PA28α/β is a regulatory complex of the 20S proteasome which consists of two IFN-γ inducible subunits. Both subunits, α and β, contribute equally to the formation of hexa- or heptameric rings which can associate with the 20S proteasome. Previously, we have shown that overexpression of the PA28α subunit enhanced the MHC class I-restricted presentation of two viral epitopes and that purified PA28α/β accelerated T cell epitope generation by the 20S proteasome in vitro, indicating a role for PA28α/β in antigen presentation. This conclusion was recently confirmed in PA28β 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 PA28α/β 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, α and β subunits of PA28. In these transfectants we found a marked enhancement in the presentation of the immunodominant H-2Ld-restricted pp89 epitope of murine cytomegalovirus, although the 20S proteasome composition was the same as in recipient cells. We, therefore, conclude that PA28α/β can enhance antigen processing independently of changes in 20S proteasome subunit composition or assembly.

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