The UBA domain of conjugating enzyme Ubc1/Ube2K facilitates assembly of K48/K63-branched ubiquitin chains.

The assembly of a specific polymeric ubiquitin chain on a target protein is a key event in the regulation of numerous cellular processes. Yet, the mechanisms that govern the selective synthesis of particular polyubiquitin signals remain enigmatic. The homologous ubiquitin-conjugating (E2) enzymes Ubc1 (budding yeast) and Ube2K (mammals) exclusively generate polyubiquitin linked through lysine 48 (K48). Uniquely among E2 enzymes, Ubc1 and Ube2K harbor a ubiquitin-binding UBA domain with unknown function. We found that this UBA domain preferentially interacts with ubiquitin chains linked through lysine 63 (K63). Based on structural modeling, in vitro ubiquitination experiments, and NMR studies, we propose that the UBA domain aligns Ubc1 with K63-linked polyubiquitin and facilitates the selective assembly of K48/K63-branched ubiquitin conjugates. Genetic and proteomics experiments link the activity of the UBA domain, and hence the formation of this unusual ubiquitin chain topology, to the maintenance of cellular proteostasis.

The CUE Domain of Cue1 Aligns Growing Ubiquitin Chains with Ubc7 for Rapid Elongation.

Ubiquitin conjugation is an essential process modulating protein function in eukaryotic cells. Surprisingly, little is known about how the progressive assembly of ubiquitin chains is managed by the responsible enzymes. Only recently has ubiquitin binding activity emerged as an important factor in chain formation. The Ubc7 activator Cue1 carries a ubiquitin binding CUE domain that substantially stimulates K48-linked polyubiquitination mediated by Ubc7. Our results from NMR-based analysis and in vitro ubiquitination reactions point out that two parameters accelerate ubiquitin chain assembly: the increasing number of CUE binding sites and the position of CUE binding within a growing chain. In particular, interactions with a ubiquitin moiety adjacent to the acceptor ubiquitin facilitate chain elongation. These data indicate a mechanism for ubiquitin binding in which Cue1 positions Ubc7 and the distal acceptor ubiquitin for rapid polyubiquitination. Disrupting this mechanism results in dysfunction of the ERAD pathway by a delayed turnover of substrates.

Efficient determination of the accessible conformation space of multi-domain complexes based on EPR PELDOR data.

Many proteins can adopt multiple conformations which are important for their function. This is also true for proteins and domains that are covalently linked to each other. One important example is ubiquitin, which can form chains of different conformations depending on which of its lysine side chains is used to form an isopeptide bond with the C-terminus of another ubiquitin molecule. Similarly, ubiquitin gets covalently attached to active-site residues of E2 ubiquitin-conjugating enzymes. Due to weak interactions between ubiquitin and its interaction partners, these covalent complexes adopt multiple conformations. Understanding the function of these complexes requires the characterization of the entire accessible conformation space and its modulation by interaction partners. Long-range (1.8-10 nm) distance restraints obtained by EPR spectroscopy in the form of probability distributions are ideally suited for this task as not only the mean distance but also information about the conformation dynamics is encoded in the experimental data. Here we describe a computational method that we have developed based on well-established structure determination software using NMR restraints to calculate the accessible conformation space using PELDOR/DEER data.

Ubiquitination in the ERAD Process.

In this review, we focus on the ubiquitination process within the endoplasmic reticulum associated protein degradation (ERAD) pathway. Approximately one third of all synthesized proteins in a cell are channeled into the endoplasmic reticulum (ER) lumen or are incorporated into the ER membrane. Since all newly synthesized proteins enter the ER in an unfolded manner, folding must occur within the ER lumen or co-translationally, rendering misfolding events a serious threat. To prevent the accumulation of misfolded protein in the ER, proteins that fail the quality control undergo retrotranslocation into the cytosol where they proceed with ubiquitination and degradation. The wide variety of misfolded targets requires on the one hand a promiscuity of the ubiquitination process and on the other hand a fast and highly processive mechanism. We present the various ERAD components involved in the ubiquitination process including the different E2 conjugating enzymes, E3 ligases, and E4 factors. The resulting K48-linked and K11-linked ubiquitin chains do not only represent a signal for degradation by the proteasome but are also recognized by the AAA+ ATPase Cdc48 and get in the process of retrotranslocation modified by enzymes bound to Cdc48. Lastly we discuss the conformations adopted in particular by K48-linked ubiquitin chains and their importance for degradation.

Structural and functional analysis of the GABARAP interaction motif (GIM).

Through the canonical LC3 interaction motif (LIR), [W/F/Y]-X1-X2-[I/L/V], protein complexes are recruited to autophagosomes to perform their functions as either autophagy adaptors or receptors. How these adaptors/receptors selectively interact with either LC3 or GABARAP families remains unclear. Herein, we determine the range of selectivity of 30 known core LIR motifs towards individual LC3s and GABARAPs. From these, we define a G ABARAP I nteraction M otif (GIM) sequence ([W/F]-[V/I]-X2-V) that the adaptor protein PLEKHM1 tightly conforms to. Using biophysical and structural approaches, we show that the PLEKHM1-LIR is indeed 11-fold more specific for GABARAP than LC3B. Selective mutation of the X1 and X2 positions either completely abolished the interaction with all LC3 and GABARAPs or increased PLEKHM1-GIM selectivity 20-fold towards LC3B. Finally, we show that conversion of p62/SQSTM1, FUNDC1 and FIP200 LIRs into our newly defined GIM, by introducing two valine residues, enhances their interaction with endogenous GABARAP over LC3B. The identification of a GABARAP-specific interaction motif will aid the identification and characterization of the expanding array of autophagy receptor and adaptor proteins and their in vivo functions.

