Current Research in Structural Biology

Tuberculosis (TB) is a major infectious disease caused by Mycobacterium tuberculosis (Mtb), leading to more than a million human deaths every year. Mammalian cell entry complexes (Mce1-4) play an essential role in the survival of Mtb during the latent stage by mediating the import of lipids, including fatty acids and cholesterol, from the host. The proper functioning of Mce-complexes requires additional proteins such as Mce-associated membrane (Mam) proteins and lipid uptake coordinator (LucA), thus making them potential candidates for the development of anti-TB drugs. Four Mam (Mam1A-1D) proteins are coded from the mce1 operon and two from the mce3 (Mam3A-3B) and mce4 (Mam4A-4B) operons. In addition, five orphaned mam (Omam) proteins have been identified, which are not part of the mce operons but are functionally relevant for the Mce complexes. Analysis of the sequences of Mam/Omam proteins suggests that they share many common secondary and tertiary structural elements despite the low sequence identity between them. Here, we have characterized a recombinantly produced Mam1A variant by small-angle X-ray and neutron scattering. The studies indicate that Mam1A is tetrameric in solution with two disulfide bridges necessary for the stability of Mam1A. Similarly, a disulfide bridge has also been identified in Mam1C. Furthermore, through coexpression and copurification, we demonstrate that Mam1A-1D and LucA interact to form stable Mam1ABCD as well as Mam1ABCD-LucA complexes. The results obtained pave the way for further understanding how the Mam1ABCD and Mam1ABCD-LucA complexes are organized and interact with the Mce complexes, leading to their stabilization.

RNA polymerase-binding protein A (RbpA) is an actinomycetes-specific protein crucial for the growth and survival of the pathogen Mycobacterium tuberculosis. Its role is essential and influences the transcription and antibiotic responses. However, the regulatory mechanisms underlying RbpA-mediated transcription remain unknown. In this study, we employed various computational techniques to investigate the role of RbpA in the formation and dynamics of the RNA polymerase complex. Our analysis reveals significant structural rearrangements in RNA polymerase happen upon interaction with RbpA. Hotspot residues, crucial amino acids in the RbpA-mediated transcriptional regulation, were identified through our examination. The study elucidates the dynamic behavior within the complex, providing insights into the flexibility and functional dynamics of the RbpA-RNA polymerase interaction. Notably, potential allosteric mechanisms, involving the interface of subunits 1 and 2 were uncovered, shedding light on how RbpA modulates transcriptional activity. Finally, potential ligands meant for the 1-2 binding site were identified through virtual screening. The outcomes of our computational study serve as a foundation for experimental investigations into inhibitors targeting the RbpA-regulated dynamics in RNA polymerase. Overall, this research contributes valuable information for understanding the intricate regulatory networks of RbpA in the context of transcription and suggests potential avenues for the development of RbpA-targeted therapeutics. Author SummaryInfection studies by Mycobacterium tuberculosis (Mtb) acquires primary importance due to its severe infection and antibiotic resistance. There is an open need for highly effective drugs and one needs to employ novel approaches such as detailed structural analysis and the possibility to focus on allosteric inhibitors. We have exploited the availability of cryo-EM structures of RNA polymerase of Mtb, with and without its transcription-activator protein namely RNA polymerase-binding protein A (RbpA). In this study, we employed various computational techniques to investigate the role of RbpA in the formation and dynamics of the RNA polymerase complex. The assemblies were subject to molecular dynamics and perturbation scanning, followed by structural comparisons and measurement of subunit interface strength. These analyses could clearly show that subunits, which are far away from the RbpA binding site, undergo differential structural changes. Hence, we focused on the site to recognize potential small molecule inhibitors using virtual screening. These analyses demonstrate that it is possible to perform comparative structural analysis of different forms of assemblies, which can be useful towards drug design.

