ROS-dependent S-palmitoylation activates cleaved and intact gasdermin D.

Gasdermin D (GSDMD) is the common effector for cytokine secretion and pyroptosis downstream of inflammasome activation and was previously shown to form large transmembrane pores after cleavage by inflammatory caspases to generate the GSDMD N-terminal domain (GSDMD-NT)1-10. Here we report that GSDMD Cys191 is S-palmitoylated and that palmitoylation is required for pore formation. S-palmitoylation, which does not affect GSDMD cleavage, is augmented by mitochondria-generated reactive oxygen species (ROS). Cleavage-deficient GSDMD (D275A) is also palmitoylated after inflammasome stimulation or treatment with ROS activators and causes pyroptosis, although less efficiently than palmitoylated GSDMD-NT. Palmitoylated, but not unpalmitoylated, full-length GSDMD induces liposome leakage and forms a pore similar in structure to GSDMD-NT pores shown by cryogenic electron microscopy. ZDHHC5 and ZDHHC9 are the major palmitoyltransferases that mediate GSDMD palmitoylation, and their expression is upregulated by inflammasome activation and ROS. The other human gasdermins are also palmitoylated at their N termini. These data challenge the concept that cleavage is the only trigger for GSDMD activation. They suggest that reversible palmitoylation is a checkpoint for pore formation by both GSDMD-NT and intact GSDMD that functions as a general switch for the activation of this pore-forming family.

Orthogonal Enzyme-Substrate Design Strategy for Discovery of Human Protein Palmitoyltransferase Substrates.

Protein palmitoylation, with more than 5000 substrates, is the most prevalent form of protein lipidation. Palmitoylated proteins participate in almost all areas of cellular physiology and have been linked to several human diseases. Twenty-three zDHHC enzymes catalyze protein palmitoylation with extensive overlap among the substrates of each zDHHC member. Currently, there is no global strategy to delineate the physiological substrates of individual zDHHC enzymes without perturbing the natural cellular pool. Here, we outline a general approach to accomplish this on the basis of synthetic orthogonal substrates that are only compatible with engineered zDHHC enzymes. We demonstrate the utility of this strategy by validating known substrates and use it to identify novel substrates of two human zDHHC enzymes. Finally, we employ this method to discover and explore conserved palmitoylation in a family of host restriction factors against pathogenic viruses, including SARS-CoV-2.

ROS-dependent palmitoylation is an obligate licensing modification for GSDMD pore formation.

Gasdermin D (GSDMD) is the common effector for cytokine secretion and pyroptosis downstream of inflammasome activation by forming large transmembrane pores upon cleavage by inflammatory caspases. Here we report the surprising finding that GSDMD cleavage is not sufficient for its pore formation. Instead, GSDMD is lipidated by S-palmitoylation at Cys191 upon inflammasome activation, and only palmitoylated GSDMD N-terminal domain (GSDMD-NT) is capable of membrane translocation and pore formation, suggesting that palmitoylation licenses GSDMD activation. Treatment by the palmitoylation inhibitor 2-bromopalmitate and alanine mutation of Cys191 abrogate GSDMD membrane localization, cytokine secretion, and cell death, without affecting GSDMD cleavage. Because palmitoylation is formed by a reversible thioester bond sensitive to free thiols, we tested if GSDMD palmitoylation is regulated by cellular redox state. Lipopolysaccharide (LPS) mildly and LPS plus the NLRP3 inflammasome activator nigericin markedly elevate reactive oxygen species (ROS) and GSDMD palmitoylation, suggesting that these two processes are coupled. Manipulation of cellular ROS by its activators and quenchers augment and abolish, respectively, GSDMD palmitoylation, GSDMD pore formation and cell death. We discover that zDHHC5 and zDHHC9 are the major palmitoyl transferases that mediate GSDMD palmitoylation, and when cleaved, recombinant and partly palmitoylated GSDMD is 10-fold more active in pore formation than bacterially expressed, unpalmitoylated GSDMD, evidenced by liposome leakage assay. Finally, other GSDM family members are also palmitoylated, suggesting that ROS stress and palmitoylation may be a general switch for the activation of this pore-forming family. One-Sentence Summary: GSDMD palmitoylation is induced by ROS and required for pore formation.

