Efficient segmental isotope labeling of multi-domain proteins using Sortase A

Protein ligation for the assembly and study of nonribosomal peptide synthetase megaenzymes

Nonribosomal peptide synthetases (NRPSs) are biosynthetic enzymes found in bacteria and fungi, that synthesize a plethora of pharmaceutically relevant compounds. NRPSs consist of repeating sets of functional domains called modules, and each module is responsible for the incorporation of a single amino acid to the growing peptidyl intermediate. The synthetic logic of an NRPS resembles an assembly line, with growing biosynthesis intermediates covalently attached to the prosthetic 4'-phosphopantetheine (ppant) moieties of T (thiolation or transfer) domains for shuttling within and between modules. Therefore, NRPSs must have each T domain phosphopantetheinylated to be functional, and host organisms encode ppant transferases that affix ppant to T domains. Ppant transferases can be promiscuous with respect to the T domain substrate and with respect to chemical modifications of the ppant thiol, which has been a useful characteristic for study of megaenzymes and other systems. However, defined studies of multimodular megaenzymes, where different analogs are required to be affixed to different T domains within the same multimodular protein, are hindered by this promiscuity. Study of NRPS peptide bond formation, for which two T domains simultaneously deliver substrates to the condensation domain, is a prime example where one would want two T domains bearing different acyl/peptidyl groups. Here, we report a strategy where two NRPS modules that are normally part of the same protein are expressed as separate constructs, modified separately with different acyl-ppants, and then ligated together by sortase A of Staphylococcus aureus or asparaginyl endopeptidase 1 of Oldenlandia affinis (OaAEP1). We assessed various reaction conditions to optimize the ligation reactions and maximize the yield of the complex of interest. Finally, we apply this method in large scale and show it allows the complex built by OaAEP1-mediated ligation to be characterized by X-ray crystallography.

Application of Sortase-Mediated Ligation for the Synthesis of Block Copolymers and Protein-Polymer Conjugates

Sortase-mediated ligation (SML) has become a powerful tool for site-specific protein modification. However, sortase A (SrtA) suffers from low catalytic efficiency and mediates an equilibrium reaction. Therefore, ligations with large macromolecules may be challenging. Here, the synthesis of polymeric building blocks for sortase-mediated ligation constituting peptide-polymers with either the recognition sequence for sortase A (LPX1TGX2) or its nucleophilic counterpart (Gx) is demonstrated. The peptide-polymers are synthesized by solid-phase peptide synthesis followed by photo-iniferter (PI) reversible addition-fragmentation chain-transfer (RAFT) polymerization of various monomers. The building blocks are subsequently utilized to investigate possibilities and limitations when using macromolecules in SML. In particular, diblock copolymers are obtained even when using the orthogonal building blocks in equimolar ratio by exploiting a technique to shift the reaction equilibrium. However, ligations of two polymers can not be achieved when the degree of polymerization exceeds 100. Subsequently, C-terminal protein-polymer conjugates are synthesized. Several polymers are utilized that can replace the omnipresent polyethylene glycol (PEG) in future therapeutics. The conjugation is exemplified with a nanobody that is known for efficient neutralization of SARS-CoV-2. The study demonstrates a universal approach to polymer-LPX1TGX2 and Gx-polymer building blocks and gives insight into their application in SML.

How does p53 work? Regulation by the intrinsically disordered domains

Defects in the tumor suppressor protein p53 are found in the majority of cancers. The p53 protein (393 amino acids long) contains the folded DNA-binding domain (DBD) and tetramerization domain (TET), with the remainder of the sequence being intrinsically disordered. Since cancer-causing mutations occur primarily in the DBD, this has been the focus of most of the research on p53. However, recent reports show that the disordered N-terminal activation domain (NTAD) and C-terminal regulatory domain (CTD) function synergistically with the DBD to regulate p53 activity. We propose a mechanistic model in which intermolecular and intramolecular interactions of the disordered regions, modulated by post-translational modifications, perform a central role in the regulation and activation of p53 in response to cellular stress.

Synthetic Amine Linkers for Efficient Sortagging

Enzymatic site-specific bioconjugation techniques, in particular sortase-mediated ligation, are increasingly used to generate conjugated proteins for a wide array of applications. Extension of the utility and practicality of sortagging for diverse purposes is critically dependent on further improvement of the efficiency of sortagging reactions with a wider structural variety of substrates. We present a comprehensive comparative mass spectrometry screening study of synthetic nonpeptidic incoming amine nucleophile substrates of Staphylococcus aureus Sortase A enzyme. We have identified the optimal structural motifs among the chemically diverse set of 452 model primary and secondary amine-containing sortagging substrates, and we demonstrate the utility of representative amine linkers for efficient C-terminal biotinylation of nanobodies.

Efficient Segmental Isotope Labeling of Integral Membrane Proteins for High-Resolution NMR Studies

High-resolution structural NMR analyses of membrane proteins are challenging due to their large size, resulting in broad resonances and strong signal overlap. Among the isotope labeling methods that can remedy this situation, segmental isotope labeling is a suitable strategy to simplify NMR spectra and retain high-resolution structural information. However, protein ligation within integral membrane proteins is complicated since the hydrophobic protein fragments are insoluble, and the removal of ligation side-products is elaborate. Here, we show that a stabilized split-intein system can be used for rapid and high-yield protein trans-splicing of integral membrane proteins under denaturing conditions. This setup enables segmental isotope labeling experiments within folded protein domains for NMR studies. We show that high-quality NMR spectra of markedly reduced complexity can be obtained in detergent micelles and lipid nanodiscs. Of note, the nanodisc insertion step specifically selects for the ligated and correctly folded membrane protein and simultaneously removes ligation byproducts. Using this tailored workflow, we show that high-resolution NMR structure determination is strongly facilitated with just two segmentally isotope-labeled membrane protein samples. The presented method will be broadly applicable to structural and dynamical investigations of (membrane-) proteins and their complexes by solution and solid-state NMR but also other structural methods where segmental labeling is beneficial.