Legionella pneumophila, the causative agent of a life-threatening pneumonia, intracellularly replicates in a specialized compartment in lung macrophages, the Legionella-containing vacuole (LCV). Secreted proteins of the pathogen govern important steps in the intracellular life cycle including bacterial egress. Among these is the type II secreted PlaA which, together with PlaC and PlaD, belongs to the GDSL phospholipase family found in L. pneumophila. PlaA shows lysophospholipase A (LPLA) activity which increases after secretion and subsequent processing by the zinc metalloproteinase ProA at residue E266/L267 located within a disulfide loop. Activity of PlaA contributes to the destabilization of the LCV in the absence of the type IVB-secreted effector SdhA. We here present the 3D structure of PlaA which shows a typical /{beta} hydrolase fold and reveals that the uncleaved disulfide loop forms a lid structure covering the catalytic triad S30/D278/H282. This leads to reduction of both substrate access and membrane interaction before activation; however, the catalytic and membrane interaction site gets more accessible when the disulfide loop is processed. After structural modelling, a similar activation process is suggested for the GDSL hydrolase PlaC, but not for PlaD. Furthermore, the size of the PlaA substrate binding site indicated preference towards phospholipids comprising ~16 carbon fatty acid residues which was verified by lipid hydrolysis, suggesting a molecular ruler mechanism. Indeed, mutational analysis changed the substrate profile with respect to fatty acid chain length. In conclusion, our analysis revealed the structural basis for the regulated activation and substrate preference of PlaA.

ATP-Pyrophosphatases (ATP-PPases) are the most primordial lineage of the large and diverse HUP (HIGH-motif proteins, Universal Stress Proteins, ATP-Pyrophosphatase) superfamily. There are four different ATP-PPase substrate-specificity groups, and members of each group show considerable sequence variation across the domains of life despite sharing the same catalytic function. Over the past decade, there has been a >20-fold expansion in the number of ATP-PPase domain structures most recently from advances in protein structure prediction (e.g. Alphafold2). Using the enriched structural information, we have characterised the two most populated ATP-PPase substrate-specificity groups, the NAD-synthases (NAD) and GMP synthases (GMPS). We performed local structural and sequence comparisons between the NADS and GMPS from different domains of life and identified taxonomic-group specific structural functional motifs. As GMPS and NADS are potential drug targets of pathogenic microorganisms including Mycobacterium tuberculosis, structural motifs specific to bacterial GMPS and NADS provide new insights that may aid antibacterial-drug design.

Trichophyton rubrum is one of the leading causes of superficial skin infections worldwide. It is a keratinolytic fungus specialized in colonization of keratinized tissue of skin, hair and nails for long periods of time. The fungus encodes a wide repertoire of secreted proteases in its genome that not only aid in nutrient acquisition but also establishment of infection on the host. The proteases are synthesized in prepro form that requires removal of the prosegment for activation. In order to gain insights into the structural association of the pro domain with the catalytic domain, we investigate the structural features of the pro domain of the secreted sedolisin member Sub16 of T. rubrum. Our results show that the pro domain of Sub16 may have inherent flexibility in the absence of the associated catalytic domain which is stabilized in complex with catalytic domain. This is the first report of structural investigation on a stand-alone pro domain of sedolisin family of subtilases that will help in design of further structural studies of this protein.

In S. cerevisiae, the uncharacterized protein Mco10 (Mitochondrial class one protein of 10 kDa) was previously found to be associated with mitochondrial ATP synthase and referred to as a new subunit l. However, recent cryo-EM structures of S. cerevisiae ATP synthase could not ascertain Mco10 as a structural subunit of the enzyme, either monomers or dimers, making questionable its role as a structural subunit. The N-terminal part of Mco10 is very similar to Atp19 (subunit k) of ATP synthase. The subunit k/Atp19, along with the subunits g/Atp20 and e/Atp21 plays a major role in stabilization of the ATP synthase dimers. In our effort to confidently define the small protein interactome of ATP synthase we similarly found Mco10 associated with ATP synthase of S. cerevisiae. We herein investigated the impact of Mco10 on ATP synthase functioning. Biochemical analysis revealed in spite of similarity in sequence and evolutionary lineage, that Mco10 and Atp19 differ significantly in function. This is the first work to show Mco10 is an auxiliary ATP synthase subunit that only functions in permeability transition.

