|M.Sc Student||Krutauz Daria|
|Subject||Unraveling a Mechanism for the Effect of UBB+1 Molecule on|
Proper Function of Ubiquitin-Proteasome System
|Department||Department of Biology||Supervisor||Professor Michael Glickman|
|Full Thesis text|
Ubiquitin is a 76-amino-acid regulatory protein, so-named for its wide distribution throughout all eukaryotic cells. Post-translational ubiquitination of proteins regulates almost all cellular processes, including protein degradation, endocytosis, stress response, DNA repair, etc. A covalent bond between ubiquitin and substrate is formed via the carboxyl group of the terminal glycine in ubiquitin and the ε-amino group of a lysine residue within the substrate. The fundamental role of the substrate-linked polyUbiquitin chain is to serve as a recognition motif for a large multi-subunit protease known as the proteasome, which is responsible for hydrolyzing the substrate’s peptide bonds, thereby reducing a folded protein into oligopeptides. The proteasome is a large (approximately 2.5MDa) dynamic cellular multi-catalytic multi-protein subunit machinery with protease function.
The aberrant ubiquitin UBB+1 protein was found to accumulate in neurons and neuritic plaques of Azheimer’s disease and the ageing brain. UBB+1 arises from standard translation of a portion of mRNAs that have a dinucleotide deletion due to a process termed “molecular misreading”. It comprises a ubiquitin moiety and a 19 amino acid C-terminal extension. This extension cannot be efficiently processed by deUbiquitination enzymes due to the glycine to tyrosine exchange in position 76 of ubiquitin.
UBB+1 is an atypical proteasome substrate that impedes the ubiquitin-proteasome system. This fusion substrate is poorly degraded and therefore accumulates in cells preventing degradation of diverse substrates of ubiquitin-proteasome system.
We set out to probe this mode of inhibition. We show that in vitro, UBB+1 is a competitive substrate interfering with proteasomes, without direct impairment or inhibition of intrinsic proteolytic or deubiquitination activities. Moreover, we show that UBB+1 competes with binding to Rpn10 - one of the shuttle proteins for proteasomal degradation.
Rpn10 is a central player in recognition of ubiquitinated substrates and their coupling to proteasomes. We propose that by occupying the ubiquitin-recognition site of Rpn10, accumulated UBB+1 limits the levels Rpn10 available to associate with its natural substrates. In this manner Rpn10, as an upstream element of UPS, forms a bottleneck on the way to the proteasome. We conclude that further exploration of the UBB+1 - Rpn10 interaction will shed light on the involvement the UPS in AD pathogenesis.