proteasome subunit composition

PA28 § / g is a regulatory complex of the 20S proteasome which consists of two IFN+ inducible subunits. Both subunits, § and g , contribute equally to the formation of hexaor heptameric rings which can associate with the 20S proteasome. Previously, we have shown that overexpression of the PA28 § subunit enhanced the MHC class I-restricted presentation of two viral epitopes and that purified PA28 § / g accelerated T cell epitope generation by the 20S proteasome in vitro, indicating a role for PA28 § / g in antigen presentation. This conclusion was recently confirmed in PA28 g gene targeted mice which were severely deficient in MHC class I-restricted antigen presentation. These mice displayed a defect in the assembly of immunoproteasomes, suggesting that a lack of the proteasome subunits LMP2, LMP7, and MECL-1 may account for the deficiency in antigen presentation. In this study we investigated whether the effect of PA28 § / g on antigen presentation is dependent on a change of proteasome subunit composition. We have analyzed the assembly and subunit composition of proteasomes in fibroblast transfectants overexpressing both, § and g subunits of PA28. In these transfectants we found a marked enhancement in the presentation of the immunodominant H-2L-restricted pp89 epitope of murine cytomegalovirus, although the 20S proteasome composition was the same as in recipient cells. We, therefore, conclude that PA28 § / g can enhance antigen processing independently of changes in 20S proteasome subunit composition or assembly.