The marine bacterium Vibrio alginolyticus possesses a polar flagellum driven by a sodium ion flow. The main components of the flagellar motor are the stator and rotor. The C-ring and MS-ring which are composed of FliG and FliF, respectively, are parts of the rotor. Here, we purified an MS-ring composed of FliF-FliG fusion proteins and solved the near-atomic resolution structure of the S-ring--the upper part of the MS-ring--using cryo-electron microscopy. This is the first report of an S-ring structure from Vibrio whereas, previously, only those from Salmonella have been reported. The Vibrio S-ring structure reveals novel features compared to that of Salmonella such as tilt angle differences of the core domain and the {beta}-collar region, the decrease of the inter-subunit interaction between core domains, and altered electrostatic inner-surface. The residues potentially interact with other flagellar components, such as FliE and FlgB, are well structurally conserved in Vibrio S-ring. These comparisons clarified the conserved and non-conserved structural features of the MS-ring across different species. IMPORTANCEUnderstanding the structure and function of the flagellar motor in bacterial species is essential for uncovering the mechanisms underlying bacterial motility and pathogenesis. Our study revealed the structure of the Vibrio S-ring, a part of its polar flagellar motor, and highlighted its unique features compared with the well-studied Salmonella S-ring. The observed differences in the inter-subunit interactions and in the tilt angles between the Vibrio and Salmonella S-rings highlighted the species-specific variations in the flagellar assembly. By concentrating on the region where the S-ring and the rod proteins interact, we uncovered conserved residues essential for the interaction. Our research contributes to advancing of bacterial flagellar biology.

The aminoacylation of lipid head group in many bacteria is carried out by bi-functional enzymes called MprF, which encode for a soluble synthase domain that typically transfers lysine or alanine from a tRNA to lipid head groups, and the modified lipid is translocated across the leaflets by a transmembrane domain. This modification of the lipids probably evolved to adapt to the environment where the microbes reside. Here, we describe the cryoEM structures of MprF enzyme from Pseudomonas aeruginosa revealing a dimeric enzyme with a distinct architecture when compared with the homologous Rhizobium enzymes and validate this arrangement with biochemical analysis. The cryoEM maps and the models in detergent micelle and nanodisc reveal a conformational change of the terminal helix of the synthase domain, highlighting the dynamic elements in the enzyme that might facilitate catalysis. Several lipid-like densities are observed in the cryoEM maps, which might indicate the path taken by the lipids and the coupling function of the two functional domains. Thus, the structure of a well-characterised PaMprF lays a platform for understanding the mechanism of amino acid transfer to a lipid head group and subsequent flipping across the leaflet that changes the property of the membrane.

Cyclic-di-nucleotide based secondary messengers regulate various physiological processes including the stress responses in bacteria. In the past decade, cyclic diadenosine monophosphate (c-di-AMP) has emerged as a crucial second messenger, implicated in fatty acid metabolism, antibiotic resistance, biofilm formation, virulence, DNA repair, ion homeostasis, sporulation etc. The level of c-di-AMP is maintained in the cell by the action of two opposing enzymes, namely diadenylate cyclase (DAC) and phosphodiesterase (PDE). In mycobacteria, this molecule is essential for its regulatory role in bacterial physiology and host-pathogen interactions. However, such modulation of c-di-AMP remains to be explored in Mycobacterium smegmatis. Here, we systematically characterised the c-di-AMP synthase (MsDisA) and a hydrolase (MsPDE) from M. smegmatis at different pH and osmolytic conditions in vitro. Our biochemical assays show that the MsDisA activity is enhanced during the alkaline stress and c-di-AMP is readily produced without any intermediates. At pH 9.4, the MsDisA promoter activity in vivo increases significantly, strengthening this observation. However, under physiological conditions, the activity of MsDisA was moderate with the formation of intermediates. To get further insights into the structural characteristics, we determined the cryo-EM structure of the MsDisA, revealing some interesting features. Biochemical analysis of individual domains shows that the N-terminal minimal region alone can form a functional octamer. Altogether, our results reveal the biochemical and structural regulation of mycobacterial c-di-AMP in response to various environmental stress.

