Structural insight into the ZFAND1-p97 interaction involved in stress granule clearance.

Arsenite-induced stress granule (SG) formation can be cleared by the ubiquitin-proteasome system aided by the ATP-dependent unfoldase p97. ZFAND1 participates in this pathway by recruiting p97 to trigger SG clearance. ZFAND1 contains two An1-type zinc finger domains (ZF1 and ZF2), followed by a ubiquitin-like domain (UBL); but their structures are not experimentally determined. To shed light on the structural basis of the ZFAND1-p97 interaction, we determined the atomic structures of the individual domains of ZFAND1 by solution-state NMR spectroscopy and X-ray crystallography. We further characterized the interaction between ZFAND1 and p97 by methyl NMR spectroscopy and cryo-EM. 15N spin relaxation dynamics analysis indicated independent domain motions for ZF1, ZF2, and UBL. The crystal structure and NMR structure of UBL showed a conserved ß-grasp fold homologous to ubiquitin and other UBLs. Nevertheless, the UBL of ZFAND1 contains an additional N-terminal helix that adopts different conformations in the crystalline and solution states. ZFAND1 uses the C-terminal UBL to bind to p97, evidenced by the pronounced line-broadening of the UBL domain during the p97 titration monitored by methyl NMR spectroscopy. ZFAND1 binding induces pronounced conformational heterogeneity in the N-terminal domain of p97, leading to a partial loss of the cryo-EM density of the N-terminal domain of p97. In conclusion, this work paved the way for a better understanding of the interplay between p97 and ZFAND1 in the context of SG clearance.

Rapid simulation of glycoprotein structures by grafting and steric exclusion of glycan conformer libraries.

Most membrane proteins are modified by covalent addition of complex sugars through N- and O-glycosylation. Unlike proteins, glycans do not typically adopt specific secondary structures and remain very mobile, shielding potentially large fractions of protein surface. High glycan conformational freedom hinders complete structural elucidation of glycoproteins. Computer simulations may be used to model glycosylated proteins but require hundreds of thousands of computing hours on supercomputers, thus limiting routine use. Here, we describe GlycoSHIELD, a reductionist method that can be implemented on personal computers to graft realistic ensembles of glycan conformers onto static protein structures in minutes. Using molecular dynamics simulation, small-angle X-ray scattering, cryoelectron microscopy, and mass spectrometry, we show that this open-access toolkit provides enhanced models of glycoprotein structures. Focusing on N-cadherin, human coronavirus spike proteins, and gamma-aminobutyric acid receptors, we show that GlycoSHIELD can shed light on the impact of glycans on the conformation and activity of complex glycoproteins.

Tying a true topological protein knot by cyclization.

Knotted proteins are fascinating to biophysicists because of their robust ability to fold into intricately defined three-dimensional structures with complex and topologically knotted arrangements. Exploring the biophysical properties of the knotted proteins is of significant interest, as they could offer enhanced chemical, thermal, and mechanostabilities. A true mathematical knot requires a closed path; in contrast, knotted protein structures have open N- and C-termini. To address the question of how a truly knotted protein differs from the naturally occurring counterpart, we enzymatically cyclized a 31 knotted YibK protein from Haemophilus influenza (HiYibK) to investigate the impact of path closure on its structure-function relationship and folding stability. Through the use of a multitude of structural and biophysical tools, including X-ray crystallography, NMR spectroscopy, small angle X-ray scattering, differential scanning calorimetry, and isothermal calorimetry, we showed that the path closure minimally perturbs the native structure and ligand binding of HiYibK. Nevertheless, the cyclization did alter the folding stability and mechanism according to chemical and thermal unfolding analysis. These molecular insights contribute to our fundamental understanding of protein folding and knotting that could have implications in the protein design with higher stabilities.

Altered Protein Dynamics and a More Reactive Catalytic Cysteine in a Neurodegeneration-associated UCHL1 Mutant.

