近日,，美國加州大學(xué)的Philip Coffino等人研究發(fā)現(xiàn),，蛋白酶體ATPase蛋白不同的同源亞型具有自己獨(dú)特的功能,。相關(guān)論文發(fā)表在4月5日的The Journal of Biological Chemistry。
蛋白酶體(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種巨型蛋白質(zhì)復(fù)合物,。在真核生物中，蛋白酶體位于細(xì)胞核和細(xì)胞質(zhì)中,。主要作用是降解細(xì)胞不需要的或受到損傷的蛋白質(zhì),。
依賴蛋白酶體的降解過程包括蛋白底物的移位及去折疊。六種不同的但同源的蛋白酶體ATPase蛋白Rpt1~6,，形成了一個(gè)作用于底物的異六聚環(huán),。這個(gè)軸向定位的環(huán)(Ar-Phi loop)的運(yùn)動(dòng)與ATP水解作用一致，吸引底物并推動(dòng)它進(jìn)入蛋白水解空腔,。
在釀酒酵母中,，存在的六種Rpts的Ar-Phi環(huán)上的芳香族殘基都是絡(luò)氨酸，這種氨基酸被認(rèn)為具有與底物有關(guān)的重要功能,。為此,，研究人員構(gòu)造了六種新的酵母菌株，這些菌株中,，Ar-Phi環(huán)上的Tyr均突變?yōu)锳la（丙氨酸）,。
結(jié)果發(fā)現(xiàn)，突變菌株能夠存活但是具有不同的表型,。rpt3,、rpt4及rpt5的Tyr/Ala突變體聚集在ATPase六聚體的一側(cè)，這是降解功能受損的表型,。
與此相反,，rpt1、rpt2及rpt6的突變體在降解能力上等同或者超過了野生型,。然而,，在泛素蛋白酶體系統(tǒng)面臨壓力的條件下，rpt1及rpt6突變體也有一些缺陷,，比如限制了細(xì)胞生長以及生存的能力,。
出乎意料的是，rpt3突變體反而能夠生長得更快但是更小,，實(shí)驗(yàn)發(fā)現(xiàn),，它有一個(gè)與G1細(xì)胞周期蛋白的錯(cuò)誤調(diào)節(jié)有關(guān)的缺陷。因此,，這種rpt3表型可能來源于細(xì)胞周期調(diào)控蛋白降解作用的改變,。
實(shí)驗(yàn)表明，5種Rpt亞型的突變,，能偶增強(qiáng)蛋白酶體ATPase的活性,，這意味著Ar-Phi環(huán)與ATP水解作用位點(diǎn)能夠雙相耦合,。
這個(gè)結(jié)果闡明了不同的Rpt蛋白所具有的不同的特殊功能，對認(rèn)識體內(nèi)獨(dú)特的蛋白酶體ATPases提供了一個(gè)新的視角,。（生物谷Deepblue編譯）
doi: 10.1074/jbc.M112.357327
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PMID:
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.
