Hirano Yuko, Kaneko Takeumi, Okamoto Kenta, Bai Minghui, Yashiroda Hideki, Furuyama Kaori, Kato Koichi, Tanaka Keiji, Murata Shigeo
Laboratory of Frontier Science, Tokyo Metropolitan Institute of Medical Science, Tokyo, Japan.
EMBO J. 2008 Aug 20;27(16):2204-13. doi: 10.1038/emboj.2008.148. Epub 2008 Jul 24.
The 20S proteasome is the catalytic core of the 26S proteasome. It comprises four stacked rings of seven subunits each, alpha(1-7)beta(1-7)beta(1-7)alpha(1-7). Recent studies indicated that proteasome-specific chaperones and beta-subunit appendages assist in the formation of alpha-rings and dimerization of half-proteasomes, but the process involved in the assembly of beta-rings is poorly understood. Here, we clarify the mechanism of beta-ring formation on alpha-rings by characterizing assembly intermediates accumulated in cells depleted of each beta-subunit. Starting from beta2, incorporation of beta-subunits occurs in an orderly manner dependent on the propeptides of beta2 and beta5, and the C-terminal tail of beta2. Unexpectedly, hUmp1, a chaperone functioning at the final assembly step, is incorporated as early as beta2 and is required for the structural integrity of early assembly intermediates. We propose a model in which beta-ring formation is assisted by both intramolecular and extrinsic chaperones, whose roles are partially different between yeast and mammals.
20S蛋白酶体是26S蛋白酶体的催化核心。它由四个堆叠的环组成，每个环有七个亚基，即α(1 - 7)β(1 - 7)β(1 - 7)α(1 - 7)。最近的研究表明，蛋白酶体特异性伴侣蛋白和β亚基附属物有助于α环的形成和半蛋白酶体的二聚化，但β环组装过程尚不清楚。在这里，我们通过表征在每个β亚基缺失的细胞中积累的组装中间体，阐明了α环上β环形成的机制。从β2开始，β亚基的掺入以有序的方式发生，这依赖于β2和β5的前肽以及β2的C末端尾巴。出乎意料的是，hUmp1，一种在最终组装步骤起作用的伴侣蛋白，早在β2时就被掺入，并且是早期组装中间体结构完整性所必需的。我们提出了一个模型，其中β环的形成由分子内和外在伴侣蛋白共同协助，它们在酵母和哺乳动物中的作用部分不同。