Bacterial pathogens such as Mycobacterium tuberculosis majorly rely on two-component signaling (TCS) systems to sense and generate adaptive responses to the dynamic and stressful host environment. TCS comprises a sensor histidine kinase (SK) that perceives the environmental signal, and a response regulator (RR) that modulates the target gene expression. TCS occurs via a phosphotransfer event from SK to RR. However, the mechanisms that regulate phosphotransfer events are not well understood. We explored the role of MutT1, originally characterized to hydrolyze oxidized GTP (8-oxo-GTP) and dGTP (8-oxo-dGTP), in TCS regulation. Unlike other MutT proteins, mycobacterial MutT1 comprises two domains (N-terminal domain, NTD; and C-terminal domain, CTD). Structurally, MutT1 NTD is like MutT proteins in other organisms. However, the MutT1 CTD is similar to E. coli SixA, a histidine phosphatase with an RHG motif. We show that MutT1 CTD dephosphorylates many SKs and impacts expression of their target genes, highlighting the role of MutT1 in regulating TCS. These novel findings are of special significance because they provide us with an extrinsic phosphatase mechanism to reset TCS signaling. The study reveals an intricate interplay between an enzyme that sanitizes the cellular nucleotide pool, and bacterial signaling pathways, offering insights into the adaptation mechanisms.

The Mycobacterium DprE2 is a NADH-dependent enzyme and converts the decaprenylphosphoryl-{beta}-D-ribose (DPX) to decaprenylphosphoryl-{beta}-D-arabinofuranose (DPA). The FAD-containing oxidoreductase MtbDprE1 and NADH-dependent reductase MtbDprE2 enzymes catalyses together the epimerization reaction, which coverts DPR to DPA. Here, MtbDprE2 enzyme was purified and structurally characterized using circular dichroism, molecular modelling and dynamics simulation techniques. The MtbDprE2 was purified, which eluted as oligomer from size exclusion column. The circular dichroism analysis yielded ~ 47.6% -helix, ~ 19.8% {beta}-sheet and ~ 32.6% random coil structures in MtbDprE2 enzyme and showed highly thermostability. The molecular modelling of MtbDprE2 and its complex with NADH showed that it contains two domains (i) the large domain consists of central twisted seven {beta}-sheets decorated by eight -helices and (ii) a small domain contains two short -helices connect by loop. Overall, the MtbDprE2 adopts a typical short-chain dehydrogenase rossmann fold and NADH binds to Asp69, Ser147, Tyr160, Lys164 of catalytic triad and Gly16, Ser19, Glu20, Ile21 of Gly-rich motif of MtbDprE2. 1 ns dynamics simulation was performed on apo and NADH bound MtbDprE2, which indicated the small conformational change in ligand binding site, which resulted more closed pocket than open pocket observed in apo enzyme. Small conformational changes were observed in active site residues and orientation between large and small domains of MtbDprE2 upon NADH binding. Current knowledge of MtbDprE2 structure and its NADH binding mechanism will contribute significantly in development of specific inhibitors against M. tuberculosis.

Ubiquitin-like protein 3 (UBL3) is a post-translational modifier that sorts proteins into small extracellular vesicles and regulates the trafficking of disease-associated proteins such as -synuclein. The structural and dynamic features of the UBL domain that underlie these functions, however, remain poorly understood. Here we performed in silico structural dynamics analysis of the UBL3 UBL domain using an NMR structure ensemble combined with anisotropic network modeling (ANM) and perturbation response scanning (PRS). Principal component analysis and residue-wise fluctuation analysis consistently revealed high flexibility in the C-terminal region of UBL3. Comparative ANM analysis across 20 ubiquitin-like proteins (UBLs) further showed that C-terminal flexibility is a conserved yet variable property within the UBL family. PRS analysis demonstrated that residues forming the central -helix of the {beta}-grasp fold exert greater dynamic control over collective motions than {beta}-sheet residues. Notably, UBL3 displayed the highest helix/sheet PRS effectiveness ratio among all UBLs analyzed, highlighting the prominent dynamic contribution of helix residues in this domain. Together, these results provide a structural basis for understanding UBL3-dependent protein interactions and disease-related mechanisms, and suggest that helix-centered dynamic control in the UBL domain may represent a potential target for modulating UBL3 function.