Sortase-Modified Cholera Toxoids Show Specific Golgi Localization

Cholera toxoid is an established tool for use in cellular tracing in neuroscience and cell biology. We use a sortase labeling approach to generate site-specific N-terminally modified variants of both the A2-B5 heterohexamer and B5 pentamer forms of the toxoid. Both forms of the toxoid are endocytosed by GM1-positive mammalian cells, and while the heterohexameric toxoid was principally localized in the ER, the B5 pentamer showed an unexpectedly specific localization in the medial/trans-Golgi. This study suggests a future role for specifically labeled cholera toxoids in live-cell imaging beyond their current applications in neuronal tracing and labeling of lipid rafts in fixed cells.

Quantitative N- or C-Terminal Labelling of Proteins with Unactivated Peptides by Use of Sortases and a d-Aminopeptidase

Quantitative and selective labelling of proteins is widely used in both academic and industrial laboratories, and catalytic labelling of proteins using transpeptidases, such as sortases, has proved to be a popular strategy for such selective modification. A major challenge for this class of enzymes is that the majority of procedures require an excess of the labelling reagent or, alternatively, activated substrates rather than simple commercially sourced peptides. We report the use of a coupled enzyme strategy which enables quantitative N- and C-terminal labelling of proteins using unactivated labelling peptides. The use of an aminopeptidase in conjunction with a transpeptidase allows sequence-specific degradation of the peptide by-product, shifting the equilibrium to favor product formation, which greatly enhances the reaction efficiency. Subsequent optimisation of the reaction allows N-terminal labelling of proteins using essentially equimolar ratios of peptide label to protein and C-terminal labelling with only a small excess. Minimizing the amount of substrate required for quantitative labelling has the potential to improve industrial processes and facilitate the use of transpeptidation as a method for protein labelling.

Three segment ligation of a 104 kDa multi-domain protein by SrtA and OaAEP1

NMR spectroscopy is an excellent tool for studying protein structure and dynamics which provides a deeper understanding of biological function. As the size of the biomolecule of interest increases, it can become advantageous to dilute the number of observed signals in the NMR spectrum to decrease spectral overlap and increase resolution. One way to limit the number of resonances in the NMR data is by selectively labeling a smaller domain within the larger macromolecule, a process called segmental isotopic labeling. Many examples of segmental isotopic labeling have been described where two segments of a protein are ligated together by chemical or enzymatic means, but there are far fewer descriptions of a three or more segment ligation reaction. Herein, we describe an enzymatic segmental labeling scheme that combines the widely used Sortase A and more recently described OaAEP1 for a two site ligation strategy. In preparation to study proposed long-range allostery in the 104 kDa DNA damage repair protein Rad50, we ligated side-chain methyl group labeled Zn Hook domain between two long segments of otherwise unlabeled P.furiosus Rad50. Enzymatic activity data demonstrated that the scars resulting from the ligation reactions did not affect Rad50 function within the Mre11-Rad50 DNA double strand break repair complex. Finally, methyl-based NMR spectroscopy confirmed the formation of the full-length ligated protein. Our strategy highlights the strengths of OaAEP1 for segmental labeling, namely faster reaction times and a smaller recognition sequence, and provides a straightforward template for using these two enzymes in multisite segmental labeling reactions.

Efficient Multiple Domain Ligation for Proteins Using Asparaginyl Endopeptidase by Selection of Appropriate Ligation Sites Based on Steric Hindrance

Three domain fragments of a multi-domain protein, ER-60, were ligated in two short linker regions using asparaginyl endopeptidase not involving denaturation. To identify appropriate ligation sites, by selecting several potential ligation sites with fewer mutations around two short linker regions, their ligation efficiencies and the functions of the ligated ER-60s were examined experimentally. To evaluate the dependence of ligation efficiencies on the ligation sites computationally, steric hinderances around the sites for the ligation were calculated through molecular dynamics simulations. Utilizing the steric hindrance, a site-dependent ligation potential index was introduced as reproducing the experimental ligation efficiency. Referring to this index, the reconstruction of ER-60 was succeeded by the ligation of the three domains for the first time. In addition, the new ligation potential index well-worked for application to other domain ligations. Therefore, the index may serve as a more time-effective tool for multi-site ligations.

Chemical tools for study and modulation of biomolecular phase transitions

Biomolecular phase transitions play an important role in organizing cellular processes in space and time. Methods and tools for studying these transitions, and the intrinsically disordered proteins (IDPs) that often drive them, are typically less developed than tools for studying their folded protein counterparts. In this perspective, we assess the current landscape of chemical tools for studying IDPs, with a specific focus on protein liquid-liquid phase separation (LLPS). We highlight methodologies that enable imaging and spectroscopic studies of these systems, including site-specific labeling with small molecules and the diverse range of capabilities offered by inteins and protein semisynthesis. We discuss strategies for introducing post-translational modifications that are central to IDP and LLPS function and regulation. We also investigate the nascent field of noncovalent small-molecule modulators of LLPS. We hope that this review of the state-of-the-art in chemical tools for interrogating IDPs and LLPS, along with an associated perspective on areas of unmet need, can serve as a valuable and timely resource for these rapidly expanding fields of study.

