Department of Chemistry and Biochemistry, Center for Biomolecular Structure and Organization, University of Maryland, College Park, 20910, USA.
Mol Cell. 2009 Dec 25;36(6):1018-33. doi: 10.1016/j.molcel.2009.11.012.
As a signal for substrate targeting, polyubiquitin meets various layers of receptors upstream to the 26S proteasome. We obtained structural information on two receptors, Rpn10 and Dsk2, alone and in complex with (poly)ubiquitin or with each other. A hierarchy of affinities emerges with Dsk2 binding monoubiquitin tighter than Rpn10 does, whereas Rpn10 prefers the ubiquitin-like domain of Dsk2 to monoubiquitin, with increasing affinities for longer polyubiquitin chains. We demonstrated the formation of ternary complexes of both receptors simultaneously with (poly)ubiquitin and found that, depending on the ubiquitin chain length, the orientation of the resulting complex is entirely different, providing for alternate signals. Dynamic rearrangement provides a chain-length sensor, possibly explaining how accessibility of Dsk2 to the proteasome is limited unless it carries a properly tagged cargo. We propose a mechanism for a malleable ubiquitin signal that depends both on chain length and combination of receptors to produce tetraubiquitin as an efficient signal threshold.
多聚泛素作为底物靶向的信号，与 26S 蛋白酶体上游的各种受体层结合。我们获得了两个受体，Rpn10 和 Dsk2 的结构信息，单独的和与（多）泛素或彼此复合物的。亲和力的层次结构出现了，Dsk2 结合单泛素比 Rpn10 更紧密，而 Rpn10 更喜欢 Dsk2 的泛素样结构域而不是单泛素，对更长的多泛素链的亲和力增加。我们证明了两种受体同时与（多）泛素形成三元复合物，并发现，根据泛素链的长度，复合物的取向完全不同，提供了替代信号。动态重排提供了一个链长传感器，可能解释了为什么 Dsk2 对蛋白酶体的可及性受到限制，除非它携带适当标记的货物。我们提出了一种灵活的泛素信号机制，该机制既依赖于链长，又依赖于受体的组合，以产生四聚泛素作为有效的信号阈值。
