title: In vivo assembly of the proteasomal complexes, implications for antigen processing.
abstract: The multicatalytic and multisubunit proteasomal complexes have been implicated in the processing of antigens to peptides presented by class I major histocompatibility complex molecules. Two structural complexes of this proteinase, 20 S and 26 S proteasomes, have been isolated from cells. By analyzing in vivo assembly of the proteasomal complexes we show that the 20 S proteasomal complexes are irreversibly assembled via 15 S assembly intermediates containing unprocessed beta-type subunits. The 20 S proteasomes further associate reversibly with proteasome activators PA28 or pre-existing ATPase complexes to form 26 S proteasomal complexes. Our findings that not all of the 20 S proteasomal complexes are assembled into 26 S proteasomal complexes within cells and that all of PA28 and ATPase complexes are associated with 20 S proteasomes strongly suggest that all proteasomal complexes coexist within cells. We further demonstrate that 26 S proteasomal complexes are predominantly present in the cytoplasm and a significant portion of the 20 S proteasomal complexes is associated with the endoplasmic reticulum membrane. Taken together, our findings suggest that depending upon their associated regulatory components, 26 S and 20 S-PA28 proteasomal complexes serve different housekeeping functions within the cells, while they degrade antigens in a cooperative manner in antigen processing.
mesh_terms: D000818:Animals; D000941:Antigens / Q000096:biosynthesis / Q000378:metabolism*; D002460:Cell Line; D003546:Cysteine Endopeptidases / Q000096:biosynthesis* / Q000737:chemistry / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006801:Humans; D008223:Lymphoma; D046911:Macromolecular Substances; D008285:Major Histocompatibility Complex; D051379:Mice; D008961:Models, Structural; D009097:Multienzyme Complexes / Q000096:biosynthesis* / Q000737:chemistry / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex; D014176:Protein Biosynthesis; D011499:Protein Processing, Post-Translational; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000737:chemistry / Q000378:metabolism; D014162:Transfection; D014407:Tumor Cells, Cultured
chemical_list: D000941:Antigens; D046911:Macromolecular Substances; D009097:Multienzyme Complexes; D010710:Phosphates; D011994:Recombinant Proteins; D003546:Cysteine Endopeptidases; C077436:LMP7 protein; D046988:Proteasome Endopeptidase Complex