A mutant of ubiquitin C-terminal hydrolase L1 (UCHL1) detected in early-onset neurodegenerative patients, UCHL1R178Q, showed higher catalytic activity than wild-type UCHL1 (UCHL1WT). Lying within the active-site pocket, the arginine is part of an interaction network that holds the catalytic histidine in an inactive arrangement. However, the structural basis and mechanism of enzymatic activation upon glutamine substitution was not understood. We combined X-ray crystallography, protein nuclear magnetic resonance (NMR) analysis, enzyme kinetics, covalent inhibition analysis, and biophysical measurements to delineate activating factors in the mutant. While the crystal structure of UCHL1R178Q showed nearly the same arrangement of the catalytic residues and active-site pocket, the mutation caused extensive alteration in the chemical environment and dynamics of more than 30 residues, some as far as 15 Å away from the site of mutation. Significant broadening of backbone amide resonances in the HSQC spectra indicates considerable backbone dynamics changes in several residues, in agreement with solution small-angle X-ray scattering (SAXS) analyses which indicate an overall increase in protein flexibility. Enzyme kinetics show the activation is due to a kcat effect despite a slightly weakened substrate affinity. In line with this, the mutant shows a higher second-order rate constant (kinact/Ki) in a reaction with a substrate-derived irreversible inhibitor, Ub-VME, compared to the wild-type enzyme, an observation indicative of a more reactive catalytic cysteine in the mutant. Together, the observations underscore structural plasticity as a factor contributing to enzyme kinetic behavior which can be modulated through mutational effects.

Structure, dynamics, and stability of the smallest and most complex 71 protein knot.

Proteins can spontaneously tie a variety of intricate topological knots through twisting and threading of the polypeptide chains. Recently developed artificial intelligence algorithms have predicted several new classes of topological knotted proteins, but the predictions remain to be authenticated experimentally. Here, we showed by X-ray crystallography and solution-state NMR spectroscopy that Q9PR55, an 89-residue protein from Ureaplasma urealyticum, possesses a novel 71 knotted topology that is accurately predicted by AlphaFold 2, except for the flexible N terminus. Q9PR55 is monomeric in solution, making it the smallest and most complex knotted protein known to date. In addition to its exceptional chemical stability against urea-induced unfolding, Q9PR55 is remarkably robust to resist the mechanical unfolding-coupled proteolysis by a bacterial proteasome, ClpXP. Our results suggest that the mechanical resistance against pulling-induced unfolding is determined by the complexity of the knotted topology rather than the size of the molecule.

Structural insights into the regulation, ligand recognition, and oligomerization of bacterial STING.

The cyclic GMP-AMP synthase (cGAS)/stimulator of interferon gene (STING) signaling pathway plays a critical protective role against viral infections. Metazoan STING undergoes multilayers of regulation to ensure specific signal transduction. However, the mechanisms underlying the regulation of bacterial STING remain unclear. In this study, we determined the crystal structure of anti-parallel dimeric form of bacterial STING, which keeps itself in an inactive state by preventing cyclic dinucleotides access. Conformational transition between inactive and active states of bacterial STINGs provides an on-off switch for downstream signaling. Some bacterial STINGs living in extreme environment contain an insertion sequence, which we show codes for an additional long lid that covers the ligand-binding pocket. This lid helps regulate anti-phage activities. Furthermore, bacterial STING can bind cyclic di-AMP in a triangle-shaped conformation via a more compact ligand-binding pocket, forming spiral-shaped protofibrils and higher-order fibril filaments. Based on the differences between cyclic-dinucleotide recognition, oligomerization, and downstream activation of different bacterial STINGs, we proposed a model to explain structure-function evolution of bacterial STINGs.

Folding and functions of knotted proteins.

Topologically knotted proteins have entangled structural elements within their native structures that cannot be disentangled simply by pulling from the N- and C-termini. Systematic surveys have identified different types of knotted protein structures, constituting as much as 1% of the total entries within the Protein Data Bank. Many knotted proteins rely on their knotted structural elements to carry out evolutionarily conserved biological functions. Being knotted may also provide mechanical stability to withstand unfolding-coupled proteolysis. Reconfiguring a knotted protein topology by circular permutation or cyclization provides insights into the importance of being knotted in the context of folding and functions. With the explosion of predicted protein structures by artificial intelligence, we are now entering a new era of exploring the entangled protein universe.

