Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut, USA.
Nat Struct Mol Biol. 2011 May;18(5):622-9. doi: 10.1038/nsmb.2027. Epub 2011 Apr 17.
Dedicated chaperones facilitate the assembly of the eukaryotic proteasome, but how they function remains largely unknown. Here we show that a yeast 20S proteasome assembly factor, Pba1-Pba2, requires a previously overlooked C-terminal hydrophobic-tyrosine-X (HbYX) motif for function. HbYX motifs in proteasome activators open the 20S proteasome entry pore, but Pba1-Pba2 instead binds inactive proteasomal precursors. We discovered an archaeal ortholog of this factor, here named PbaA, that also binds preferentially to proteasomal precursors in a HbYX motif-dependent fashion using the same proteasomal α-ring surface pockets as are bound by activators. PbaA and the related PbaB protein can be induced to bind mature 20S proteasomes if the active sites in the central chamber are occupied by inhibitors. Our data are consistent with an allosteric mechanism in which the maturation of the proteasome active sites determines the binding of assembly chaperones, potentially shielding assembly intermediates or misassembled complexes from nonproductive associations until assembly is complete.
专门的分子伴侣有助于真核生物蛋白酶体的组装，但它们的功能仍在很大程度上未知。在这里，我们表明酵母 20S 蛋白酶体组装因子 Pba1-Pba2 需要一个以前被忽视的 C 末端疏水性酪氨酸-X（HbYX）基序才能发挥功能。蛋白酶体激活剂中的 HbYX 基序打开 20S 蛋白酶体入口孔，但 Pba1-Pba2 反而结合无活性的蛋白酶体前体。我们发现了这种因子的一个古菌同源物，在这里称为 PbaA，它也以依赖 HbYX 基序的方式优先结合蛋白酶体前体，使用与激活剂结合的相同蛋白酶体α环表面口袋。如果中央腔中的活性位点被抑制剂占据，则可以诱导 PbaA 和相关的 PbaB 蛋白结合成熟的 20S 蛋白酶体。我们的数据与变构机制一致，其中蛋白酶体活性位点的成熟决定了组装伴侣的结合，可能会保护组装中间体或组装错误的复合物免受非生产性结合，直到组装完成。