Site-specific inhibition of the small ubiquitin-like modifier (SUMO)-conjugating enzyme Ubc9 selectively impairs SUMO chain formation.

Posttranslational modifications by small ubiquitin-like modifiers (SUMOs) regulate many cellular processes, including genome integrity, gene expression, and ribosome biogenesis. The E2-conjugating enzyme Ubc9 catalyzes the conjugation of SUMOs to ϵ-amino groups of lysine residues in target proteins. Attachment of SUMO moieties to internal lysines in Ubc9 itself can further lead to the formation of polymeric SUMO chains. Mono- and poly-SUMOylations of target proteins provide docking sites for distinct adapter and effector proteins important for regulating discrete SUMO-regulated pathways. However, molecular tools to dissect pathways depending on either mono- or poly-SUMOylation are largely missing. Using a protein-engineering approach, we generated high-affinity SUMO2 variants by phage display that bind the back side binding site of Ubc9 and function as SUMO-based Ubc9 inhibitors (SUBINs). Importantly, we found that distinct SUBINs primarily inhibit poly-SUMO chain formation, whereas mono-SUMOylation was not impaired. Proof-of-principle experiments demonstrated that in a cellular context, SUBINs largely prevent heat shock-triggered poly-SUMOylation. Moreover, SUBINs abrogated arsenic-induced degradation of promyelocytic leukemia protein. We propose that the availability of the new chain-selective SUMO inhibitors reported here will enable a thorough investigation of poly-SUMO-mediated cellular processes, such as DNA damage responses and cell cycle progression.

Structural investigation of glycan recognition by the ERAD quality control lectin Yos9.

Yos9 is an essential component of the endoplasmic reticulum associated protein degradation (ERAD) system that is responsible for removing terminally misfolded proteins from the ER lumen and mediating proteasomal degradation in the cytosol. Glycoproteins that fail to attain their native conformation in the ER expose a distinct oligosaccharide structure, a terminal α1,6-linked mannose residue, that is specifically recognized by the mannose 6-phoshate receptor homology (MRH) domain of Yos9. We have determined the structure of the MRH domain of Yos9 in its free form and complexed with 3α, 6α-mannopentaose. We show that binding is achieved by loops between ß-strands performing an inward movement and that this movement also affects the entire ß-barrel leading to a twist. These rearrangements may facilitate the processing of client proteins by downstream acting factors. In contrast, other oligosaccharides such as 2α-mannobiose bind weakly with only locally occurring chemical shift changes underscoring the specificity of this substrate selection process within ERAD.

Structural basis for phosphorylation-triggered autophagic clearance of Salmonella.

Selective autophagy is mediated by the interaction of autophagy modifiers and autophagy receptors that also bind to ubiquitinated cargo. Optineurin is an autophagy receptor that plays a role in the clearance of cytosolic Salmonella. The interaction between receptors and modifiers is often relatively weak, with typical values for the dissociation constant in the low micromolar range. The interaction of optineurin with autophagy modifiers is even weaker, but can be significantly enhanced through phosphorylation by the TBK1 {TANK [TRAF (tumour-necrosis-factor-receptor-associated factor)-associated nuclear factor κB activator]-binding kinase 1}. In the present study we describe the NMR and crystal structures of the autophagy modifier LC3B (microtubule-associated protein light chain 3 beta) in complex with the LC3 interaction region of optineurin either phosphorylated or bearing phospho-mimicking mutations. The structures show that the negative charge induced by phosphorylation is recognized by the side chains of Arg¹¹ and Lys5¹ in LC3B. Further mutational analysis suggests that the replacement of the canonical tryptophan residue side chain of autophagy receptors with the smaller phenylalanine side chain in optineurin significantly weakens its interaction with the autophagy modifier LC3B. Through phosphorylation of serine residues directly N-terminally located to the phenylalanine residue, the affinity is increased to the level normally seen for receptor-modifier interactions. Phosphorylation, therefore, acts as a switch for optineurin-based selective autophagy.

Chain Assembly and Disassembly Processes Differently Affect the Conformational Space of Ubiquitin Chains.

Ubiquitination is the most versatile posttranslational modification. The information is encoded by linkage type as well as chain length, which are translated by ubiquitin binding domains into specific signaling events. Chain topology determines the conformational space of a ubiquitin chain and adds an additional regulatory layer to this ubiquitin code. In particular, processes that modify chain length will be affected by chain conformations as they require access to the elongation or cleavage sites. We investigated conformational distributions in the context of chain elongation and disassembly using pulsed electron-electron double resonance spectroscopy in combination with molecular modeling. Analysis of the conformational space of diubiquitin revealed conformational selection or remodeling as mechanisms for chain recognition during elongation or hydrolysis, respectively. Chain elongation to tetraubiquitin increases the sampled conformational space, suggesting that a high intrinsic flexibility of K48-linked chains may contribute to efficient proteasomal degradation.