IN-VIVO ASSEMBLY OF THE PROTEASOMAL COMPLEXES, IMPLICATIONS FOR ANTIGEN-PROCESSING

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. Ey 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.