A simple protocol for the production of highly deuterated proteins for biophysical studies

Highly deuterated protein samples expand the biophysics and biological tool kit by providing, among other qualities, contrast matching in neutron diffraction experiments and reduction of dipolar spin interactions from normally protonated proteins in magnetic resonance studies, impacting both electron paramagnetic resonance and NMR spectroscopy. In NMR applications, deuteration is often combined with other isotopic labeling patterns to expand the range of conventional NMR spectroscopy research in both solution and solid-state conditions. However, preparation of deuterated proteins is challenging. We present here a simple, effective, and user-friendly protocol to produce highly deuterated proteins in Escherichia coli cells. The protocol utilizes the common shaker flask growth method and the well-known pET system (which provides expression control via the T7 promotor) for large-scale recombinant protein expression. One liter expression typically yields 5 to 50 mg of highly deuterated protein. Our data demonstrate that the optimized procedure produces a comparable quantity of protein in deuterium (²H₂O) oxide M9 medium compared with that in ¹H₂O M9 medium. The protocol will enable a broader utilization of deuterated proteins in a number of biophysical techniques.

A Chemical Biology Primer for NMR Spectroscopists

Among structural biology techniques, NMR spectroscopy offers unique capabilities that enable the atomic resolution studies of dynamic and heterogeneous biological systems under physiological and native conditions. Complex biological systems, however, often challenge NMR spectroscopists with their low sensitivity, crowded spectra or large linewidths that reflect their intricate interaction patterns and dynamics. While some of these challenges can be overcome with the development of new spectroscopic approaches, chemical biology can also offer elegant and efficient solutions at the sample preparation stage. In this tutorial, we aim to present several chemical biology tools that enable the preparation of selectively and segmentally labeled protein samples, as well as the introduction of site-specific spectroscopic probes and post-translational modifications. The four tools covered here, namely cysteine chemistry, inteins, native chemical ligation, and unnatural amino acid incorporation, have been developed and optimized in recent years to be more efficient and applicable to a wider range of proteins than ever before. We briefly introduce each tool, describe its advantages and disadvantages in the context of NMR experiments, and offer practical advice for sample preparation and analysis. We hope that this tutorial will introduce beginning researchers in the field to the possibilities chemical biology can offer to NMR spectroscopists, and that it will inspire new and exciting applications in the quest to understand protein function in health and disease.

Challenges in the use of sortase and other peptide ligases for site-specific protein modification

Site-specific protein modification is a widely-used biochemical tool. However, there are many challenges associated with the development of protein modification techniques, in particular, achieving site-specificity, reaction efficiency and versatility. The engineering of peptide ligases and their substrates has been used to address these challenges. This review will focus on sortase, peptidyl asparaginyl ligases (PALs) and variants of subtilisin; detailing how their inherent specificity has been utilised for site-specific protein modification. The review will explore how the engineering of these enzymes and substrates has led to increased reaction efficiency mainly due to enhanced catalytic activity and reduction of reversibility. It will also describe how engineering peptide ligases to broaden their substrate scope is opening up new opportunities to expand the biochemical toolkit, particularly through the development of techniques to conjugate multiple substrates site-specifically onto a protein using orthogonal peptide ligases.

We highlight chemical and biochemical strategies taken to optimise peptide and protein modification using peptide ligases.

The NMR structure of the engineered halophilic DnaE intein for segmental isotopic labeling using conditional protein splicing

Protein trans-splicing catalyzed by split inteins has been used for segmental isotopic labeling of proteins for alleviating the complexity of NMR signals. Whereas inteins spontaneously trigger protein splicing upon protein folding, inteins from extremely halophilic organisms require a high salinity condition to induce protein splicing. We designed and created a salt-inducible intein from the widely used DnaE intein from Nostoc punctiforme by introducing 29 mutations, which required a lower salt concentration than naturally occurring halo-obligate inteins. We determined the NMR solution structure of the engineered salt-inducible DnaE intein in 2 M NaCl, showing the essentially identical three-dimensional structure to the original one, albeit it unfolds without salts. The NMR structure of a halo-obligate intein under high salinity suggests that the stabilization of the active folded conformation is not a mere result of various intramolecular interactions but the subtle energy balance from the complex interactions, including the solvation energy, which involve waters, ions, co-solutes, and protein polypeptide chains.

Mobile origin-licensing factors confer resistance to conflicts with RNA polymerase

Fundamental to our understanding of chromosome duplication is the idea that replication origins function both as sites where MCM helicases are loaded during the G1 phase and where synthesis begins in S phase. However, the temporal delay between phases exposes the replisome assembly pathway to potential disruption prior to replication. Using multicolor, single-molecule imaging, we systematically study the consequences of encounters between actively transcribing RNA polymerases (RNAPs) and replication initiation intermediates in the context of chromatin. We demonstrate that RNAP can push multiple licensed MCM helicases over long distances with nucleosomes ejected or displaced. Unexpectedly, we observe that MCM helicase loading intermediates also can be repositioned by RNAP and continue origin licensing after encounters with RNAP, providing a web of alternative origin specification pathways. Taken together, our observations reveal a surprising mobility in origin-licensing factors that confers resistance to the complex challenges posed by diverse obstacles encountered on chromosomes.

nightshift: A Python program for plotting simulated NMR spectra from assigned chemical shifts from the Biological Magnetic Resonance Data Bank