Access and utilization of long chain fatty acyl-CoA by zDHHC protein acyltransferases.

S-acylation is a reversible posttranslational modification, where a long-chain fatty acid is attached to a protein through a thioester linkage. Being the most abundant form of lipidation in humans, a family of twenty-three human zDHHC integral membrane enzymes catalyze this reaction. Previous structures of the apo and lipid bound zDHHCs shed light into the molecular details of the active site and binding pocket. Here, we delve further into the details of fatty acyl-CoA recognition by zDHHC acyltransferases using insights from the recent structure. We additionally review indirect evidence that suggests acyl-CoAs do not diffuse freely in the cytosol, but are channeled into specific pathways, and comment on the suggested mechanisms for fatty acyl-CoA compartmentalization and intracellular transport, to finally speculate about the potential mechanisms that underlie fatty acyl-CoA delivery to zDHHC enzymes.

New Horizons in Structural Biology of Membrane Proteins: Experimental Evaluation of the Role of Conformational Dynamics and Intrinsic Flexibility.

A plethora of membrane proteins are found along the cell surface and on the convoluted labyrinth of membranes surrounding organelles. Since the advent of various structural biology techniques, a sub-population of these proteins has become accessible to investigation at near-atomic resolutions. The predominant bona fide methods for structure solution, X-ray crystallography and cryo-EM, provide high resolution in three-dimensional space at the cost of neglecting protein motions through time. Though structures provide various rigid snapshots, only an amorphous mechanistic understanding can be inferred from interpolations between these different static states. In this review, we discuss various techniques that have been utilized in observing dynamic conformational intermediaries that remain elusive from rigid structures. More specifically we discuss the application of structural techniques such as NMR, cryo-EM and X-ray crystallography in studying protein dynamics along with complementation by conformational trapping by specific binders such as antibodies. We finally showcase the strength of various biophysical techniques including FRET, EPR and computational approaches using a multitude of succinct examples from GPCRs, transporters and ion channels.

S-acylation of SARS-CoV-2 spike protein: Mechanistic dissection, in vitro reconstitution and role in viral infectivity.

S-acylation, also known as palmitoylation, is the most widely prevalent form of protein lipidation, whereby long-chain fatty acids get attached to cysteine residues facing the cytosol. In humans, 23 members of the zDHHC family of integral membrane enzymes catalyze this modification. S-acylation is critical for the life cycle of many enveloped viruses. The Spike protein of SARS-CoV-2, the causative agent of COVID-19, has the most cysteine-rich cytoplasmic tail among known human pathogens in the closely related family of ß-coronaviruses; however, it is unclear which of the cytoplasmic cysteines are S-acylated, and what the impact of this modification is on viral infectivity. Here we identify specific cysteine clusters in the Spike protein of SARS-CoV-2 that are targets of S-acylation. Interestingly, when we investigated the effect of the cysteine clusters using pseudotyped virus, mutation of the same three clusters of cysteines severely compromised viral infectivity. We developed a library of expression constructs of human zDHHC enzymes and used them to identify zDHHC enzymes that can S-acylate SARS-CoV-2 Spike protein. Finally, we reconstituted S-acylation of SARS-CoV-2 Spike protein in vitro using purified zDHHC enzymes. We observe a striking heterogeneity in the S-acylation status of the different cysteines in our in cellulo experiments, which, remarkably, was recapitulated by the in vitro assay. Altogether, these results bolster our understanding of a poorly understood posttranslational modification integral to the SARS-CoV-2 Spike protein. This study opens up avenues for further mechanistic dissection and lays the groundwork toward developing future strategies that could aid in the identification of targeted small-molecule modulators.

Production of Recombinant Transmembrane Proteins from Mammalian Cells for Biochemical and Structural Analyses.