Abstract/SummaryPhospholipase C gamma 2 (PLC{gamma}2) plays important roles in cell signaling downstream of various membrane receptors. PLC{gamma}2 contains a multi-domain inhibitory region critical for its regulation, while it has remained unclear how these domains contribute to PLC{gamma}2 activity modulation. Here we determined three structures of human PLC{gamma}2 in autoinhibited states, which reveal dynamic interactions at the autoinhibition interface, involving the conformational flexibility of the SH3 domain in the inhibitory region, and its previously unknown interaction with a C-terminal helical domain in the core region. We also determined a structure of PLC{gamma}2 bound to the kinase domain of fibroblast growth factor receptor 1 (FGFR1), which demonstrates the recognition of FGFR1 by the nSH2 domain in the inhibitory region of PLC{gamma}2. Our results provide new structural insights into PLC{gamma}2 regulation that will facilitate future mechanistic studies to understand the entire activation process.

Biological polypeptides are known to contain cis-linkage in their main chain as a minor but important feature. Such anomalous connection of amino acids has different structural and functional effects on proteins. Experimental evidence of cis-bonds in proteins is mainly obtained using X-ray crystallography and other methods in the field of structural biology. To date, extensive analyses have been carried out on the experimentally found cis-bonds using the Protein Data Bank entry bases and/or residue bases; however, their consistency in each protein has not been examined on a global scale. Data accumulation and advances in methodology enable the use of new approaches from a proteomic point of view. Here, we sought to describe a simple procedure for the detection and confirmation of cis-bonds from a set of experimental coordinates for a protein to discriminate this type of bond from isomerizable and/or misassigned bonds. The resulting set of consistent cis bonds provides unprecedented insights into the trend of "high cis content" proteins and the upper limit of consistent cis bonds per polypeptide length. Recognizing such limit would not only be important for a practical check of upcoming structures, but also for the design of novel protein folds beyond the evolutionally-acquired repertoire.

Wag31, or DivIVA, is an essential protein and a drug target in human pathogen Mycobacterium tuberculosis that self-assembles at the negatively curved membrane surface to form a higher-order structural scaffold, maintains rod-shaped cellular morphology, and localizes key cell-wall synthesizing proteins at the pole for exclusive polar growth. We determined the crystal structure of N-terminal membrane anchoring domain of mycobacterial Wag31 at 2.3 [A] resolution using molecular replacement method. Crystal packing analysis revealed a previously unseen dimer-of-dimer assembly state of N-terminal Wag31 with C2 point group symmetry, which is formed by antiparallel stacking of two coiled coil dimers. Size-exclusion column chromatography-coupled small angle solution X-ray scattering data showed a tetrameric form as a major assembly state of N-terminal Wag31 in solution, further supporting the crystal structure. Plausible models of linear self-assembling, and branching, of Wag31 filaments consistent with available data are suggested.

Furin cleavage-site (CS) present between the S1/S2 junction in SARS-CoV2 spike (S) protein is critical to drive the fusion of SARS-CoV2 with the host cell. SARS-CoV2 falls in the sarbecovirus lineage that doesnt comprise of furin CS and therefore makes its origin enigmatic. The available wild-type (Wt) SARS-CoV2 S protein with PDB ID: 6yvb lacks a stretch of amino acid including furin CS as well. All investigators till date have shown this stretch existing in the form of a loop. We are for the first time reporting that this stretch comprises of 14 amino acid residues (677QTNSPRRARSVASQ689), forming an antiparallel {beta}-sheet comprising of PRRAR furin CS. We observed the presence of this antiparallel {beta}-sheet in MERS spike protein as well. While switching over from Wt. SARS-CoV2 with PRRAR furin CS to B.1.1.7 variant with HRRAR furin CS, we found 3% increase in the percentage content of {beta} stands. Interestingly, we found that the change of B.1.1.7 to B.1.617 variant comprising of RRRAR furin CS shifted the percentage secondary structure back to that found in Wt. SARS-CoV2. We anticipate that this {beta}-sheet is used as a docking site by host cell proteases to act on furin-CS. Additionally, we studied the interaction of modeled SARS-CoV2 S protein with transmembrane protease, serine 2 (TMPRSS2), and furin proteases, which clearly highlighted that these proteases exclusively uses furin CS located in {beta}-sheet to cleave the SARS-CoV2 S protein at its S1/S2 junction.

