PA28 alpha beta double and PA28 beta single transfectant mouse B8 cell lines reveal enhanced presentation of a mouse cytomegalovirus (MCMV) pp89 MHC class I epitope

PA28 is an interferon-gamma inducible modulator of proteasome function comp osed of two subunits, i.e. PA28 alpha and PA28 beta. Previously we showed t hat stabile overexpression of the PA28 alpha subunit alone supported MHC cl ass I antigen presentation of two viral epitopes, However, no information w as obtained on the consequences when PA28 alpha and PA28 beta function in c oncert or when PA28 beta is overexpressed on its own, Here we demonstrate t hat overexpression of PA28 alpha and beta together is similarly efficient i n supporting MHC class I antigen presentation of the MCMV pp89 9mer epitope as PA28a alone, excluding a potentially potentiating role of PA28 beta. Su rprisingly, and despite the fact that PA28 beta alone was thought to be ina ctive and to only stabilize PA28 activity, overexpression of PA28 beta also resulted in improved antigen presentation. However, by northernblot and im munoprecipitation experiments we show that while PA28a alpha is able to act alone the observed effect in the PA28 beta and PA28 alpha beta transfectan t cell lines is due to increased levels of PA28 alpha beta complexes. (C) 2 000 Elsevier Science Ltd. All rights reserved.