Eukaryotic integral membrane proteins are key components of various biological processes. Because they are implicated in multiple diseases, it is important to understand their mechanism of action by elucidating their structure and function. Complex technical challenges associated with the generation of recombinant membrane proteins severely impair our ability to understand them using structural and biochemical methods. Here, we provide a detailed procedure to address and mitigate difficulties involved in the large-scale heterologous overexpression and purification of eukaryotic membrane proteins using HEK293S GnTi- cells transduced with baculovirus. Two human proteins, hDHHC15 and hPORCN, are presented as examples, with step-by-step instructions for transient transfection and generation of baculoviruses, followed by overexpression and purification from HEK293S GnTi- cells. © 2020 Wiley Periodicals LLC. Basic Protocol 1: Small-scale protein expression in mammalian HEK293T cells Basic Protocol 2: Generation of baculovirus from Sf9 (insect) cells Alternate Protocol: Enumeration-free method for generating P2 viral stock Support Protocol 1: Small-scale transduction of HEK293T cells with P2 baculovirus Basic Protocol 3: Large-scale viral transduction of HEK293S GnTi- cells Support Protocol 2: Large-scale membrane preparation from HEK293S GnTi- cells Basic Protocol 4: Large-scale purification of membrane proteins from HEK293S GnTi- cells.

Solution NMR: A powerful tool for structural and functional studies of membrane proteins in reconstituted environments.

A third of the genes in prokaryotic and eukaryotic genomes encode membrane proteins that are either essential for signal transduction and solute transport or function as scaffold structures. Unlike many of their soluble counterparts, the overall structural and functional organization of membrane proteins is sparingly understood. Recent advances in X-ray crystallography, cryo-EM, and nuclear magnetic resonance (NMR) are closing this gap by enabling an in-depth view of these ever-elusive proteins at atomic resolution. Despite substantial technological advancements, however, the overall proportion of membrane protein entries in the Protein Data Bank (PDB) remains <4%. This paucity is mainly attributed to difficulties associated with their expression and purification, propensity to form large multisubunit complexes, and challenges pertinent to identification of an ideal detergent, lipid, or detergent/lipid mixture that closely mimic their native environment. NMR is a powerful technique to obtain atomic-resolution and dynamic details of a protein in solution. This is accomplished through an assortment of isotopic labeling schemes designed to acquire multiple spectra that facilitate deduction of the final protein structure. In this review, we discuss current approaches and technological developments in the determination of membrane protein structures by solution NMR and highlight recent structural and mechanistic insights gained with this technique. We also discuss strategies for overcoming size limitations in NMR applications, and we explore a plethora of membrane mimetics available for the structural and mechanistic understanding of these essential cellular proteins.

The major outer sheath protein forms distinct conformers and multimeric complexes in the outer membrane and periplasm of Treponema denticola.

The major outer sheath protein (MOSP) is a prominent constituent of the cell envelope of Treponema denticola (TDE) and one of its principal virulence determinants. Bioinformatics predicts that MOSP consists of N- and C-terminal domains, MOSPN and MOSPC. Biophysical analysis of constructs refolded in vitro demonstrated that MOSPC, previously shown to possess porin activity, forms amphiphilic trimers, while MOSPN forms an extended hydrophilic monomer. In TDE and E. coli expressing MOSP with a PelB signal sequence (PelB-MOSP), MOSPC is OM-embedded and surface-exposed, while MOSPN resides in the periplasm. Immunofluorescence assay, surface proteolysis, and novel cell fractionation schemes revealed that MOSP in TDE exists as outer membrane (OM) and periplasmic trimeric conformers; PelB-MOSP, in contrast, formed only OM-MOSP trimers. Although both conformers form hetero-oligomeric complexes in TDE, only OM-MOSP associates with dentilisin. Mass spectrometry (MS) indicated that OM-MOSP interacts with proteins in addition to dentilisin, most notably, oligopeptide-binding proteins (OBPs) and the ß-barrel of BamA. MS also identified candidate partners for periplasmic MOSP, including TDE1658, a spirochete-specific SurA/PrsA ortholog. Collectively, our data suggest that MOSP destined for the TDE OM follows the canonical BAM pathway, while formation of a stable periplasmic conformer involves an export-related, folding pathway not present in E. coli.

