Section of Structural Biology, Institute of Cancer Research, Chester Beatty Laboratories, London, UK.
EMBO J. 2010 Nov 3;29(21):3733-44. doi: 10.1038/emboj.2010.247. Epub 2010 Oct 5.
The anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase responsible for controlling cell cycle transitions, is a multisubunit complex assembled from 13 different proteins. Numerous APC/C subunits incorporate multiple copies of the tetratricopeptide repeat (TPR). Here, we report the crystal structure of Schizosaccharomyces pombe Cut9 (Cdc16/Apc6) in complex with Hcn1 (Cdc26), showing that Cdc16/Cut9 is a contiguous TPR superhelix of 14 TPR units. A C-terminal block of TPR motifs interacts with Hcn1, whereas an N-terminal TPR block mediates Cdc16/Cut9 self-association through a homotypic interface. This dimer interface is structurally related to the N-terminal dimerization domain of Cdc27, demonstrating that both Cdc16/Cut9 and Cdc27 form homo-dimers through a conserved mechanism. The acetylated N-terminal Met residue of Hcn1 is enclosed within a chamber created from the Cut9 TPR superhelix. Thus, in complex with Cdc16/Cut9, the N-acetyl-Met residue of Hcn1, a putative degron for the Doa10 E3 ubiquitin ligase, is inaccessible for Doa10 recognition, protecting Hcn1/Cdc26 from ubiquitin-dependent degradation. This finding may provide a structural explanation for a mechanism to control the stoichiometry of proteins participating in multisubunit complexes.
后期促进复合物/细胞周期体（APC/C）是一种负责控制细胞周期转变的 E3 泛素连接酶，由 13 种不同的蛋白质组装而成。许多 APC/C 亚基包含多个四肽重复序列（TPR）的拷贝。在这里，我们报告了裂殖酵母 Cut9（Cdc16/Apc6）与 Hcn1（Cdc26）复合物的晶体结构，显示 Cdc16/Cut9 是由 14 个 TPR 单元组成的连续 TPR 超螺旋。TPR 基序的 C 端块与 Hcn1 相互作用，而 N 端 TPR 块通过同源界面介导 Cdc16/Cut9 自身缔合。该二聚体界面在结构上与 Cdc27 的 N 端二聚化结构域相关，表明 Cdc16/Cut9 和 Cdc27 均通过保守机制形成同源二聚体。Hcn1 的 N 端乙酰化 Met 残基被封闭在由 Cut9 TPR 超螺旋形成的腔室内。因此，在与 Cdc16/Cut9 复合物中，Hcn1 的 N-乙酰基-Met 残基，即 Doa10 E3 泛素连接酶的假定降解基序，无法被 Doa10 识别，从而保护 Hcn1/Cdc26 免受泛素依赖性降解。这一发现可能为控制参与多亚基复合物的蛋白质的化学计量比的机制提供了结构解释。