High Density of N- and O-Glycosylation Shields and Defines the Structural Dynamics of the Intrinsically Disordered Ectodomain of Receptor-type Protein Tyrosine Phosphatase Alpha.

The intracellular phosphatase domain of the receptor-type protein tyrosine phosphatase alpha (PTPRA) is known to regulate various signaling pathways related to cell adhesion through c-Src kinase activation. In contrast, the functional significance of its relatively short, intrinsically disordered, and heavily glycosylated ectodomain remains unclear. Through detailed mass spectrometry analyses of a combination of protease and glycosidase digests, we now provide the first experimental evidence for its site-specific glycosylation pattern. This includes the occurrence of O-glycan at the N-glycosylation sequon among the more than 30 O-glycosylation sites confidently identified beside the 7 N-glycosylation sites. The closely spaced N- and O-glycans appear to have mutually limited the extent of further galactosylation and sialylation. An immature smaller form of full-length PTPRA was found to be deficient in O-glycosylation, most likely due to failure to transit the Golgi. N-glycosylation, on the other hand, is dispensable for cell surface expression and contributes less than the extensive O-glycosylation to the overall solution structure of the ectodomain. The glycosylation information is combined with the overall structural features of the ectodomain derived from small-angle X-ray scattering and high-speed atomic force microscopy monitoring to establish a dynamic structural model of the densely glycosylated PTPRA ectodomain. The observed high structural flexibility, as manifested by continuous transitioning from fully to partially extended and fold-back conformations, suggests that the receptor-type phosphatase is anchored to the membrane and kept mostly at a monomeric state through an ectodomain shaped and fully shielded by glycosylation.

An N-glycopeptide MS/MS data analysis workflow leveraging two complementary glycoproteomic software tools for more confident identification and assignments.

Complete coverage of all N-glycosylation sites on the SARS-CoV2 spike protein would require the use of multiple proteases in addition to trypsin. Subsequent identification of the resulting glycopeptides by searching against database often introduces assignment errors due to similar mass differences between different permutations of amino acids and glycosyl residues. By manually interpreting the individual MS2 spectra, we report here the common sources of errors in assignment, especially those introduced by the use of chymotrypsin. We show that by applying a stringent threshold of acceptance, erroneous assignment by the commonly used Byonic software can be controlled within 15%, which can be reduced further if only those also confidently identified by a different search engine, pGlyco3, were considered. A representative site-specific N-glycosylation pattern could be constructed based on quantifying only the overlapping subset of N-glycopeptides identified at higher confidence. Applying the two complimentary glycoproteomic software in a concerted data analysis workflow, we found and confirmed that glycosylation at several sites of an unstable Omicron spike protein differed significantly from those of the stable trimeric product of the parental D614G variant.

Elucidation of the folding pathway of a circular permutant of topologically knotted YbeA by tryptophan substitutions.

CP74 is an engineered circular permutant of a deep trefoil knotted SpoU-TrmD (SPOUT) RNA methyl transferase protein YbeA from E. coli. We have previously established that the circular permutation unties the knotted topology of YbeA and CP74 forms a domain-swapped dimer with a large dimeric interface of ca. 4600 Å2. To understand the impact of domain-swapping and the newly formed hinge region joining the two folded domains on the folding and stability of CP74, the five equally spaced tryptophan residues were individually substituted into phenylalanine to monitor their conformational and stability changes by a battery of biophysical tools. Far-UV circular dichroism, intrinsic fluorescence, and small-angle X-ray scattering dictated minimal global conformational perturbations to the native structures in the tryptophan variants. The structures of the tryptophan variants also showed the conservation of the domain-swapped ternary structure with the exception that the W72F exhibited significant asymmetry in the α-helix 5. Comparative global thermal and chemical stability analyses indicated the pivotal role of W100 in the folding of CP74 followed by W19 and W72. Solution-state NMR spectroscopy and hydrogen-deuterium exchange mass spectrometry further revealed the accumulation of a native-like intermediate state in which the hinge region made important contributions to maintain the domain-swapped ternary structure of CP74.