The butyrophilin 3A1 intracellular domain undergoes a conformational change involving the juxtamembrane region.

Small isoprenoid diphosphates, such as (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate (HMBPP), are ligands of the internal domain of BTN3A1. Ligand binding in target cells promotes activation of Vγ9Vδ2 T cells. We demonstrate by small-angle X-ray scattering (SAXS) that HMBPP binding to the internal domain of BTN3A1 induces a conformational change in the position of the B30.2 domain relative to the juxtamembrane (JM) region. To better understand the molecular details of this conformational rearrangement, NMR spectroscopy was used to discover that the JM region interacts with HMBPP, specifically at the diphosphate. The spectral location of the affected amide peaks, partial NMR assignments, and JM mutants (ST296AA or T304A) investigated, confirm that the backbone amide of at least one Thr (Thr304), adjacent to conserved Ser, comes close to the HMBPP diphosphate, whereas double mutation of nonconserved residues (Ser/Thr296/297) may perturb the local fold. Cellular mutation of either of the identified Thr residues reduces the activation of Vγ9Vδ2 T cells by HMBPP, zoledronate, and POM2-C-HMBP, but not by a partial agonist BTN3 antibody. Taken together, our results show that ligand binding to BTN3A1 induces a conformational change within the intracellular domain that involves the JM region and is required for full activation.-Nguyen, K., Li, J., Puthenveetil, R., Lin, X., Poe, M. M., Hsiao, C.-H. C., Vinogradova, O., Wiemer, A. J. The butyrophilin 3A1 intracellular domain undergoes a conformational change involving the juxtamembrane region.

Nanodiscs and Solution NMR: preparation, application and challenges.

Nanodiscs provide an excellent system for the structure-function investigation of membrane proteins. Its direct advantage lies in presenting a water soluble form of an otherwise hydrophobic molecule, making it amenable to a plethora of solution techniques. Nuclear Magnetic Resonance is one such high resolution approach that looks at the structure and dynamics of a protein with atomic level precision. Recently, there has been a breakthrough in making nanodiscs more susceptible for structure determination by solution NMR, yet it still remains to become the preferred choice for a membrane mimetic. In this practical review, we provide a general discourse on nanodisc and its application to solution NMR. We also offer potential solutions to remediate the technical challenges associated with nanodisc preparation and the choice of proper experimental set-ups. Along with discussing several structural applications, we demonstrate an alternative use of nanodiscs for functional studies, where we investigated the phosphorylation of a cell surface receptor, Integrin. This is the first successful manifestation of observing activated receptor phosphorylation in nanodiscs through NMR. We additionally present an on-column method for nanodisc preparation with multiple strategies and discuss the potential use of alternative nanoscale phospholipid bilayer systems like SMA lipid discs and Saposin-A lipoprotein discs.

Investigation of the adaptor protein PLIC-2 in multiple pathways.

PLIC, Protein Linking IAP (CD47) to Cytoskeleton, have long since been implicated in connecting the extracellular membrane to the intracellular cell cytoskeleton. This phenomenon is supposedly achieved by bridging a receptor protein CD47 to vimentin, an intermediate filament, which in turn regulates integrin dependent cell spreading. Since the discovery of these proteins, the molecular details of the above-mentioned interactions and the underlying complexes are yet to be characterized. Several independent studies have together emphasized PLIC/Ubiquilin's role in the proteasomal degradation pathway. This seems to be in contrast to the purported initial discovery of PLIC as a cytoskeletal adaptor protein. In an effort to reconcile the different roles associated with the ubiquitous PLIC proteins, we tested the involvement of PLIC-2 both in the proteasomal degradation pathway and as a protein linking the cell cytoskeleton to the cytoplasmic tail of CD47. This was achieved thorough an in vitro investigation of their binding interface using a combination of biophysical techniques. Our results show that the two terminal domains of PLIC-2 interact weakly with each other, while the C-terminal UBA domain interacts strongly with ubiquitin. Interestingly, no perceptible interaction was observed for PLIC-2 with the cytoplasmic tail of CD47 questioning its role as a "PLIC" protein linking the cell membrane to the cytoskeleton.

