COORDINATED DUAL CLEAVAGES INDUCED BY THE PROTEASOME REGULATOR PA28 LEAD TO DOMINANT MHC LIGANDS

The eukaryotic 20S proteasome is known to associate with the IfN gamma -inducible regulator PA28. We analyzed the kinetics of product generat ion by 20S proteasomes with and without PA28. In the absence of PA28, the 20S proteasome rapidly generates peptides that have been cleaved o nly once, while internal fragments accumulate only slowly. In the pres ence of PA28, products generated by two flanking cleavages appear imme diately as main products while the generation of single-cleavage produ cts is strongly reduced. Kinetic data support a PA28-induced, coordina ted double-cleavage mechanism. In particular, degradation of peptides derived from mouse cytomegalovirus pp89 and JAK1 kinase in the presenc e of PA28 leads to strongly enhanced production of the respective majo r histocompatibility complex ligands and potential precursors. These r esults show that PA28 profoundly alters the cleavage mechanism of the proteasome and appears to optimize the generation of dominant T-cell e pitopes.