Application of hybrid biophysical-biochemical methods to unravel the molecular basis for auto-inhibition and activation of protein tyrosine phosphatase TCPTP/PTPN2.

Since the discovery of protein tyrosine phosphorylation as one of the critical post-translational modifications, it has been well known that the activity of protein tyrosine kinases (PTKs) is tightly regulated. On the other hand, protein tyrosine phosphatases (PTPs) are often regarded to act constitutively active, but recently we and others have shown that many PTPs are expressed in an inactive form due to allosteric inhibition by their unique structural features. Furthermore, their cellular activity is highly regulated in a spatiotemporal manner. In general, PTPs share a conserved catalytic domain comprising about 280 residues that is flanked by either an N-terminal or a C-terminal non-catalytic segment, which differs significantly in size and structure from each other and is known to regulate specific PTP's catalytic activity. The well-characterized non-catalytic segments can be globular or intrinsically disordered. In this work, we have focused on the T-Cell Protein Tyrosine Phosphatase (TCPTP/PTPN2) and demonstrated how the hybrid biophysical-biochemical methods can be applied to unravel the underlying mechanism through which TCPTP's catalytic activity is regulated by the non-catalytic C-terminal segment. Our analysis showed that TCPTP is auto-inhibited by its intrinsically disordered tail and trans-activated by Integrin alpha-1's cytosolic region.

Elucidation of folding pathways of knotted proteins.

Understanding the mechanisms by which proteins fold and thread into topologically knotted conformations has been challenging because of the apparent complexity associated with the folding and threading events. Nevertheless, many experimental and computational studies have provided insights into the folding pathways of knotted proteins and showed that most of the knotted proteins could spontaneously and reversibly fold into knotted topologies with highly populated intermediates and, at times, through multiple folding pathways. Our laboratory has reported the folding mechanisms of a variety of knotted proteins that have different knot types, ranging from the simplest trefoil 31 knot to the most complex Stevedore's 61 knot. Therefore, we focused on using multiplex thermodynamics and kinetics measurements to tease out unique information associated with different structural probes to obtain a more comprehensive overview of the folding mechanisms of the knotted proteins of interest. In this chapter, we shall discuss the use of different biophysical tools and analytical models to glean mechanistic insights into how intricate polypeptides attain knotted topologies.

Biophysical and structural characterizations of the effects of mutations on the structure-activity relationships of SARS-CoV-2 spike protein.

Mutations on the spike (S) protein of SARS-CoV-2 could induce structural changes that help increase viral transmissibility and enhance resistance to antibody neutralization. Here, we report a robust workflow to prepare recombinant S protein variants and its host receptor angiotensin-convert enzyme 2 (ACE2) by using a mammalian cell expression system. The functional states of the S protein variants are investigated by cryo-electron microscopy (cryo-EM) and negative staining electron microscopy (NSEM) to visualize their molecular structures in response to mutations, receptor binding, antibody binding, and environmental changes. The folding stabilities of the S protein variants can be deduced from morphological changes based on NSEM imaging analysis. Differential scanning calorimetry provides thermodynamic information to complement NSEM. Impacts of the mutations on host receptor binding and antibody neutralization are in vitro by kinetic binding analyses in addition to atomic insights gleaned from cryo-electron microscopy (cryo-EM). This experimental strategy is generally applicable to studying the molecular basis of host-pathogen interactions.

In situ structure and dynamics of an alphacoronavirus spike protein by cryo-ET and cryo-EM.

