Akt Kinase Activation Mechanisms Revealed Using Protein Semisynthesis.

Akt is a critical protein kinase that drives cancer proliferation, modulates metabolism, and is activated by C-terminal phosphorylation. The current structural model for Akt activation by C-terminal phosphorylation has centered on intramolecular interactions between the C-terminal tail and the N lobe of the kinase domain. Here, we employ expressed protein ligation to produce site-specifically phosphorylated forms of purified Akt1 that are well suited for mechanistic analysis. Using biochemical, crystallographic, and cellular approaches, we determine that pSer473-Akt activation is driven by an intramolecular interaction between the C-tail and the pleckstrin homology (PH)-kinase domain linker that relieves PH domain-mediated Akt1 autoinhibition. Moreover, dual phosphorylation at Ser477/Thr479 activates Akt1 through a different allosteric mechanism via an apparent activation loop interaction that reduces autoinhibition by the PH domain and weakens PIP3 affinity. These results provide a new framework for understanding how Akt is controlled in cell signaling and suggest distinct functions for differentially modified Akt forms.

Enzyme-catalyzed expressed protein ligation.

Expressed protein ligation is a valuable method for protein semisynthesis that involves the reaction of recombinant protein C-terminal thioesters with N-terminal cysteine (N-Cys)-containing peptides, but the requirement of a Cys residue at the ligation junction can limit the utility of this method. Here we employ subtiligase variants to efficiently ligate Cys-free peptides to protein thioesters. Using this method, we have more accurately determined the effect of C-terminal phosphorylation on the tumor suppressor protein PTEN.

Getting the Most Out of Your Crystals: Data Collection at the New High-Flux, Microfocus MX Beamlines at NSLS-II.

Advances in synchrotron technology are changing the landscape of macromolecular crystallography. The two recently opened beamlines at NSLS-II-AMX and FMX-deliver high-flux microfocus beams that open new possibilities for crystallographic data collection. They are equipped with state-of-the-art experimental stations and automation to allow data collection on previously intractable crystals. Optimized data collection strategies allow users to tailor crystal positioning to optimally distribute the X-ray dose over its volume. Vector data collection allows the user to define a linear trajectory along a well diffracting volume of the crystal and perform rotational data collection while moving along the vector. This is particularly well suited to long, thin crystals. We describe vector data collection of three proteins-Akt1, PI3Kα, and CDP-Chase-to demonstrate its application and utility. For smaller crystals, we describe two methods for multicrystal data collection in a single loop, either manually selecting multiple centers (using H108A-PHM as an example), or "raster-collect", a more automated approach for a larger number of crystals (using CDP-Chase as an example).

A C-terminal membrane anchor affects the interactions of prion proteins with lipid membranes.

Membrane attachment via a C-terminal glycosylphosphatidylinositol anchor is critical for conversion of PrP(C) into pathogenic PrP(Sc). Therefore the effects of the anchor on PrP structure and function need to be deciphered. Three PrP variants, including full-length PrP (residues 23-231, FL_PrP), N-terminally truncated PrP (residues 90-231, T_PrP), and PrP missing its central hydrophobic region (Δ105-125, ΔCR_PrP), were equipped with a C-terminal membrane anchor via a semisynthesis strategy. Analyses of the interactions of lipidated PrPs with phospholipid membranes demonstrated that C-terminal membrane attachment induces a different binding mode of PrP to membranes, distinct from that of non-lipidated PrPs, and influences the biochemical and conformational properties of PrPs. Additionally, fluorescence-based assays indicated pore formation by lipidated ΔCR_PrP, a variant that is known to be highly neurotoxic in transgenic mice. This finding was supported by using patch clamp electrophysiological measurements of cultured cells. These results provide new evidence for the role of the membrane anchor in PrP-lipid interactions, highlighting the importance of the N-terminal and the central hydrophobic domain in these interactions.

Mechanistic analysis of ghrelin-O-acyltransferase using substrate analogs.

Ghrelin-O-Acyltransferase (GOAT) is an 11-transmembrane integral membrane protein that octanoylates the metabolism-regulating peptide hormone ghrelin at Ser3 and may represent an attractive target for the treatment of type II diabetes and the metabolic syndrome. Protein octanoylation is unique to ghrelin in humans, and little is known about the mechanism of GOAT or of related protein-O-acyltransferases HHAT or PORC. In this study, we explored an in vitro microsomal ghrelin octanoylation assay to analyze its enzymologic features. Measurement of Km for 10-mer, 27-mer, and synthetic Tat-peptide-containing ghrelin substrates provided evidence for a role of charge interactions in substrate binding. Ghrelin substrates with amino-alanine in place of Ser3 demonstrated that GOAT can catalyze the formation of an octanoyl-amide bond at a similar rate compared with the natural reaction. A pH-rate comparison of these substrates revealed minimal differences in acyltransferase activity across pH 6.0-9.0, providing evidence that these reactions may be relatively insensitive to the basicity of the substrate nucleophile. The conserved His338 residue was required both for Ser3 and amino-Ala3 ghrelin substrates, suggesting that His338 may have a key catalytic role beyond that of a general base.