Nuclear magnetic resonance (NMR) provides site specific information on local environments through chemical shifts. NMR is widely used in the study of proteins, ranging from determination of three-dimensional (3D) structures to characterizing dynamics and binding of small molecules and other proteins or ligands. Assigned chemical shift data for the atoms within proteins is a treasure trove of information that can facilitate a broad range of biochemical and biophysical studies. The Biological Magnetic Resonance Data Bank (BMRB) is a publicly accessible database that contains a large number of assigned chemical shifts; however, translating this wealth of knowledge into a practical application is not straightforward. Herein we present nightshift: a Python command line utility and library for plotting simulated two-dimensional (2D) and 3D NMR spectra from assigned chemical shifts in the BMRB. This tool allows users to simulate routinely collected amide and methyl fingerprint spectra, backbone triple-resonance assignment spectra, and user-defined custom correlations, including ones that do not necessarily correspond to published experiments. This tool enables experienced NMR spectroscopists, those learning the craft, and interested scientists seeking to utilize NMR the ability to preview or examine a wide range of spectra for proteins whose assignments are deposited in the BMRB, irrespective of whether those experiments have been executed or reported. The tool applies equally to folded and intrinsically disordered proteins, limited only by the existence of a BMRB deposition. The features of nightshift are described along with applications that illustrate the ease with which complicated correlation spectra and binding events can be simulated.

Sortase-mediated segmental labeling: A method for segmental assignment of intrinsically disordered regions in proteins

A significant number of proteins possess sizable intrinsically disordered regions (IDRs). Due to the dynamic nature of IDRs, NMR spectroscopy is often the tool of choice for characterizing these segments. However, the application of NMR to IDRs is often hindered by their instability, spectral overlap and resonance assignment difficulties. Notably, these challenges increase considerably with the size of the IDR. In response to these issues, here we report the use of sortase-mediated ligation (SML) for segmental isotopic labeling of IDR-containing samples. Specifically, we have developed a ligation strategy involving a key segment of the large IDR and adjacent folded headpiece domain comprising the C-terminus of A. thaliana villin 4 (AtVLN4). This procedure significantly reduces the complexity of NMR spectra and enables group identification of signals arising from the labeled IDR fragment, a process we refer to as segmental assignment. The validity of our segmental assignment approach is corroborated by backbone residue-specific assignment of the IDR using a minimal set of standard heteronuclear NMR methods. Using segmental assignment, we further demonstrate that the IDR region adjacent to the headpiece exhibits nonuniform spectral alterations in response to temperature. Subsequent residue-specific characterization revealed two segments within the IDR that responded to temperature in markedly different ways. Overall, this study represents an important step toward the selective labeling and probing of target segments within much larger IDR contexts. Additionally, the approach described offers significant savings in NMR recording time, a valuable advantage for the study of unstable IDRs, their binding interfaces, and functional mechanisms.

Convenient Auto-Processing Vector Based on Bamboo Mosaic Virus for Presentation of Antigens Through Enzymatic Coupling

We have developed a new binary epitope-presenting CVP platform based on bamboo mosaic virus (BaMV) by using the sortase A (SrtA)-mediated ligation technology. The reconstructed BaMV genome harbors two modifications: 1) a coat protein (CP) with N-terminal extension of the tobacco etch virus (TEV) protease recognition site plus 4 extra glycine (G) residues as the SrtA acceptor; and 2) a TEV protease coding region replacing that of the triple-gene-block proteins. Inoculation of such construct, pKB5G, on Nicotiana benthamiana resulted in the efficient production of filamentous CVPs ready for SrtA-mediated ligation with desired proteins. The second part of the binary platform includes an expression vector for the bacterial production of donor proteins. We demonstrated the applicability of the platform by using the recombinant envelope protein domain III (rEDIII) of Japanese encephalitis virus (JEV) as the antigen. Up to 40% of the BaMV CP subunits in each CVP were loaded with rEDIII proteins in 1 min. The rEDIII-presenting BaMV CVPs (BJLPET5G) could be purified using affinity chromatography. Immunization assays confirmed that BJLPET5G could induce the production of neutralizing antibodies against JEV infections. The binary platform could be adapted as a useful alternative for the development and mass production of vaccine candidates.

Segmental and site-specific isotope labelling strategies for structural analysis of posttranslationally modified proteins

Posttranslational modifications can alter protein structures, functions and locations, and are important cellular regulatory and signalling mechanisms. Spectroscopic techniques such as nuclear magnetic resonance, infrared and Raman spectroscopy, as well as small-angle scattering, can provide insights into the structural and dynamic effects of protein posttranslational modifications and their impact on interactions with binding partners. However, heterogeneity of modified proteins from natural sources and spectral complexity often hinder analyses, especially for large proteins and macromolecular assemblies. Selective labelling of proteins with stable isotopes can greatly simplify spectra, as one can focus on labelled residues or segments of interest. Employing chemical biology tools for modifying and isotopically labelling proteins with atomic precision provides access to unique protein samples for structural biology and spectroscopy. Here, we review site-specific and segmental isotope labelling methods that are employed in combination with chemical and enzymatic tools to access posttranslationally modified proteins. We discuss illustrative examples in which these methods have been used to facilitate spectroscopic studies of posttranslationally modified proteins, providing new insights into biology.

