UBE2A and UBE2B are recruited by an atypical E3 ligase module in UBR4.

UBR4 is a 574 kDa E3 ligase (E3) of the N-degron pathway with roles in neurodevelopment, age-associated muscular atrophy and cancer. The catalytic module that carries out ubiquitin (Ub) transfer remains unknown. Here we identify and characterize a distinct E3 module within human UBR4 consisting of a 'hemiRING' zinc finger, a helical-rich UBR zinc-finger interacting (UZI) subdomain, and an N-terminal region that can serve as an affinity factor for the E2 conjugating enzyme (E2). The structure of an E2-E3 complex provides atomic-level insight into the specificity determinants of the hemiRING toward the cognate E2s UBE2A/UBE2B. Via an allosteric mechanism, the UZI subdomain modestly activates the Ub-loaded E2 (E2∼Ub). We propose attenuated activation is complemented by the intrinsically high lysine reactivity of UBE2A, and their cooperation imparts a reactivity profile important for substrate specificity and optimal degradation kinetics. These findings reveal the mechanistic underpinnings of a neuronal N-degron E3, its specific recruitment of UBE2A, and highlight the underappreciated architectural diversity of cross-brace domains with Ub E3 activity.

Tools for the discovery of biopolymer producing cysteine relays.

Cysteine relays, where a protein or small molecule is transferred multiple times via transthiolation, are central to the production of biological polymers. Enzymes that utilise relay mechanisms display broad substrate specificity and are readily engineered to produce new polymers. In this review, I discuss recent advances in the discovery, engineering and biophysical characterisation of cysteine relays. I will focus on eukaryotic ubiquitin (Ub) cascades and prokaryotic polyhydroxyalkanoate (PHA) synthesis. These evolutionarily distinct processes employ similar chemistry and are readily modified for biotechnological applications. Both processes have been studied intensively for decades, yet recent studies suggest we do not fully understand their mechanistic diversity or plasticity. I will discuss the important role that activity-based probes (ABPs) and other chemical tools have had in identifying and delineating Ub cysteine-relays and the potential for ABPs to be applied to PHA synthases. Finally, I will offer a personal perspective on the potential of engineering cysteine-relays for non-native polymer production.

Deubiquitinating enzyme amino acid profiling reveals a class of ubiquitin esterases.

The reversibility of ubiquitination by the action of deubiquitinating enzymes (DUBs) serves as an important regulatory layer within the ubiquitin system. Approximately 100 DUBs are encoded by the human genome, and many have been implicated with pathologies, including neurodegeneration and cancer. Non-lysine ubiquitination is chemically distinct, and its physiological importance is emerging. Here, we couple chemically and chemoenzymatically synthesized ubiquitinated lysine and threonine model substrates to a mass spectrometry-based DUB assay. Using this platform, we profile two-thirds of known catalytically active DUBs for threonine esterase and lysine isopeptidase activity and find that most DUBs demonstrate dual selectivity. However, with two anomalous exceptions, the ovarian tumor domain DUB class demonstrates specific (iso)peptidase activity. Strikingly, we find the Machado-Joseph disease (MJD) class to be unappreciated non-lysine DUBs with highly specific ubiquitin esterase activity rivaling the efficiency of the most active isopeptidases. Esterase activity is dependent on the canonical catalytic triad, but proximal hydrophobic residues appear to be general determinants of non-lysine activity. Our findings also suggest that ubiquitin esters have appreciable cellular stability and that non-lysine ubiquitination is an integral component of the ubiquitin system. Its regulatory sophistication is likely to rival that of canonical ubiquitination.

Structural basis for RING-Cys-Relay E3 ligase activity and its role in axon integrity.

