AbstractsBiology & Animal Science

Functional aspects of the ubiquitin-proteasome system

by Michael Seeger

Institution: Freie Universität Berlin
Year: 1014
Record ID: 1117312
Full text PDF: http://edocs.fu-berlin.de/diss/receive/FUDISS_thesis_000000099216


Synthesis and maturation of endoplasmic reticulum (ER) proteins is tightly regulated by an extensive quality control system. Perturbation of ER homeostasis may lead to an accumulation of aberrant proteins that trigger a number of signaling pathways known as unfolded protein response (UPR). The UPR results in a transient inhibition of general translation followed by an enhanced expression of genes that encode molecular chaperones and factors involved in ER-associated protein degradation (ERAD). One of the genes that are induced by the UPR encodes a protein called Herp. Herp resides at the ER membrane and associates with the ubiquitin-protein ligase Hrd1, which is a central component of membrane complexes required for ERAD. In the absence of Herp, Hrd1-dependent ubiquitylation and degradation of specific ER proteins is compromised, while the positive effect of Herp on ERAD requires its N-terminal ubiquitin-like (UBL) domain. Hence, it is suggested that Herp acts as a positive regulator of Hrd1-mediated protein degradation, counteracting the accumulation of aberrant proteins in the ER. Hrd1-mediated ubiquitylation enables extraction of proteins from the ER to the cytosol by p97, as well as their proteasome dependent degradation. Specificity of the p97 ATPase complex towards certain cellular processes is ensured by a number of cofactors. One of these cofactors is UBXD6/Rep8, a transmembrane protein that binds p97 as well as Hrd1. Inhibition of UBXD6 expression leads to a reduced amount of ER membrane-associated p97, accompanied by an impairment of ERAD. It is therefore proposed that UBXD6 recruits p97 to Hrd1 based ERAD complexes, enabling efficient extraction of ubiquitylated ER proteins to the cytosol and their degradation by the 26S proteasome. The 26S proteasome comprises the barrel shaped 20S proteasome that is connected to one or two 19S regulator complexes. The 19S regulator complex is responsible for substrate protein binding, deubiquitylation, as well as ATP dependent unfolding and translocation to the catalytic sites in the lumen of the 20S proteasome. Recognition and binding of multi-ubiquitylated proteins by the 26S proteasome has been attributed to the ubiquitin interacting motif (UIM) of the 19S regulator subunit Rpn10/Pus1. Experiments in fission yeast revealed that ubiquitin-associated (UBA) domains also display a binding preference for multi-ubiquitin chains. The proteins Rhp23 and Dph1 contain such UBA domains as well as a UBL domain, which, in contrast to the UBL domain of Herp, is able to bind the proteasome. While phenotypes of fission yeast cells carrying single deletions of Pus1, Rhp23 or Dph1 are similar to wild type cells, double deletions of Rhp23 and either Pus1 or Dph1 lead to a stabilization of proteasome substrates, accumulation of ubiquitylated proteins and severe growth defects. The data therefore suggest that Rhp23 and Dph1 represent a group of proteins that act as adapters to recruit multi-ubiquitylated substrate proteins to the proteasome. Die Synthese und Reifung von Proteinen…