Assembly Mechanisms of Specialized Core Particles of the Proteasome

The 26S proteasome has a highly complicated structure comprising the 20S core particle (CP) and the 19S regulatory particle (RP). Along with the standard CP in all eukaryotes, vertebrates have two more subtypes of CP called the immunoproteasome and the thymoproteasome. The immunoproteasome has catalytic subunits beta 1i, beta 2i, and beta 5i replacing beta 1, beta 2, and beta 5 and enhances production of major histocompatibility complex I ligands. The thymoproteasome contains thymus-specific subunit beta 5t in place of beta 5 or beta 5i and plays a pivotal role in positive selection of CD8(+) T cells. Here we investigate the assembly pathways of the specialized CPs and show that beta 1i and beta 2i are incorporated ahead of all the other beta-subunits and that both beta 5i and beta 5t can be incorporated immediately after the assembly of beta 3 in the absence of beta 4, distinct from the assembly of the standard CP in which beta-subunits are incorporated in the order of beta 2, beta 3, beta 4, beta 5, beta 6, beta 1, and beta 7. The propeptide of beta 5t is a key factor for this earlier incorporation, whereas the body sequence seems to be important for the earlier incorporation of beta 5i. This unique feature of beta 5t and beta 5i may account for preferential assembly of the immunoproteasome and the thymoproteasome over the standard type even when both the standard and specialized subunits are co-expressed.