Conserved allosteric ensembles in disordered proteins using TROSY/anti-TROSY R2-filtered spectroscopy

Defining the role of intrinsic disorder in proteins in the myriad of biological processes with which it is involved represents a significant goal in modern biophysics. Toward this end, NMR is uniquely suited for molecular studies of dynamic and disordered regions, but studying these regions in concert with their more structured domains and binding partners presents spectroscopic challenges. Here, we investigate the interactions between the structured and disordered regions of the human glucocorticoid receptor (GR). To do this, we developed an NMR strategy that relies on a novel relaxation filter for the simultaneous study of structured and unstructured regions. Using this approach, we conducted a comparative analysis of three translational isoforms of GR containing a folded DNA-binding domain (DBD) and two disordered regions that flank the DBD, one of which varies in size in the different isoforms. Notably, we were able to assign resonances that had previously been inaccessible because of the spectral complexity of the translational isoforms, which in turn allowed us to 1) identify a region of the structured DBD that undergoes significant changes in the local chemical environment in the presence of the disordered region and 2) determine differences in the conformational ensembles of the disordered regions of the translational isoforms. Furthermore, an ensemble-based thermodynamic analysis of the isoforms reveals conserved patterns of stability within the N-terminal domain of GR that persist despite low sequence conservation. These studies provide an avenue for further investigations of the mechanistic underpinnings of the functional relevance of the translational isoforms of GR while also providing a general NMR strategy for studying systems containing both structured and disordered regions.

Deuteration Aiming for Neutron Scattering

The distinguished feature of neutron as a scattering probe is an isotope effect, especially the large difference in neutron scattering length between hydrogen and deuterium. The difference renders the different visibility between hydrogenated and deuterated proteins. Therefore, the combination of deuterated protein and neutron scattering enables the selective visualization of a target domain in the complex or a target protein in the multi-component system. Despite of this fascinating character, there exist several problems for the general use of this method: difficulty and high cost for protein deuteration, and control and determination of deuteration ratio of the sample. To resolve them, the protocol of protein deuteration techniques is presented in this report. It is strongly expected that this protocol will offer more opportunity for conducting the neutron scattering studies with deuterated proteins.

Laboratory evolution of a sortase enzyme that modifies amyloid-β protein

Epitope-specific enzymes are powerful tools for site-specific protein modification but generally require genetic manipulation of the target protein. Here, we describe the laboratory evolution of the bacterial transpeptidase sortase A to recognize the LMVGG sequence in endogenous amyloid-β (Aβ) protein. Using a yeast display selection for covalent bond formation, we evolved a sortase variant that prefers LMVGG substrates from a starting enzyme that prefers LPESG substrates, resulting in a >1,400-fold change in substrate preference. We used this evolved sortase to label endogenous Aβ in human cerebrospinal fluid, enabling the detection of Aβ with sensitivities rivaling those of commercial assays. The evolved sortase can conjugate a hydrophilic peptide to Aβ₄₂, greatly impeding the ability of the resulting protein to aggregate into higher-order structures. These results demonstrate laboratory evolution of epitope-specific enzymes toward endogenous targets as a strategy for site-specific protein modification without target gene manipulation and enable potential future applications of sortase-mediated labeling of Aβ peptides.

Micellar TIA1 with folded RNA binding domains as a model for reversible stress granule formation

TIA1, a protein critical for eukaryotic stress response and stress granule formation, is structurally characterized in full-length form. TIA1 contains three RNA recognition motifs (RRMs) and a C-terminal low-complexity domain, sometimes referred to as a "prion-related domain" or associated with amyloid formation. Under mild conditions, full-length (fl) mouse TIA1 spontaneously oligomerizes to form a metastable colloid-like suspension. RRM2 and RRM3, known to be critical for function, are folded similarly in excised domains and this oligomeric form of apo fl TIA1, based on NMR chemical shifts. By contrast, the termini were not detected by NMR and are unlikely to be amyloid-like. We were able to assign the NMR shifts with the aid of previously assigned solution-state shifts for the RRM2,3 isolated domains and homology modeling. We present a micellar model of fl TIA1 wherein RRM2 and RRM3 are colocalized, ordered, hydrated, and available for nucleotide binding. At the same time, the termini are disordered and phase separated, reminiscent of stress granule substructure or nanoscale liquid droplets.

Deciphering protein post-translational modifications using chemical biology tools

Proteins carry out a wide variety of catalytic, regulatory, signalling and structural functions in living systems. Following their assembly on ribosomes and throughout their lifetimes, most eukaryotic proteins are modified by post-translational modifications; small functional groups and complex biomolecules are conjugated to amino acid side chains or termini, and the protein backbone is cleaved, spliced or cyclized, to name just a few examples. These modifications modulate protein activity, structure, location and interactions, and, thereby, control many core biological processes. Aberrant post-translational modifications are markers of cellular stress or malfunction and are implicated in several diseases. Therefore, gaining an understanding of which proteins are modified, at which sites and the resulting biological consequences is an important but complex challenge requiring interdisciplinary approaches. One of the key challenges is accessing precisely modified proteins to assign functional consequences to specific modifications. Chemical biologists have developed a versatile set of tools for accessing specifically modified proteins by applying robust chemistries to biological molecules and developing strategies for synthesizing and ligating proteins. This Review provides an overview of these tools, with selected recent examples of how they have been applied to decipher the roles of a variety of protein post-translational modifications. Relative advantages and disadvantages of each of the techniques are discussed, highlighting examples where they are used in combination and have the potential to address new frontiers in understanding complex biological processes.

Efficient Sortase-Mediated Ligation Using a Common C-Terminal Fusion Tag

Sortase-mediated ligation is a powerful method for generating site-specifically modified proteins. However, this process is limited by the inherent reversibility of the ligation reaction. To address this, here we report the continued development and optimization of an experimentally facile strategy for blocking reaction reversibility. This approach, which we have termed metal-assisted sortase-mediated ligation (MA-SML), relies on the use of a solution additive (Ni2+) and a C-terminal tag (LPXTGGHH5) that is widely used for converting protein targets into sortase substrates. In a series of model systems utilizing a 1:1 molar ratio of sortase substrate and glycine amine nucleophile, we find that MA-SML consistently improves the extent of ligation. This enables the modification of proteins with fluorophores, PEG, and a bioorthogonal cyclooctyne moiety without the need to use precious reagents in excess. Overall, these results demonstrate the potential of MA-SML as a general strategy for improving reaction efficiency in a broad range of sortase-based protein engineering applications.