Porcine epidemic diarrhea (PED) is a highly contagious swine disease caused by porcine epidemic diarrhea virus (PEDV). PED causes enteric disorders with an exceptionally high fatality in neonates, bringing substantial economic losses in the pork industry. The trimeric spike (S) glycoprotein of PEDV is responsible for virus-host recognition, membrane fusion, and is the main target for vaccine development and antigenic analysis. The atomic structures of the recombinant PEDV S proteins of two different strains have been reported, but they reveal distinct N-terminal domain 0 (D0) architectures that may correspond to different functional states. The existence of the D0 is a unique feature of alphacoronavirus. Here we combined cryo-electron tomography (cryo-ET) and cryo-electron microscopy (cryo-EM) to demonstrate in situ the asynchronous S protein D0 motions on intact viral particles of a highly virulent PEDV Pintung 52 strain. We further determined the cryo-EM structure of the recombinant S protein derived from a porcine cell line, which revealed additional domain motions likely associated with receptor binding. By integrating mass spectrometry and cryo-EM, we delineated the complex compositions and spatial distribution of the PEDV S protein N-glycans, and demonstrated the functional role of a key N-glycan in modulating the D0 conformation.

Tumor suppressor BAP1 nuclear import is governed by transportin-1.

Subcellular localization of the deubiquitinating enzyme BAP1 is deterministic for its tumor suppressor activity. While the monoubiquitination of BAP1 by an atypical E2/E3-conjugated enzyme UBE2O and BAP1 auto-deubiquitination are known to regulate its nuclear localization, the molecular mechanism by which BAP1 is imported into the nucleus has remained elusive. Here, we demonstrated that transportin-1 (TNPO1, also known as Karyopherin ß2 or Kapß2) targets an atypical C-terminal proline-tyrosine nuclear localization signal (PY-NLS) motif of BAP1 and serves as the primary nuclear transporter of BAP1 to achieve its nuclear import. TNPO1 binding dissociates dimeric BAP1 and sequesters the monoubiquitination sites flanking the PY-NLS of BAP1 to counteract the function of UBE2O that retains BAP1 in the cytosol. Our findings shed light on how TNPO1 regulates the nuclear import, self-association, and monoubiquitination of BAP1 pertinent to oncogenesis.

Impacts of Cancer-associated Mutations on the Structure-Activity Relationship of BAP1.

BRAC1 associated protein-1 (BAP1) is a major tumor suppressor involved in many cancers. The deubiquitinase (DUB) activity of BAP1 is essential for its nuclear localization, histone remodeling and proteostasis associated with mitochondrial calcium flux. Loss of the DUB activity due to catalytic mutations within the ubiquitin C-terminal hydrolase (UCH) domain of BAP1 (BAP1-UCH) directly contributes to oncogenesis. Nevertheless, it is non-trivial to rationalize how the other high-frequency but non-catalytic mutations within the BAP1-UCH lead to malignancies. Here we used multiplex spectroscopic, thermodynamic and biophysical analyses to investigate the impacts of eleven high-occurrence mutations within BAP1-UCH on the structure, folding and function. Several mutations significantly destabilize BAP1-UCH and increase its aggregation propensity. Hydrogen-deuterium exchange mass spectrometry data revealed allosteric destabilizations caused by mutations distant from the catalytic site. Our findings gave a comprehensive and multiscale account of the molecular basis of how these non-catalytic mutations within BAP1-UCH may be implicated in oncogenesis.

BARD1 is an ATPase activating protein for OLA1.

OLA1 is a P-loop ATPase, implicated in centrosome duplication through the interactions with tumor suppressors BRCA1 and BARD1. Disruption of the interaction of OLA1 with BARD1 results in centrosome amplification. However, the molecular interplay and mechanism of the OLA1-BARD1 complex remain elusive. Here, we use a battery of biophysical, biochemical, and structural analyses to elucidate the molecular basis of the OLA1-BARD1 interaction. Our structural and enzyme kinetics analyses show this nucleotide-dependent interaction enhances the ATPase activity of OLA1 by increasing the turnover number (kcat). Unlike canonical GTPase activating proteins that act directly on the catalytic G domain, the BARD1 BRCT domain binds to the OLA1 TGS domain via a highly conserved BUDR motif. A cancer related mutation V695L on BARD1 is known to associate with centrosome abnormality. The V695L mutation reduces the BARD1 BRCT-mediated activation of OLA1. Crystallographic snapshot of the BRCT V695L mutant at 1.88 Å reveals this mutation perturbs the OLA1 binding site, resulting in reduced interaction. Altogether, our findings suggest the BARD1 BRCT domain serves as an ATPase activating protein to control OLA1 allosterically.

