Enhanced protein degradation by branched ubiquitin chains

Posttranslational modification of cell cycle regulators with ubiquitin chains is essential for eukaryotic cell division. Such chains can be connected through seven lysine residues or the amino-terminus of ubiquitin, thereby allowing the assembly of eight homogenous and multiple mixed or branched conjugates. While functions of homogenous chain types have been described, physiological roles of branched structures are unknown. The anaphase-promoting complex (APC/C) catalyzes degradation of key regulators during the cell cycle causing proper transition through the cell cycle. Here, I report that the APC/C efficiently synthesizes substrate-attached branched conjugates and enhances recognition of ubiquitylated substrates by the proteasome, thereby driving the degradation of cell cycle regulators during early mitosis. I showed several APC/C-substrates require the ubiquitin-conjugating enzyme E2 S (Ube2S) for degradation, e.g. the Never in mitosis A-related kinase 2 (Nek2A). The reconstitution of Nek2A ubiquitylation revealed that Ube2S does not simply extend a conjugate, but instead branches multiple lysine11-linked chains off the ubiquitin chains produced by the ubiquitin-conjugating enzyme E2 C (Ube2C). Therefore, I identified an enzyme and substrates for modification with branched ubiquitin chains, which points to an important role of these conjugates in providing an improved signal for proteasomal degradation. Compared to homogenous chains, branched conjugates synthesized by the APC/C increase the efficiency of proteasomal substrate recognition, and accordingly, are required for the degradation of cell cycle regulators at times of limited APC/C activity such as prometaphase. Hence, the APC/C is an enzyme that synthesizes branched ubiquitin chains, which provide an improved signal for proteasomal degradation.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2014

Author dc:creator

• Meyer, Hermann-Josef

Contributors dc:contributor

• Stemmann, Olaf

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/1736/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:1736