20S PROTEASOME FROM LMP7 KNOCK OUT MICE REVEALS ALTERED PROTEOLYTIC ACTIVITIES AND CLEAVAGE SITE PREFERENCES

20S proteasomes of tissues from LMP7 knock out mice which show reduced MHC class I restricted antigen presentation were analyzed with regard to their subunit composition, peptide hydrolyzing activity and their ability to cleave a synthetic 25-mer polypeptide. LMP7 deficiency resu lts in an enhanced incorporation of subunit MB1 and in a 2-3.8-fold in crease in V-max for the Suc-LLVY-MCA hydrolyzing activity. Since LMP7 deficiency also affects the cleavage site preference of 20S proteasome s the reduced MHC class I antigen presentation of LMP7 knock out mice is most likely due to an impairment in peptide generation.