The rapid spread of acquired multidrug resistance (MDR) in bacteria is a world-wide health threat. The MexR protein regulates the expression of the MexAB-OprM efflux pump, which actively extrudes chemical compounds with high toxicity to the host organism Pseudomonas Aeruginosa. In repression mode, two MexR dimers bind to an operator with two homologous pseudo-palindromic boxes located in proximity (named PI and PII). Here we report a first structural characterization of the complex in solution using small angle X-ray scattering (SAXS), small-angle neutron scattering (SANS) and rigid body modelling. The spacing between the PI and PII boxes is rich in AT base pairs indicate possible flexibility between the two MexR dimer binding sites. In agreement, our best modelling fits show a requirement for DNA bending between the two MexR binding sites to optimally fit SAS data as well as known biological properties of the MexR operons. Taken together, this study contributes to better understanding of the structural properties of bacterial operators and their repressor proteins.

Mycobacterium tuberculosis topoisomerase I (MtbTOP1) is essential for the viability of the causative agent of TB. There are still significant unanswered questions regarding the dynamic conformations during catalysis of relaxation of negatively supercoiled DNA by MtbTOP1. We aim to study the flexible hinge residues that control the dynamics of inter-domain rearrangements involved in the enzyme conformational changes that allow the opening-closing of the topoisomerase gate. We used the online server PACKMAN to predict possible hinges from the MtbTOP1 crystal structure. The predicted region "PRO506 to LEU526" at the border between domains D2 and D4 with a p-value <0.05 was then studied as a potential hinge. The highly conserved ARG516 from this region interacts with the DNA inside the protein toroidal cavity. This arginine maintains inter-domain interaction with GLU207 of D4 and ASP691 of D5 domains. After introducing alanine substitutions, we further studied the mutant topoisomerases in biochemical experiments. The results showed a significant loss in DNA relaxation activity without affecting DNA binding and cleavage after mutating GLU207 and ARG516, consistent with their role as hinge residues in domain rearrangements.

The Mediator complex plays an essential and multi-faceted role in regulation of RNA polymerase II transcription in all eukaryotes. Structural analysis of yeast Mediator has provided an understanding of the conserved core of the complex and its interaction with RNA polymerase II but failed to reveal the structure of the Tail module that contains most subunits targeted by activators and repressors. Here we present a molecular model of mammalian (Mus musculus) Mediator, derived from a 4.0 [A] resolution cryo-EM map of the complex. The mammalian Mediator structure reveals that the previously unresolved Tail module, which includes a number of metazoan specific subunits, interacts extensively with core Mediator and has the potential to influence its conformation and interactions.

The Endoplasmic Reticulum (ER) is the entry site to the secretory pathway, serving as the targeting destination for [~]30% of the proteome. The mechanisms for targeting soluble or integral membrane secretory pathway proteins are well-studied. However, it is currently unknown how the tens of ER surface proteins (SuPs), central for organelle function, reach the outer leaflet of the membrane. It was previously shown that an amphipathic helix (AH) from the Brome mosaic virus protein 1a, is both necessary and sufficient for targeting to the ER surface in bakers yeast. We therefore utilized this helix as a model substrate and performed a high-content screen to uncover factors that affect SuP targeting. Our results suggest a role for membrane lipid composition in targeting specificity. To see if the presence of an AH is a more general mechanism for SuP targeting, we searched for their presence within SuPs of both yeast and humans. Five endogenous yeast SuPs contained AHs, and of these four were sufficient for ER localization. Moreover, the presence of an AH was conserved to human SuP orthologs. By altering helix features we determine the parameters that affect this new targeting motif. Hence our work demonstrates how specific properties of AHs encode affinity for the ER membrane. More globally, understanding how SuPs are targeted correctly takes us a step forward in determining the underlying mechanisms of cellular localization and secretory pathway functions. The authors declare that they have no conflict of interest.