Yi Haiqing, Friedman Julie L, Ferreira Paulo A
Department of Ophthalmology, Duke University, Medical Center, Durham, North Carolina 27710, USA.
J Biol Chem. 2007 Nov 30;282(48):34770-8. doi: 10.1074/jbc.M706903200. Epub 2007 Oct 2.
The ubiquitin-proteasome system (UPS) plays a critical role in protein degradation. The 19S regulatory particle (RP) of the 26S proteasome mediates the recognition, deubiquitylation, unfolding, and channeling of ubiquitylated substrates to the 20S proteasome. Several subunits of the 19S RP interact with a growing number of factors. The cyclophilin-like domain (CLD) of Ran-binding protein-2 (RanBP2/Nup358) associates specifically with at least one subunit, S1, of the base subcomplex of the 19S RP, but the functional implications of this interaction on the UPS activity are elusive. This study shows the CLD of RanBP2 promotes selectively the accumulation of a subset of reporter substrates of the UPS, such as the ubiquitin (Ub)-fusion yellow fluorescent protein (YFP) degradation substrate, Ub(G76V)-YFP, and the N-end rule substrate, Ub-R-YFP. Conversely, the degradation of endoplasmic reticulum and misfolded proteins, and of those linked to UPS-independent degradation, is not affected by CLD. The selective effect of CLD on the UPS in vivo is independent of, and synergistic with, proteasome inhibitors, and CLD does not affect the intrinsic proteolytic activity of the 20S proteasome. The inhibitory activity of CLD on the UPS resides in a purported SUMO binding motif. We also found two RanBP2 substrates, RanGTPase-activating protein and retinitis pigmentosa GTPase regulator interacting protein-1alpha1, whose steady-state levels are selectively modulated by CLD. Hence, the CLD of RanBP2 acts as a novel auxiliary modulator of the UPS activity; it may contribute to the molecular and subcellular compartmentation of the turnover of properly folded proteins and modulation of the expressivity of several neurological diseases.
泛素-蛋白酶体系统（UPS）在蛋白质降解中起关键作用。26S蛋白酶体的19S调节颗粒（RP）介导泛素化底物的识别、去泛素化、解折叠以及将其转运至20S蛋白酶体。19S RP的几个亚基与越来越多的因子相互作用。Ran结合蛋白2（RanBP2/Nup358）的亲环蛋白样结构域（CLD）特异性地与19S RP基础亚复合物的至少一个亚基S1相互作用，但这种相互作用对UPS活性的功能影响尚不清楚。本研究表明，RanBP2的CLD选择性地促进UPS的一部分报告底物的积累，如泛素（Ub）-融合黄色荧光蛋白（YFP）降解底物Ub（G76V）-YFP和N端规则底物Ub-R-YFP。相反，内质网和错误折叠蛋白以及与UPS非依赖性降解相关的蛋白的降解不受CLD影响。CLD在体内对UPS的选择性作用独立于蛋白酶体抑制剂并与之协同，且CLD不影响20S蛋白酶体的内在蛋白水解活性。CLD对UPS的抑制活性存在于一个所谓的SUMO结合基序中。我们还发现了两个RanBP2底物，即Ran鸟苷三磷酸酶激活蛋白和视网膜色素变性GTP酶调节蛋白相互作用蛋白-1α1，其稳态水平受到CLD的选择性调节。因此，RanBP2的CLD作为UPS活性的一种新型辅助调节剂；它可能有助于正确折叠蛋白周转的分子和亚细胞分隔以及几种神经疾病表达性的调节。