Combining molecular dynamics simulations with small-angle X-ray and neutron scattering data to study multi-domain proteins in solution

Many proteins contain multiple folded domains separated by flexible linkers, and the ability to describe the structure and conformational heterogeneity of such flexible systems pushes the limits of structural biology. Using the three-domain protein TIA-1 as an example, we here combine coarse-grained molecular dynamics simulations with previously measured small-angle scattering data to study the conformation of TIA-1 in solution. We show that while the coarse-grained potential (Martini) in itself leads to too compact conformations, increasing the strength of protein-water interactions results in ensembles that are in very good agreement with experiments. We show how these ensembles can be refined further using a Bayesian/Maximum Entropy approach, and examine the robustness to errors in the energy function. In particular we find that as long as the initial simulation is relatively good, reweighting against experiments is very robust. We also study the relative information in X-ray and neutron scattering experiments and find that refining against the SAXS experiments leads to improvement in the SANS data. Our results suggest a general strategy for studying the conformation of multi-domain proteins in solution that combines coarse-grained simulations with small-angle X-ray scattering data that are generally most easy to obtain. These results may in turn be used to design further small-angle neutron scattering experiments that exploit contrast variation through 1H/2H isotope substitutions.

NMR Structure and Dynamics of TonB Investigated by Scar-Less Segmental Isotopic Labeling Using a Salt-Inducible Split Intein

The growing understanding of partially unfolded proteins increasingly points to their biological relevance in allosteric regulation, complex formation, and protein design. However, the structural characterization of disordered proteins remains challenging. NMR methods can access both the dynamics and structures of such proteins, yet suffering from a high degeneracy of NMR signals. Here, we overcame this bottleneck utilizing a salt-inducible split intein to produce segmentally isotope-labeled samples with the native sequence, including the ligation junction. With this technique, we investigated the NMR structure and conformational dynamics of TonB from Helicobacter pylori in the presence of a proline-rich low complexity region. Spin relaxation experiments suggest that the several nano-second time scale dynamics of the C-terminal domain (CTD) is almost independent of the faster pico-to-nanosecond dynamics of the low complexity central region (LCCR). Our results demonstrate the utility of segmental isotopic labeling for proteins with heterogenous dynamics such as TonB and could advance NMR studies of other partially unfolded proteins.

Staufen1 reads out structure and sequence features in ARF1 dsRNA for target recognition

Staufen1 (STAU1) is a dsRNA binding protein mediating mRNA transport and localization, translational control and STAU1-mediated mRNA decay (SMD). The STAU1 binding site (SBS) within human ADP-ribosylation factor1 (ARF1) 3′UTR binds STAU1 and this downregulates ARF1 cytoplasmic mRNA levels by SMD. However, how STAU1 recognizes specific mRNA targets is still under debate. Our structure of the ARF1 SBS–STAU1 complex uncovers target recognition by STAU1. STAU1 dsRNA binding domain (dsRBD) 4 interacts with two pyrimidines and one purine from the minor groove side via helix α1, the β1–β2 loop anchors the dsRBD at the end of the dsRNA and lysines in helix α2 bind to the phosphodiester backbone from the major groove side. STAU1 dsRBD3 displays the same binding mode with specific recognition of one guanine base. Mutants disrupting minor groove recognition of ARF1 SBS affect in vitro binding and reduce SMD in vivo. Our data thus reveal how STAU1 recognizes minor groove features in dsRNA relevant for target selection.

Methyl TROSY spectroscopy: A versatile NMR approach to study challenging biological systems

A major goal in structural biology is to unravel how molecular machines function in detail. To that end, solution-state NMR spectroscopy is ideally suited as it is able to study biological assemblies in a near natural environment. Based on methyl TROSY methods, it is now possible to record high-quality data on complexes that are far over 100 kDa in molecular weight. In this review, we discuss the theoretical background of methyl TROSY spectroscopy, the information that can be extracted from methyl TROSY spectra and approaches that can be used to assign methyl resonances in large complexes. In addition, we touch upon insights that have been obtained for a number of challenging biological systems, including the 20S proteasome, the RNA exosome, molecular chaperones and G-protein-coupled receptors. We anticipate that methyl TROSY methods will be increasingly important in modern structural biology approaches, where information regarding static structures is complemented with insights into conformational changes and dynamic intermolecular interactions.

Optimization of sortase A ligation for flexible engineering of complex protein systems

Engineering and bioconjugation of proteins is a critically valuable tool that can facilitate a wide range of biophysical and structural studies. The ability to orthogonally tag or label a domain within a multidomain protein may be complicated by undesirable side reactions to noninvolved domains. Furthermore, the advantages of segmental (or domain-specific) isotopic labeling for NMR, or deuteration for neutron scattering or diffraction, can be realized by an efficient ligation procedure. Common methods-expressed protein ligation, protein trans-splicing, and native chemical ligation-each have specific limitations. Here, we evaluated the use of different variants of Staphylococcus aureus sortase A for a range of ligation reactions and demonstrate that conditions can readily be optimized to yield high efficiency (i.e. completeness of ligation), ease of purification, and functionality in detergents. These properties may enable joining of single domains into multidomain proteins, lipidation to mimic posttranslational modifications, and formation of cyclic proteins to aid in the development of nanodisc membrane mimetics. We anticipate that the method for ligating separate domains into a single functional multidomain protein reported here may enable many applications in structural biology.