MYCBP2 is a ubiquitin (Ub) E3 ligase (E3) that is essential for neurodevelopment and regulates axon maintenance. MYCBP2 transfers Ub to nonlysine substrates via a newly discovered RING-Cys-Relay (RCR) mechanism, where Ub is relayed from an upstream cysteine to a downstream substrate esterification site. The molecular bases for E2-E3 Ub transfer and Ub relay are unknown. Whether these activities are linked to the neural phenotypes is also unclear. We describe the crystal structure of a covalently trapped E2~Ub:MYCBP2 transfer intermediate revealing key structural rearrangements upon E2-E3 Ub transfer and Ub relay. Our data suggest that transfer to the dynamic upstream cysteine, whilst mitigating lysine activity, requires a closed-like E2~Ub conjugate with tempered reactivity, and Ub relay is facilitated by a helix-coil transition. Furthermore, neurodevelopmental defects and delayed injury-induced degeneration in RCR-defective knock-in mice suggest its requirement, and that of substrate esterification activity, for normal neural development and programmed axon degeneration.

Overcoming insecticide resistance through computational inhibitor design.

Insecticides allow control of agricultural pests and disease vectors and are vital for global food security and health. The evolution of resistance to insecticides, such as organophosphates (OPs), is a serious and growing concern. OP resistance often involves sequestration or hydrolysis of OPs by carboxylesterases. Inhibiting carboxylesterases could, therefore, restore the effectiveness of OPs for which resistance has evolved. Here, we use covalent virtual screening to produce nano-/picomolar boronic acid inhibitors of the carboxylesterase αE7 from the agricultural pest Lucilia cuprina as well as a common Gly137Asp αE7 mutant that confers OP resistance. These inhibitors, with high selectivity against human acetylcholinesterase and low to no toxicity in human cells and in mice, act synergistically with the OPs diazinon and malathion to reduce the amount of OP required to kill L. cuprina by up to 16-fold and abolish resistance. The compounds exhibit broad utility in significantly potentiating another OP, chlorpyrifos, against the common pest, the peach-potato aphid (Myzus persicae). These compounds represent a solution to OP resistance as well as to environmental concerns regarding overuse of OPs, allowing significant reduction of use without compromising efficacy.

Denisovan, modern human and mouse TNFAIP3 alleles tune A20 phosphorylation and immunity.

Resisting and tolerating microbes are alternative strategies to survive infection, but little is known about the evolutionary mechanisms controlling this balance. Here genomic analyses of anatomically modern humans, extinct Denisovan hominins and mice revealed a TNFAIP3 allelic series with alterations in the encoded immune response inhibitor A20. Each TNFAIP3 allele encoded substitutions at non-catalytic residues of the ubiquitin protease OTU domain that diminished IκB kinase-dependent phosphorylation and activation of A20. Two TNFAIP3 alleles encoding A20 proteins with partial phosphorylation deficits seemed to be beneficial by increasing immunity without causing spontaneous inflammatory disease: A20 T108A;I207L, originating in Denisovans and introgressed in modern humans throughout Oceania, and A20 I325N, from an N-ethyl-N-nitrosourea (ENU)-mutagenized mouse strain. By contrast, a rare human TNFAIP3 allele encoding an A20 protein with 95% loss of phosphorylation, C243Y, caused spontaneous inflammatory disease in humans and mice. Analysis of the partial-phosphorylation A20 I325N allele in mice revealed diminished tolerance of bacterial lipopolysaccharide and poxvirus inoculation as tradeoffs for enhanced immunity.

The Structure and Stability of the Disulfide-Linked γS-Crystallin Dimer Provide Insight into Oxidation Products Associated with Lens Cataract Formation.

The reducing environment in the eye lens diminishes with age, leading to significant oxidative stress. Oxidation of lens crystallin proteins is the major contributor to their destabilization and deleterious aggregation that scatters visible light, obscures vision, and ultimately leads to cataract. However, the molecular basis for oxidation-induced aggregation is unknown. Using X-ray crystallography and small-angle X-ray scattering, we describe the structure of a disulfide-linked dimer of human γS-crystallin that was obtained via oxidation of C24. The γS-crystallin dimer is stable at glutathione concentrations comparable to those in aged and cataractous lenses. Moreover, dimerization of γS-crystallin significantly increases the protein's propensity to form large insoluble aggregates owing to non-cooperative domain unfolding, as is observed in crystallin variants associated with early-onset cataract. These findings provide insight into how oxidative modification of crystallins contributes to cataract and imply that early-onset and age-related forms of the disease share comparable development pathways.

