Gauss Robert, Sommer Thomas, Jarosch Ernst
Max-Delbrück Center for Molecular Medicine, Berlin, Germany.
EMBO J. 2006 May 3;25(9):1827-35. doi: 10.1038/sj.emboj.7601088. Epub 2006 Apr 13.
Misfolded proteins of the endoplasmic reticulum (ER) are targeted to the cytoplasm for proteasomal degradation. Key components of this process are ER membrane-bound ubiquitin ligases. These ligases associate with the cytoplasmic AAA-ATPase Cdc48p/p97, which is thought to support the release of malfolded proteins from the ER. Here, we characterize a yeast protein complex containing the ubiquitin ligase Hrd1p and the ER membrane proteins Hrd3p and Der1p. Hrd3p binds malfolded proteins in the ER lumen enabling their delivery to downstream components. Therefore, we propose that Hrd3p acts as a substrate recruitment factor for the Hrd1p ligase complex. Hrd3p function is also required for the association of Cdc48p with Hrd1p. Moreover, our data demonstrate that recruitment of Cdc48p depends on substrate processing by the Hrd1p ligase complex. Thus, the Hrd1p ligase complex unites substrate selection in the ER lumen and polyubiquitination in the cytoplasm and links these processes to the release of ER proteins via the Cdc48p complex.
内质网（ER）中错误折叠的蛋白质会被转运到细胞质中进行蛋白酶体降解。这一过程的关键组成部分是内质网膜结合的泛素连接酶。这些连接酶与细胞质中的AAA-ATP酶Cdc48p/p97相关联，Cdc48p/p97被认为有助于错误折叠的蛋白质从内质网中释放出来。在此，我们对一种酵母蛋白复合物进行了表征，该复合物包含泛素连接酶Hrd1p以及内质网蛋白Hrd3p和Der1p。Hrd3p在内质网腔中结合错误折叠的蛋白质，使其能够传递给下游组分。因此，我们提出Hrd3p作为Hrd1p连接酶复合物的底物招募因子发挥作用。Cdc48p与Hrd1p的结合也需要Hrd3p的功能。此外，我们的数据表明，Cdc48p的招募依赖于Hrd1p连接酶复合物对底物的处理。因此，Hrd1p连接酶复合物将内质网腔中的底物选择和细胞质中的多聚泛素化结合起来，并通过Cdc48p复合物将这些过程与内质网蛋白的释放联系起来。