Small-Angle Neutron Scattering of RNA-Protein Complexes

Small-angle neutron scattering (SANS) provides structural information on biomacromolecules and their complexes in dilute solutions at the nanometer length scale. The overall dimensions, shapes, and interactions can be probed and compared to information obtained by complementary structural biology techniques such as crystallography, NMR, and EM. SANS, in combination with solvent H₂O/D₂O exchange and/or deuteration, is particularly well suited to probe the internal structure of RNA-protein (RNP) complexes since neutrons are more sensitive than X-rays to the difference in scattering length densities of proteins and RNA, with respect to an aqueous solvent. In this book chapter we provide a practical guide on how to carry out SANS experiments on RNP complexes, as well as possibilities of data analysis and interpretation.

Natural Occurring and Engineered Enzymes for Peptide Ligation and Cyclization

The renaissance of peptides as prospective therapeutics has fostered the development of novel strategies for their synthesis and modification. In this context, besides the development of new chemical peptide ligation approaches, especially the use of enzymes as a versatile tool has gained increased attention. Nowadays, due to their inherent properties such as excellent regio- and chemoselectivity, enzymes represent invaluable instruments in both academic and industrial laboratories. This mini-review focuses on natural- and engineered peptide ligases that can form a new peptide (amide) bond between the C-terminal carboxy and N-terminal amino group of a peptide and/or protein. The pro's and cons of several enzyme classes such as Sortases, Asparaginyl Endoproteases, Trypsin related enzymes and as a central focus subtilisin-derived variants are summarized. Most recent developments with regards to ligation and cyclization are highlighted.

Domain selective labeling for NMR studies of multidomain proteins by domain ligation using highly active sortase A

Structural study of multidomain proteins using NMR is an emerging issue for understanding biological functions. To this end, domain-specific labeling is expected to be a key technology for facilitating the NMR-assignment process and for collecting distance information via spin labeling. To obtain domain-specific labeled samples, use of sortase A as a protein ligation tool is a viable approach. Sortase A enables ligation of separately expressed proteins (domains) through the Leu-Pro-X-Thr-Gly linker. However, the ligation reaction mediated by sortase A is not efficient. Poor yield and long reaction times hamper large-scale preparation using sortase A. Here we report the application of highly active sortases to NMR analyses. Optimal yields can be achieved within several hours when the ligation reaction are mediated by highly active sortases at 4 °C. We propose that this protocol can contribute to structural analyses of multidomain proteins by NMR.

Protein Structure Determination in Living Cells

To date, in-cell NMR has elucidated various aspects of protein behaviour by associating structures in physiological conditions. Meanwhile, current studies of this method mostly have deduced protein states in cells exclusively based on 'indirect' structural information from peak patterns and chemical shift changes but not 'direct' data explicitly including interatomic distances and angles. To fully understand the functions and physical properties of proteins inside cells, it is indispensable to obtain explicit structural data or determine three-dimensional (3D) structures of proteins in cells. Whilst the short lifetime of cells in a sample tube, low sample concentrations, and massive background signals make it difficult to observe NMR signals from proteins inside cells, several methodological advances help to overcome the problems. Paramagnetic effects have an outstanding potential for in-cell structural analysis. The combination of a limited amount of experimental in-cell data with software for ab initio protein structure prediction opens an avenue to visualise 3D protein structures inside cells. Conventional nuclear Overhauser effect spectroscopy (NOESY)-based structure determination is advantageous to elucidate the conformations of side-chain atoms of proteins as well as global structures. In this article, we review current progress for the structure analysis of proteins in living systems and discuss the feasibility of its future works.

Broadening the scope of sortagging

Sortases are enzymes occurring in the cell wall of Gram-positive bacteria. Sortase A (SrtA), the best studied sortase class, plays a key role in anchoring surface proteins with the recognition sequence LPXTG covalently to oligoglycine units of the bacterial cell wall. This unique transpeptidase activity renders SrtA attractive for various purposes and motivated researchers to study multiple in vivo and in vitro ligations in the last decades. This ligation technique is known as sortase-mediated ligation (SML) or sortagging and developed to a frequently used method in basic research. The advantages are manifold: extremely high substrate specificity, simple access to substrates and enzyme, robust nature and easy handling of sortase A. In addition to the ligation of two proteins or peptides, early studies already included at least one artificial (peptide equipped) substrate into sortagging reactions - which demonstrates the versatility and broad applicability of SML. Thus, SML is not only a biology-related technique, but has found prominence as a major interdisciplinary research tool. In this review, we provide an overview about the use of sortase A in interdisciplinary research, mainly for protein modification, synthesis of protein-polymer conjugates and immobilization of proteins on surfaces.

Neutron scattering in the biological sciences: progress and prospects

The scattering of neutrons can be used to provide information on the structure and dynamics of biological systems on multiple length and time scales. Pursuant to a National Science Foundation-funded workshop in February 2018, recent developments in this field are reviewed here, as well as future prospects that can be expected given recent advances in sources, instrumentation and computational power and methods. Crystallography, solution scattering, dynamics, membranes, labeling and imaging are examined. For the extraction of maximum information, the incorporation of judicious specific deuterium labeling, the integration of several types of experiment, and interpretation using high-performance computer simulation models are often found to be particularly powerful.

