Roles of molecular chaperones in protein import into mitochondria.

大多数核编码的线粒体前体蛋白合成的氨基末端的前序列，其特征是正电荷和两亲性。线粒体前体翻译后靶向线粒体通常由包括hsp70应激蛋白家族成员的胞质伴侣促进。我们研究了各种合成肽与未折叠蛋白和酵母胞浆hsp70，Ssalp的相互作用。我们发现线粒体前序列可以诱导未折叠蛋白的聚集，这表明线粒体前序列可以在折叠过程中与前体的成熟部分相互作用并引起聚集。我们接下来发现，线粒体前序列可以结合到Ssalp，并且前序列与Ssalp的相互作用与前序列的两亲性直接相关。综上所述，本研究结果表明，热休克蛋白70可以通过结合到前序列以及未折叠的成熟部分来防止聚集。虽然与hsp70相关，但应防止前序列与新合成的前体的未折叠部分相互作用，并且不能诱导这些蛋白质在胞质溶胶中的聚集。（细菌hsp70）由另外两种热休克蛋白DnaJ和GrpE调节。我们在这里揭示了Ygelp，一种酵母GrpE同源物，是线粒体基质中的可溶性蛋白质。酵母细胞中Ygelp的缺失导致线粒体前体蛋白的积累，表明Ygelp参与蛋白质输入线粒体。这些结果和其他结果表明，Ygelp的功能与Ssclp，线粒体热休克蛋白70合作，并促进蛋白质输入到线粒体。

Most nuclear encoded mitochondrial precursor proteins are synthesized with amino-terminal presequences that are characterized by positive charge and amphiphilicity. The post-translational targeting of mitochondrial precursors to mitochondria is often facilitated by cytosolic chaperones including members of hsp70 stress protein family. We have studied the interactions of various synthetic peptides with unfolded proteins and yeast cytosolic hsp70, Ssalp. We found that mitochandrial presequences could induce aggregation of the unfolded proteins, suggesting that mitochondrial presequences can interact with the mature part of a precursor during the folding process and cause aggregation. We next found that mitochondrial presequences could bind to Ssalp and that the interactions of presequences with Ssalp are directly correlated with the amphiphilicity of the presequences. Taken together, the present results suggest that hsp70 can prevent aggregation by binding to the presequence as well as the unfolded mature part. While associated with hsp70, the presequence should be prevented from interacting with the newly synthesized, unfolded part of the precursor and would not be able to induce aggregation of these proteins in the cytosol.It is known that in E.coli, the function of DnaK (bacterial hsp70) is regulated by two other heat shock proteins, DnaJ and GrpE.We have revealed here that Ygelp, a yeast GrpE homolog, is a soluble protein in the mitochondrial matrix. Depletion of Ygelp in yeast cells resulted in accumulation of the mitochondrial precursor proteins, suggesting that Ygelp is involved in protein import into mitochondria. These results and others suggest that Ygelp functions in cooperation with Ssclp, a mitochondrial hsp70, and facilitates protein import into mitochondria.