Bipartite Topology of Treponema pallidum Repeat Proteins C/D and I: OUTER MEMBRANE INSERTION, TRIMERIZATION, AND PORIN FUNCTION REQUIRE A C-TERMINAL ß-BARREL DOMAIN.

We previously identified Treponema pallidum repeat proteins TprC/D, TprF, and TprI as candidate outer membrane proteins (OMPs) and subsequently demonstrated that TprC is not only a rare OMP but also forms trimers and has porin activity. We also reported that TprC contains N- and C-terminal domains (TprC(N) and TprC(C)) orthologous to regions in the major outer sheath protein (MOSP(N) and MOSP(C)) of Treponema denticola and that TprC(C) is solely responsible for ß-barrel formation, trimerization, and porin function by the full-length protein. Herein, we show that TprI also possesses bipartite architecture, trimeric structure, and porin function and that the MOSP(C)-like domains of native TprC and TprI are surface-exposed in T. pallidum, whereas their MOSP(N)-like domains are tethered within the periplasm. TprF, which does not contain a MOSP(C)-like domain, lacks amphiphilicity and porin activity, adopts an extended inflexible structure, and, in T. pallidum, is tightly bound to the protoplasmic cylinder. By thermal denaturation, the MOSP(N) and MOSP(C)-like domains of TprC and TprI are highly thermostable, endowing the full-length proteins with impressive conformational stability. When expressed in Escherichia coli with PelB signal sequences, TprC and TprI localize to the outer membrane, adopting bipartite topologies, whereas TprF is periplasmic. We propose that the MOSP(N)-like domains enhance the structural integrity of the cell envelope by anchoring the ß-barrels within the periplasm. In addition to being bona fide T. pallidum rare outer membrane proteins, TprC/D and TprI represent a new class of dual function, bipartite bacterial OMP.

Structural insights into the recognition of ß3 integrin cytoplasmic tail by the SH3 domain of Src kinase.

Src kinase plays an important role in integrin signaling by regulating cytoskeletal organization and cell remodeling. Previous in vivo studies have revealed that the SH3 domain of c-Src kinase directly associates with the C-terminus of ß3 integrin cytoplasmic tail. Here, we explore this binding interface with a combination of different spectroscopic and computational methods. Chemical shift mapping, PRE, transferred NOE and CD data were used to obtain a docked model of the complex. This model suggests a different binding mode from the one proposed through previous studies wherein, the C-terminal end of ß3 spans the region in between the RT and n-Src loops of SH3 domain. Furthermore, we show that tyrosine phosphorylation of ß3 prevents this interaction, supporting the notion of a constitutive interaction between ß3 integrin and Src kinase.

Optimization of the design and preparation of nanoscale phospholipid bilayers for its application to solution NMR.

Despite arduous efforts and recent technological developments structural investigation of integral membrane proteins remains a challenge. The primary deterrents include difficulties with their expression, low inherent solubility, and problems associated with existing membrane mimicking systems. A relatively new class of membrane mimetics, nanodiscs, is emerging as a promising alternative. Although nanodiscs have been proven successful for several biophysical applications, they yet remain to become the system of preferred choice for structure determination. We have hereby made nanodiscs more suitable for solution NMR applications by reducing the diameter of the self-assembly complex to its potential limit. We achieved a noticeable improvement in the quality of NMR spectra obtained for the transmembrane and cytoplasmic domains of integrin αIIb incorporated into these smaller discs rendering them susceptible for a thorough structural investigation. In addition, we also present an on-column method for a rapid, efficient, single-step preparation of protein incorporated nanodiscs at high concentrations. These discs have been fully characterized by transmission electron microscopy, dynamic light scattering, and differential scanning calorimetry.