Conversion of an amide to a high-energy thioester by Staphylococcus aureus sortase A is powered by variable binding affinity for calcium

Thioesters are key intermediates in biology, which often are generated from less energy-rich amide precursors. Staphylococcus aureus sortase A (SrtA) is an enzyme widely used in biotechnology for peptide ligation. The reaction proceeds in two steps, where the first step involves the conversion of an amide bond of substrate peptide into a thioester intermediate with the enzyme. Here we show that the free energy required for this step is matched by an about 30-fold increase in binding affinity of a calcium ion at the calcium binding site of SrtA, which is remote from the thioester bond. The magnitude of this allosteric effect highlights the importance of calcium for the activity of SrtA. The increase in calcium binding affinity upon binding of substrate not only achieves catalytic formation of an energy-rich intermediate in the absence of nucleotide triphosphates or any tight non-covalent enzyme-substrate interactions, but is also accompanied by accumulation of the labile thioester intermediate, which makes it directly observable in nuclear magnetic resonance (NMR) spectra.

The role of small-angle scattering in structure-based screening applications

In many biomolecular interactions, changes in the assembly states and structural conformations of participants can act as a complementary reporter of binding to functional and thermodynamic assays. This structural information is captured by a number of structural biology and biophysical techniques that are viable either as primary screens in small-scale applications or as secondary screens to complement higher throughput methods. In particular, small-angle X-ray scattering (SAXS) reports the average distance distribution between all atoms after orientational averaging. Such information is important when for example investigating conformational changes involved in inhibitory and regulatory mechanisms where binding events do not necessarily cause functional changes. Thus, we summarise here the current and prospective capabilities of SAXS-based screening in the context of other methods that yield structural information. Broad guidelines are also provided to assist readers in preparing screening protocols that are tailored to available X-ray sources.

The MinDE system is a generic spatial cue for membrane protein distribution in vitro

The E. coli MinCDE system has become a paradigmatic reaction-diffusion system in biology. The membrane-bound ATPase MinD and ATPase-activating protein MinE oscillate between the cell poles followed by MinC, thus positioning the main division protein FtsZ at midcell. Here we report that these energy-consuming MinDE oscillations may play a role beyond constraining MinC/FtsZ localization. Using an in vitro reconstitution assay, we show that MinDE self-organization can spatially regulate a variety of functionally completely unrelated membrane proteins into patterns and gradients. By concentration waves sweeping over the membrane, they induce a direct net transport of tightly membrane-attached molecules. That the MinDE system can spatiotemporally control a much larger set of proteins than previously known, may constitute a MinC-independent pathway to division site selection and chromosome segregation. Moreover, the here described phenomenon of active transport through a traveling diffusion barrier may point to a general mechanism of spatiotemporal regulation in cells.

Sortase A: A Model for Transpeptidation and Its Biological Applications

Molecular biologists and chemists alike have long sought to modify proteins with substituents that cannot be installed by standard or even advanced genetic approaches. We here describe the use of transpeptidases to achieve these goals. Living systems encode a variety of transpeptidases and peptide ligases that allow for the enzyme-catalyzed formation of peptide bonds, and protein engineers have used directed evolution to enhance these enzymes for biological applications. We focus primarily on the transpeptidase sortase A, which has become popular over the past few years for its ability to perform a remarkably wide variety of protein modifications, both in vitro and in living cells.

Capturing dynamic conformational shifts in protein–ligand recognition using integrative structural biology in solution

In recent years, a dynamic view of the structure and function of biological macromolecules is emerging, highlighting an essential role of dynamic conformational equilibria to understand molecular mechanisms of biological functions. The structure of a biomolecule, i.e. protein or nucleic acid in solution, is often best described as a dynamic ensemble of conformations, rather than a single structural state. Strikingly, the molecular interactions and functions of the biological macromolecule can then involve a shift between conformations that pre-exist in such an ensemble. Upon external cues, such population shifts of pre-existing conformations allow gradually relaying the signal to the downstream biological events. An inherent feature of this principle is conformational dynamics, where intrinsically disordered regions often play important roles to modulate the conformational ensemble. Unequivocally, solution-state NMR spectroscopy is a powerful technique to study the structure and dynamics of such biomolecules in solution. NMR is increasingly combined with complementary techniques, including fluorescence spectroscopy and small angle scattering. The combination of these techniques provides complementary information about the conformation and dynamics in solution and thus affords a comprehensive description of biomolecular functions and regulations. Here, we illustrate how an integrated approach combining complementary techniques can assess the structure and dynamics of proteins and protein complexes in solution.

A molecular engineering toolbox for the structural biologist

Exciting new technological developments have pushed the boundaries of structural biology, and have enabled studies of biological macromolecules and assemblies that would have been unthinkable not long ago. Yet, the enhanced capabilities of structural biologists to pry into the complex molecular world have also placed new demands on the abilities of protein engineers to reproduce this complexity into the test tube. With this challenge in mind, we review the contents of the modern molecular engineering toolbox that allow the manipulation of proteins in a site-specific and chemically well-defined fashion. Thus, we cover concepts related to the modification of cysteines and other natural amino acids, native chemical ligation, intein and sortase-based approaches, amber suppression, as well as chemical and enzymatic bio-conjugation strategies. We also describe how these tools can be used to aid methodology development in X-ray crystallography, nuclear magnetic resonance, cryo-electron microscopy and in the studies of dynamic interactions. It is our hope that this monograph will inspire structural biologists and protein engineers alike to apply these tools to novel systems, and to enhance and broaden their scope to meet the outstanding challenges in understanding the molecular basis of cellular processes and disease.