The evolution of multiple active site configurations in a designed enzyme.

Developments in computational chemistry, bioinformatics, and laboratory evolution have facilitated the de novo design and catalytic optimization of enzymes. Besides creating useful catalysts, the generation and iterative improvement of designed enzymes can provide valuable insight into the interplay between the many phenomena that have been suggested to contribute to catalysis. In this work, we follow changes in conformational sampling, electrostatic preorganization, and quantum tunneling along the evolutionary trajectory of a designed Kemp eliminase. We observe that in the Kemp Eliminase KE07, instability of the designed active site leads to the emergence of two additional active site configurations. Evolutionary conformational selection then gradually stabilizes the most efficient configuration, leading to an improved enzyme. This work exemplifies the link between conformational plasticity and evolvability and demonstrates that residues remote from the active sites of enzymes play crucial roles in controlling and shaping the active site for efficient catalysis.

Activity-based E3 ligase profiling uncovers an E3 ligase with esterification activity.

Ubiquitination is initiated by transfer of ubiquitin (Ub) from a ubiquitin-activating enzyme (E1) to a ubiquitin-conjugating enzyme (E2), producing a covalently linked intermediate (E2-Ub) 1 . Ubiquitin ligases (E3s) of the 'really interesting new gene' (RING) class recruit E2-Ub via their RING domain and then mediate direct transfer of ubiquitin to substrates 2 . By contrast, 'homologous to E6-AP carboxy terminus' (HECT) E3 ligases undergo a catalytic cysteine-dependent transthiolation reaction with E2-Ub, forming a covalent E3-Ub intermediate3,4. Additionally, RING-between-RING (RBR) E3 ligases have a canonical RING domain that is linked to an ancillary domain. This ancillary domain contains a catalytic cysteine that enables a hybrid RING-HECT mechanism 5 . Ubiquitination is typically considered a post-translational modification of lysine residues, as there are no known human E3 ligases with non-lysine activity. Here we perform activity-based protein profiling of HECT or RBR-like E3 ligases and identify the neuron-associated E3 ligase MYCBP2 (also known as PHR1) as the apparent single member of a class of RING-linked E3 ligase with esterification activity and intrinsic selectivity for threonine over serine. MYCBP2 contains two essential catalytic cysteine residues that relay ubiquitin to its substrate via thioester intermediates. Crystallographic characterization of this class of E3 ligase, which we designate RING-Cys-relay (RCR), provides insights into its mechanism and threonine selectivity. These findings implicate non-lysine ubiquitination in cellular regulation of higher eukaryotes and suggest that E3 enzymes have an unappreciated mechanistic diversity.

Laboratory evolution of protein conformational dynamics.

This review focuses on recent work that has begun to establish specific functional roles for protein conformational dynamics, specifically how the conformational landscapes that proteins can sample can evolve under laboratory based evolutionary selection. We discuss recent technical advances in computational and biophysical chemistry, which have provided us with new ways to dissect evolutionary processes. Finally, we offer some perspectives on the emerging view of conformational dynamics and evolution, and the challenges that we face in rationally engineering conformational dynamics.

Structure of an Insecticide Sequestering Carboxylesterase from the Disease Vector Culex quinquefasciatus: What Makes an Enzyme a Good Insecticide Sponge?