Oxidation of catalytic cysteine of human deubiquitinase BAP1 triggers misfolding and aggregation in addition to functional loss.

Deubiquitinating enzymes (DUBs) form a large protease family involved in a myriad of biological and pathological processes, including ROS sensors. ROS-mediated inhibition of their DUB activities is critical for fine-tuning the stress-activated signaling pathways. Here, we demonstrate that the ubiquitin C-terminal hydrolase (UCH) domain of BAP1 (BAP1-UCH) is highly sensitive to moderate oxidative stress. Oxidation of the catalytic C91 significantly destabilizes BAP1-UCH and increases the population of partially unfolded form, which is prone to aggregation. Unlike other DUBs, the oxidation-induced structural and functional loss of BAP1-UCH cannot be fully reversed by reducing agents. The oligomerization of oxidized BAP1-UCH is attributed to inter-molecular disulfide bond formation. Hydrogen-deuterium mass exchange spectrometry (HDX-MS) reveals increased fluctuations of the central ß-sheet upon oxidation. Our findings suggest that oxidation-mediated functional loss and increased aggregation propensity may contribute to oncogenesis associated with BAP1.

The catalytic activity of TCPTP is auto-regulated by its intrinsically disordered tail and activated by Integrin alpha-1.

T-Cell Protein Tyrosine Phosphatase (TCPTP, PTPN2) is a non-receptor type protein tyrosine phosphatase that is ubiquitously expressed in human cells. TCPTP is a critical component of a variety of key signaling pathways that are directly associated with the formation of cancer and inflammation. Thus, understanding the molecular mechanism of TCPTP activation and regulation is essential for the development of TCPTP therapeutics. Under basal conditions, TCPTP is largely inactive, although how this is achieved is poorly understood. By combining biomolecular nuclear magnetic resonance spectroscopy, small-angle X-ray scattering, and chemical cross-linking coupled with mass spectrometry, we show that the C-terminal intrinsically disordered tail of TCPTP functions as an intramolecular autoinhibitory element that controls the TCPTP catalytic activity. Activation of TCPTP is achieved by cellular competition, i.e., the intrinsically disordered cytosolic tail of Integrin-α1 displaces the TCPTP autoinhibitory tail, allowing for the full activation of TCPTP. This work not only defines the mechanism by which TCPTP is regulated but also reveals that the intrinsically disordered tails of two of the most closely related PTPs (PTP1B and TCPTP) autoregulate the activity of their cognate PTPs via completely different mechanisms.

Sialic acid-containing glycolipids mediate binding and viral entry of SARS-CoV-2.

Emerging evidence suggests that host glycans influence severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Here, we reveal that the receptor-binding domain (RBD) of the spike (S) protein on SARS-CoV-2 recognizes oligosaccharides containing sialic acid (Sia), with preference for monosialylated gangliosides. Gangliosides embedded within an artificial membrane also bind to the RBD. The monomeric affinities (Kd = 100-200 µM) of gangliosides for the RBD are similar to another negatively charged glycan ligand of the RBD proposed as a viral co-receptor, heparan sulfate (HS) dp2-dp6 oligosaccharides. RBD binding and infection of SARS-CoV-2 pseudotyped lentivirus to angiotensin-converting enzyme 2 (ACE2)-expressing cells is decreased following depletion of cell surface Sia levels using three approaches: sialyltransferase (ST) inhibition, genetic knockout of Sia biosynthesis, or neuraminidase treatment. These effects on RBD binding and both pseudotyped and authentic SARS-CoV-2 viral entry are recapitulated with pharmacological or genetic disruption of glycolipid biosynthesis. Together, these results suggest that sialylated glycans, specifically glycolipids, facilitate viral entry of SARS-CoV-2.