近日，美国加州大学的Philip Coffino等人研究发现，蛋白酶体ATPase蛋白不同的同源亚型具有自己独特的功能。相关论文发表在4月5日的The Journal of Biological Chemistry。
蛋白酶体(proteasomes) 是在真核生物和古菌中普遍存在的，在一些原核生物中也存在的一种巨型蛋白质复合物。在真核生物中，蛋白酶体位于细胞核和细胞质中。主要作用是降解细胞不需要的或受到损伤的蛋白质。
依赖蛋白酶体的降解过程包括蛋白底物的移位及去折叠。六种不同的但同源的蛋白酶体ATPase蛋白Rpt1~6，形成了一个作用于底物的异六聚环。这个轴向定位的环(Ar-Phi loop)的运动与ATP水解作用一致，吸引底物并推动它进入蛋白水解空腔。
在酿酒酵母中，存在的六种Rpts的Ar-Phi环上的芳香族残基都是络氨酸，这种氨基酸被认为具有与底物有关的重要功能。为此，研究人员构造了六种新的酵母菌株，这些菌株中，Ar-Phi环上的Tyr均突变为Ala（丙氨酸）。
结果发现，突变菌株能够存活但是具有不同的表型。rpt3、rpt4及rpt5的Tyr/Ala突变体聚集在ATPase六聚体的一侧，这是降解功能受损的表型。
与此相反，rpt1、rpt2及rpt6的突变体在降解能力上等同或者超过了野生型。然而，在泛素蛋白酶体系统面临压力的条件下，rpt1及rpt6突变体也有一些缺陷，比如限制了细胞生长以及生存的能力。
出乎意料的是，rpt3突变体反而能够生长得更快但是更小，实验发现，它有一个与G1细胞周期蛋白的错误调节有关的缺陷。因此，这种rpt3表型可能来源于细胞周期调控蛋白降解作用的改变。
实验表明，5种Rpt亚型的突变，能偶增强蛋白酶体ATPase的活性，这意味着Ar-Phi环与ATP水解作用位点能够双相耦合。
这个结果阐明了不同的Rpt蛋白所具有的不同的特殊功能，对认识体内独特的蛋白酶体ATPases提供了一个新的视角。（生物谷Deepblue编译）
doi: 10.1074/jbc.M112.357327
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Functional asymmetries of the proteasome translocase pore
Jenny Erales, Martin A. Hoyt, Fabian Troll and Philip Coffino.
Degradation by proteasomes involves coupled translocation and unfolding of its protein substrates. Six distinct but paralogous proteasome ATPase proteins,Rpt1-6, form a heterohexameric ring which acts on substrates.An axially-positioned loop (Ar-Phi loop) moves in concert with ATP hydrolysis, engages substrate and propels it into a proteolytic chamber.The aromatic (Ar) residue of the Ar-Phi loop in all six Rpts of S. cerevisiae is tyrosine; this amino acid is thought to have important functional contacts with substrate. Six yeast strains were constructed and characterized in which Tyr was individually mutated to Ala.The mutant cells were viable and had distinct phenotypes. rpt3, rpt4 and rpt5 Tyr/Ala mutants, which cluster on one side of the ATPase hexamer, were substantially impaired in their capacity to degrade substrates.In contrast, rpt1, rpt2 and rpt6 mutants equaled or exceeded wild type in degradation activity. However rpt1 and rpt6 mutants had defects that limited cell growth or viability under conditions that stressed the ubiquitin proteasome system. In contrast, the rpt3 mutant grew faster than wild type and to a smaller size, a defect that has previously been associated with misregulation of G1 cyclins.This rpt3 phenotype likely results from altered degradation of cell cycle regulatory proteins.Finally, mutation of five of the Rpt subunits, increased proteasome ATPase activity, implying bidirectional coupling between the Ar-Phi loop and the ATP hydrolysis site.The present observations assign specific functions to individual Rpt proteins and provide insights into the diverse roles of the axial loops of individual proteasome ATPases.