Carboxylesterase (CBE)-mediated metabolic resistance to organophosphate and carbamate insecticides is a major problem for the control of insect disease vectors, such as the mosquito. The most common mechanism involves overexpression of CBEs that bind to the insecticide with high affinity, thereby sequestering them before they can interact with their target. However, the absence of any structure for an insecticide-sequestering CBE limits our understanding of the molecular basis for this process. We present the first structure of a CBE involved in sequestration, Cqestß21, from the mosquito disease vector Culex quinquefasciatus. Lysine methylation was used to obtain the crystal structure of Cqestß21, which adopts a canonical α/ß-hydrolase fold that has high similarity to the target of organophosphate and carbamate insecticides, acetylcholinesterase. Sequence similarity networks of the insect carboxyl/cholinesterase family demonstrate that CBEs associated with metabolic insecticide resistance across many species share a level of similarity that distinguishes them from a variety of other classes. This is further emphasized by the structural similarities and differences in the binding pocket and active site residues of Cqestß21 and other insect carboxyl/cholinesterases. Stopped-flow and steady-state inhibition studies support a major role for Cqestß21 in organophosphate resistance and a minor role in carbamate resistance. Comparison with another isoform associated with insecticide resistance, Cqestß1, showed both enzymes have similar affinity to insecticides, despite 16 amino acid differences between the two proteins. This provides a molecular understanding of pesticide sequestration by insect CBEs and could facilitate the design of CBE-specific inhibitors to circumvent this resistance mechanism in the future.

Sensing and signaling of oxidative stress in chloroplasts by inactivation of the SAL1 phosphoadenosine phosphatase.

Intracellular signaling during oxidative stress is complex, with organelle-to-nucleus retrograde communication pathways ill-defined or incomplete. Here we identify the 3'-phosphoadenosine 5'-phosphate (PAP) phosphatase SAL1 as a previously unidentified and conserved oxidative stress sensor in plant chloroplasts. Arabidopsis thaliana SAL1 (AtSAL1) senses changes in photosynthetic redox poise, hydrogen peroxide, and superoxide concentrations in chloroplasts via redox regulatory mechanisms. AtSAL1 phosphatase activity is suppressed by dimerization, intramolecular disulfide formation, and glutathionylation, allowing accumulation of its substrate, PAP, a chloroplast stress retrograde signal that regulates expression of plastid redox associated nuclear genes (PRANGs). This redox regulation of SAL1 for activation of chloroplast signaling is conserved in the plant kingdom, and the plant protein has evolved enhanced redox sensitivity compared with its yeast ortholog. Our results indicate that in addition to sulfur metabolism, SAL1 orthologs have evolved secondary functions in oxidative stress sensing in the plant kingdom.

Mapping the Accessible Conformational Landscape of an Insect Carboxylesterase Using Conformational Ensemble Analysis and Kinetic Crystallography.

The proper function of enzymes often depends upon their efficient interconversion between particular conformational sub-states on a free-energy landscape. Experimentally characterizing these sub-states is challenging, which has limited our understanding of the role of protein dynamics in many enzymes. Here, we have used a combination of kinetic crystallography and detailed analysis of crystallographic protein ensembles to map the accessible conformational landscape of an insect carboxylesterase (LcαE7) as it traverses all steps in its catalytic cycle. LcαE7 is of special interest because of its evolving role in organophosphate insecticide resistance. Our results reveal that a dynamically coupled network of residues extends from the substrate-binding site to a surface loop. Interestingly, the coupling of this network that is apparent in the apoenzyme appears to be reduced in the phosphorylated enzyme intermediate. Altogether, the results of this work highlight the importance of protein dynamics to enzyme function and the evolution of new activity.

Evolution of Protein Quaternary Structure in Response to Selective Pressure for Increased Thermostability.

Oligomerization has been suggested to be an important mechanism for increasing or maintaining the thermostability of proteins. Although it is evident that protein-protein contacts can result in substantial stabilization in many extant proteins, evidence for evolutionary selection for oligomerization is largely indirect and little is understood of the early steps in the evolution of oligomers. A laboratory-directed evolution experiment that selected for increased thermostability in the αE7 carboxylesterase from the Australian sheep blowfly, Lucilia cuprina, resulted in a thermostable variant, LcαE7-4a, that displayed increased levels of dimeric and tetrameric quaternary structure. A trade-off between activity and thermostability was made during the evolution of thermostability, with the higher-order oligomeric species displaying the greatest thermostability and lowest catalytic activity. Analysis of monomeric and dimeric LcαE7-4a crystal structures revealed that only one of the oligomerization-inducing mutations was located at a potential protein-protein interface. This work demonstrates that by imposing a selective pressure demanding greater thermostability, mutations can lead to increased oligomerization and stabilization, providing support for the hypothesis that oligomerization is a viable evolutionary strategy for protein stabilization.

