Ubiquitin Signaling
Dr. Koraljka Husnjak
Koraljka was born in Croatia, where she studied molecular biology at the Faculty of Science, University of Zagreb. She received her Master of Science and PhD training at the Division of Molecular Medicine at the Rudjer Boskovic Institute headed by Kresimir Pavelic. Koraljka initially worked on oncogenic viruses and molecular mechanisms of cancer development. After joining the group of Ivan Dikic as a postdoctoral fellow, she shifted her focus on identification and characterization of novel ubiquitin- and ubiquitin-like binding domains by various approaches, including the identification of Rpn13 as a novel proteasomal ubiquitin receptor. She started her independent career in 2010 as a group leader at IBC2. Her group is interested in various aspects of atypical ubiquitination. In particular, her laboratory aims to decipher the functional roles of various ubiquitin-binding proteins in protein degradation, signal transduction and immune response. Her most recent work relates to the role of linear ubiquitination and novel linear ubiquitin-binding domains in TNFR1 signaling and cell death. Since January 2019 Koraljka is a team leader of the FCSC.
We identified and characterized Rpn13 as a proteasomal ubiquitin receptor (Figure 1) and determined its cellular function and structure of its ubiquitin-binding domain (UBD) Pru on the collaborative basis (1, 2). For the last several years the laboratory is working on the knock-out and ubiquitin-binding deficient knock-in models of Rpn13. Additionally, we could show how E3 ligase Parkin, relevant for the development of Parkinson disease, gets recruited and activated at the 19S proteasome through the interaction with Rpn13. We demonstrated that turnover of Parkin and several of its substrates involved in mitophagy, are impaired in the absence of Rpn13 (3). On the collaborative basis, we recently showed how Sestrin 2 protein acts as a positive regulator of proteasomal function and activates transcription of Nrf2-regulated antioxidant genes (4). We could also show that binding of SGTA (cytosolic factor implicated in the quality control of mislocalized membrane proteins) to Rpn13 selectively modulates protein quality control (5).
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Figure 1. Complex structures of mouse Rpn13 (22-130) and ubiquitin (PDB entry 2z59) |
References
1. Husnjak K, Elsasser S, Zhang N, Chen X, Randles L, Shi Y, Hofmann K, Walters K, Finley D, Dikic I (2008) Proteasome subunit Rpn13 is a novel ubiquitin receptor. Nature 2008; 453 (7194): 481-488.
2. Schreiner P, Chen X, Husnjak K, Randles L, Zhang N, Elsasser S, Finley D, Dikic I, Walters K, Groll M. Ubiquitin docking at the proteasome via a novel PH domain interaction. Nature 2008; 453 (7194): 548-552.
3. Aguileta MA, Korac J, Durcan TM, Trempe JF, Haber M, Gehring K, Elsasser S, Waidmann O, Fon EA, Husnjak K. The E3 Ubiquitin ligase parkin is recruited to the 26S proteasome via the proteasomal ubiquitin receptor Rpn13. J Biol Chem 2015; 290 (12): 7492-7505.
4. Tomasovic A, Kurrle N, Sürün D, Heidler J, Husnjak K, Poser I, Schnütgen F, Scheibe S, Seimetz M, Jaksch P, Hyman A, Weissmann N, von Melchner H. Sestrin 2 protein regulates platelet-derived growth factor receptor β (Pdgfrβ) expression by modulating proteasomal and Nrf2 transcription factor functions. J Biol Chem 2015; 290(15): 9738-9752.
5. Leznicki P, Prlic JK, Kliza K, Husnjak K, Nyathi Y, Dikic I, High S. SGTA binding to Rpn13 selectively modulates protein quality control. J Cell Sci 2015; 128(17): 3187–3196.
The most recently discovered modification by ubiquitin is linear polyubiquitination, which (unlike lysine-specific ubiquitination) relies on the formation of the peptide bond between the N-terminal methionine of one ubiquitin and the terminal glycine of another ubiquitin moiety. Even though only a single E3 ligase complex (LUBAC), one highly specific deubiquitinase (OTULIN) and a small number of substrates are currently known for linear polyubiquitination, multiple studies confirmed the crucial role of linear polyubiquitination in innate and adaptive immune signaling. The number of identified linear polyubiquitin-modified targets is rather small due to technical and experimental limitations.
Our aim is to identify novel players in linear polyubiquitination, with emphasis on novel linear polyubiquitin-modified substrates. We therefore developed, validated and applied lysine-less internally tagged ubiquitin (INT-Ub.7KR) and combined it with mass spectrometry, in order to discover linear polyubiquitin-modified substrates, as well as novel processes where these proteins play a role (1).
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Figure 1. The design and in vitro validation of internally tagged ubiquitin (INT-Ub) (from Kliza et al, 2017) |
Reference
Polyubiquitin chains modifying target proteins are recognized by more than 20 families of ubiquitin receptors, i.e. proteins containing ubiquitin-binding domains (UBDs), which decode ubiquitinated target signals into biochemical cascades within the cell (1).
A very few linear polyubiquitin-specific UBDs have been described so far. In order to further expand the functional analysis of linear ubiquitin receptors that recognize and propagate linear ubiquitin signalling in the cells, my laboratory has performed yeast two-hybrid assays with tandem repeats of ubiquitin to identify novel LUBIDs and we are currently studying the physiological functions of identified LUBID- containing proteins.
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| Figure 1. Various ubiquitin receptors specifically recognize various ubiquitin linkages (from Husnjak and Dikic, 2012) |