Prefoldin Subunits Are Protected from Ubiquitin-Proteasome System-mediated Degradation by Forming Complex with Other Constituent Subunits
Prefoldin Subunits Are Protected from Ubiquitin-Proteasome System-mediated Degradation by Forming Complex with Other Constituent Subunits
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DOI：
10.1074/jbc.m110.216259
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发表时间：
2011-06-03
影响因子：
4.8
通讯作者：
Ariga, Hiroyoshi
中科院分区：
文献类型：
--
作者：
Miyazawa, Makoto;Tashiro, Erika;Ariga, Hiroyoshi
分子伴侣前折叠蛋白（PFD）是由六个不同的亚基组成的复合物，PFD 1-PFD 6，并将新合成的未折叠蛋白质递送到胞质伴侣TRiC/CCT以促进蛋白质的折叠。PFD亚基也具有与PFD复合物不同的功能。我们先前将MM-1 alpha/PFD 5鉴定为新型c-Myc结合蛋白，并发现MM-1 alpha抑制c-Myc的转化活性。然而，目前尚不清楚细胞如何调节单个亚基的蛋白质水平，以及什么机制改变了亚基及其复合物之间的活性比例。在这项研究中，我们发现敲除一个亚基会降低其他亚基的蛋白水平，而将MM-1 α以外的五个亚基转染到细胞中会增加内源性MM-1 α的水平。我们还发现，用蛋白酶体抑制剂MG 132处理细胞，增加了转染/过表达的MM-1 α的水平，但不增加内源性MM-1 α的水平，表明过表达的MM-1 α而不是内源性MM-1 α被泛素蛋白酶体系统（UPS）降解。使用其他PFD亚基的实验表明，UPS降解的PFD亚基的单体，但不同的亚基之间的降解程度。此外，一个亚基的水平在与相应的亚基共转染后增加，表明亚基之间存在待稳定的特定组合。这些结果表明，PFD亚基之间的蛋白质水平的相互调节，并显示如何单独的亚基形成的PFD复合物没有降解。
The molecular chaperone prefoldin (PFD) is a complex comprised of six different subunits, PFD1-PFD6, and delivers newly synthesized unfolded proteins to cytosolic chaperonin TRiC/CCT to facilitate the folding of proteins. PFD subunits also have functions different from the function of the PFD complex. We previously identified MM-1 alpha/PFD5 as a novel c-Myc-binding protein and found that MM-1 alpha suppresses transformation activity of c-Myc. However, it remains unclear how cells regulate protein levels of individual subunits and what mechanisms alter the ratio of their activities between subunits and their complex. In this study, we found that knockdown of one subunit decreased protein levels of other subunits and that transfection of five subunits other than MM-1 alpha into cells increased the level of endogenous MM-1 alpha. We also found that treatment of cells with MG132, a proteasome inhibitor, increased the level of transfected/overexpressed MM-1 alpha but not that of endogenous MM-1 alpha, indicating that overexpressed MM-1 alpha, but not endogenous MM-1 alpha, was degraded by the ubiquitin proteasome system (UPS). Experiments using other PFD subunits showed that the UPS degraded a monomer of PFD subunits, though extents of degradation varied among subunits. Furthermore, the level of one subunit was increased after cotransfection with the respective subunit, indicating that there are specific combinations between subunits to be stabilized. These results suggest mutual regulation of protein levels among PFD subunits and show how individual subunits form the PFD complex without degradation.