Conformational Disorganization within the Active Site of a Recently Evolved Organophosphate Hydrolase Limits Its Catalytic Efficiency.

The evolution of new enzymatic activity is rarely observed outside of the laboratory. In the agricultural pest Lucilia cuprina, a naturally occurring mutation (Gly137Asp) in α-esterase 7 (LcαE7) results in acquisition of organophosphate hydrolase activity and confers resistance to organophosphate insecticides. Here, we present an X-ray crystal structure of LcαE7:Gly137Asp that, along with kinetic data, suggests that Asp137 acts as a general base in the new catalytic mechanism. Unexpectedly, the conformation of Asp137 observed in the crystal structure obstructs the active site and is not catalytically productive. Molecular dynamics simulations reveal that alternative, catalytically competent conformers of Asp137 are sampled on the nanosecond time scale, although these states are less populated. Thus, although the mutation introduces the new reactive group responsible for organophosphate detoxification, the catalytic efficiency appears to be limited by conformational disorganization: the frequent sampling of low-energy nonproductive states. This result is consistent with a model of molecular evolution in which initial function-changing mutations can result in enzymes that display only a fraction of their catalytic potential due to conformational disorganization.

Structure and function of the hydrophilic Photosystem II assembly proteins: Psb27, Psb28 and Ycf48.

Photosystem II (PS II) is a macromolecular complex responsible for light-driven oxidation of water and reduction of plastoquinone as part of the photosynthetic electron transport chain found in thylakoid membranes. Each PS II complex is composed of at least 20 protein subunits and over 80 cofactors. The biogenesis of PS II requires further hydrophilic and membrane-spanning proteins which are not part of the active holoenzyme. Many of these biogenesis proteins make transient interactions with specific PS II assembly intermediates: sometimes these are essential for biogenesis while in other examples they are required for optimizing assembly of the mature complex. In this review the function and structure of the Psb27, Psb28 and Ycf48 hydrophilic assembly factors is discussed by combining structural, biochemical and physiological information. Each of these assembly factors has homologues in all oxygenic photosynthetic organisms. We provide a simple overview for the roles of these protein factors in cyanobacterial PS II assembly emphasizing their participation in both photosystem biogenesis and recovery from photodamage.

Mutational analysis of the stability of Psb27 from Synechocystis sp. PCC 6803: implications for models of Psb27 structure and binding to CP43.

Psb27 associates with the CP43 subunit of photosystem II during biogenesis of the photosystem. Several models have been proposed for the interaction between Psb27 and CP43. The utility of predictions and hypotheses arising from these models depends on the accuracy of the Psb27 structure used in the model. Two of the Psb27 structures used to model the Psb27-CP43 interaction place residue E98 on the surface of Psb27 and D14 in a position to form hydrogen bonds that stabilise the fold of the protein; however, a third structure questions the surface exposure of E98 and does not identify significant interactions of D14. Here we present evidence that D14 contributes to the thermal stability of Psb27 and that E98 is located on the surface. A D14A mutation was shown to reduce the apparent midpoint of unfolding of Psb27 by 16 °C. Four highly conserved surface residues and E98 were subject to charge-reversal mutations (R54E, R94E, E98R, E103R, R108E). The stabilities of the charge-reversal variants and the unmodified control were similar, suggesting E98 is a surface residue. Placing E98 in the correct, surface position will support more reliable models of the interaction of Psb27 with CP43.

Solution structure of Psb27 from cyanobacterial photosystem II.

Psb27 is a highly conserved component of photosystem II. The three-dimensional structure has a well-defined helical core, composed of four helices arranged in a right-handed up-down-up-down fold, with a less ordered region of the structure located at the N-terminus. The position of conserved residues on the surface suggests conserved functional roles for distinct interconnected features encompassing a P-phi-P loop, a polar patch spanning helices alpha3 and alpha4, and the N-terminal sequence.