Sung Min-Kyung, Reitsma Justin M, Sweredoski Michael J, Hess Sonja, Deshaies Raymond J
Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA 91125.
Proteome Exploration Laboratory, Division of Biology and Biological Engineering, Beckman Institute, California Institute of Technology, Pasadena, CA 91125.
Mol Biol Cell. 2016 Sep 1;27(17):2642-52. doi: 10.1091/mbc.E16-05-0290. Epub 2016 Jul 6.
Ribosome assembly is an essential process that consumes prodigious quantities of cellular resources. Ribosomal proteins cannot be overproduced in Saccharomyces cerevisiae because the excess proteins are rapidly degraded. However, the responsible quality control (QC) mechanisms remain poorly characterized. Here we demonstrate that overexpression of multiple proteins of the small and large yeast ribosomal subunits is suppressed. Rpl26 overexpressed from a plasmid can be detected in the nucleolus and nucleoplasm, but it largely fails to assemble into ribosomes and is rapidly degraded. However, if the endogenous RPL26 loci are deleted, plasmid-encoded Rpl26 assembles into ribosomes and localizes to the cytosol. Chemical and genetic perturbation studies indicate that overexpressed ribosomal proteins are degraded by the ubiquitin-proteasome system and not by autophagy. Inhibition of the proteasome led to accumulation of multiple endogenous ribosomal proteins in insoluble aggregates, consistent with the operation of this QC mechanism in the absence of ribosomal protein overexpression. Our studies reveal that ribosomal proteins that fail to assemble into ribosomes are rapidly distinguished from their assembled counterparts and ubiquitinated and degraded within the nuclear compartment.
核糖体组装是一个消耗大量细胞资源的基本过程。在酿酒酵母中，核糖体蛋白不能过量产生，因为过量的蛋白会迅速降解。然而，负责的质量控制（QC）机制仍未得到充分表征。在这里，我们证明了酵母核糖体小亚基和大亚基的多种蛋白的过表达受到抑制。从质粒上过表达的Rpl26可以在核仁和核质中被检测到，但它基本上无法组装到核糖体中并迅速降解。然而，如果内源性RPL26基因座被删除，质粒编码的Rpl26会组装到核糖体中并定位于细胞质。化学和遗传扰动研究表明，过表达的核糖体蛋白是通过泛素-蛋白酶体系统降解的，而不是通过自噬。蛋白酶体的抑制导致多种内源性核糖体蛋白在不溶性聚集体中积累，这与在没有核糖体蛋白过表达的情况下这种质量控制机制的运作一致。我们的研究表明，未能组装到核糖体中的核糖体蛋白会迅速与其已组装的对应物区分开来，并在核区内被泛素化和降解。
