Laboratory of Protein Metabolism, Graduate School of Pharmaceutical Sciences, The University of Tokyo, Tokyo, Japan.
Genes Cells. 2018 Oct;23(10):839-848. doi: 10.1111/gtc.12631. Epub 2018 Aug 22.
The proteasome core particle (CP) is a cytoplasmic and nuclear protease complex and is comprised of two α-rings and two β-rings stacked in order of αββα. The assembly of CP proceeds by ordered recruitment of β-subunits on an α-ring with help of assembly chaperones PAC1-PAC2, PAC3-PAC4, and UMP1. However, the mechanism of α-ring formation remains unsolved. Here, we show that α4, α5, α6, and α7 form a core intermediate as the initial process of α-ring assembly, which requires PAC3-PAC4. α1 and α3 can be incorporated independently into the core α4-α7 intermediate, whereas α2 incorporation is dependent on preceding incorporation of α1. Through these processes, PAC1-PAC2 prevents nonproductive dimerization of α-ring assembly intermediates. We also found that PAC1-PAC2 overrides the effect of nuclear localization signals of α-subunits and retains α-ring assembly intermediates in the cytoplasm. Our results first show a detailed assembly pathway of proteasomal α-ring and explain the mechanism by which CP assembly occurs in the cytoplasm.
蛋白酶体核心颗粒（CP）是一种细胞质和核蛋白酶复合物，由两个α环和两个β环按αβαβ的顺序堆叠而成。CP 的组装通过在组装伴侣 PAC1-PAC2、PAC3-PAC4 和 UMP1 的帮助下，有序地将β亚基募集到α环上进行。然而，α环形成的机制仍未解决。在这里，我们表明α4、α5、α6 和α7 形成一个核心中间物作为α环组装的初始过程，这需要 PAC3-PAC4。α1 和 α3 可以独立地掺入到核心α4-α7 中间物中，而α2 的掺入依赖于α1 的先前掺入。通过这些过程，PAC1-PAC2 可以防止α环组装中间体的非生产性二聚化。我们还发现，PAC1-PAC2 可以忽略α亚基的核定位信号的影响，并将α环组装中间体保留在细胞质中。我们的结果首次展示了蛋白酶体α环的详细组装途径，并解释了 CP 在细胞质中组装的机制。