Honey bee odorant-binding protein 14: effects on thermal stability upon odorant binding revealed by FT-IR spectroscopy and CD measurements.

In the present work, we study the effect of odorant binding on the thermal stability of honey bee (Apis mellifera L.) odorant-binding protein 14. Thermal denaturation of the protein in the absence and presence of different odorant molecules was monitored by Fourier transform infrared spectroscopy (FT-IR) and circular dichroism (CD). FT-IR spectra show characteristic bands for intermolecular aggregation through the formation of intermolecular ß-sheets during the heating process. Transition temperatures in the FT-IR spectra were evaluated using moving-window 2D correlation maps and confirmed by CD measurements. The obtained results reveal an increase of the denaturation temperature of the protein when bound to an odorant molecule. We could also discriminate between high- and low-affinity odorants by determining transition temperatures, as demonstrated independently by the two applied methodologies. The increased thermal stability in the presence of ligands is attributed to a stabilizing effect of non-covalent interactions between odorant-binding protein 14 and the odorant molecule.

Analyzing protein posttranslational modifications using enzyme-catalyzed expressed protein ligation.

Expressed protein ligation (EPL) allows for the attachment of a synthetic peptide into the N- or C-terminus of a recombinant protein fragment to generate a site-specifically modified protein with substantial yields for biochemical and biophysical studies. In this method, multiple posttranslational modifications (PTMs) can be incorporated into a synthetic peptide containing an N-terminal Cysteine, which selectively reacts with a protein C-terminal thioester to afford an amide bond formation. However, the requirement of a Cysteine at the ligation site can limit EPL's potential applications. Here, we describe a method called enzyme-catalyzed EPL, which uses subtiligase to ligate protein thioesters with Cysteine-free peptides. The procedure includes generating protein C-terminal thioester and peptide, performing the enzymatic EPL reaction, and purifying the protein ligation product. We exemplify this method by generating phospholipid phosphatase PTEN with site-specific phosphorylations installed onto its C-terminal tail for biochemical assays.

Utilizing a Baculovirus/Insect Cell Expression System and Expressed Protein Ligation (EPL) for Protein Semisynthesis.

Protein semisynthesis has been used for the chemoselective linking of synthetic peptides and recombinant protein fragments to generate complete native proteins in good yield. The ability to site-selectively incorporate multiple post-translational chemical modifications (PTMs) into proteins via this approach shows great potential for enhancing understanding of the molecular basis of protein function and regulation. Protein semisynthesis, however, often requires high expression efficiency of the recombinant protein fragments (i.e., high expression yield and ability to preserve protein biological functions), which can be hard to achieve for some human enzymes when using bacterial expression systems. Here, we describe how to use a baculovirus/insect cell expression system and a protein semisynthesis strategy known as expressed protein ligation (EPL) to produce workable levels of proteins of interest containing site-specific chemical modifications. The protocol provides detailed guidance for generating protein C-terminal thioesters for use with the EPL reaction, performing the EPL reaction, and purifying the protein ligation product. We exemplify the protocols by generating protein kinase Akt1 with site-specific phosphorylations installed into its C-terminal tail, for kinetic kinase assays. We hope these methods will help increase the use of protein semisynthesis for elucidating the post-translational regulation of human enzymes involved in cell signaling. © 2022 Wiley Periodicals LLC Basic Protocol 1: Generation of the N-terminal protein of interest (POI) fragment containing a C-terminal thioester moiety Basic Protocol 2: Expressed protein ligation (EPL) of the protein thioester with a synthetic peptide and purification of the protein ligation product Basic Protocol 3: Semisynthesis and biochemical analysis of site-specifically phosphorylated Akt1.

PH domain-mediated autoinhibition and oncogenic activation of Akt.

Akt is a Ser/Thr protein kinase that plays a central role in metabolism and cancer. Regulation of Akt's activity involves an autoinhibitory intramolecular interaction between its pleckstrin homology (PH) domain and its kinase domain that can be relieved by C-tail phosphorylation. PH domain mutant E17K Akt is a well-established oncogene. Previously, we reported that the conformation of autoinhibited Akt may be shifted by small molecule allosteric inhibitors limiting the mechanistic insights from existing X-ray structures that have relied on such compounds (Chu et al., 2020). Here, we discover unexpectedly that a single mutation R86A Akt exhibits intensified autoinhibitory features with enhanced PH domain-kinase domain affinity. Structural and biochemical analysis uncovers the importance of a key interaction network involving Arg86, Glu17, and Tyr18 that controls Akt conformation and activity. Our studies also shed light on the molecular basis for E17K Akt activation as an oncogenic driver.

Multifaceted Regulation of Akt by Diverse C-Terminal Post-translational Modifications.

Akt is a Ser/Thr protein kinase that regulates cell growth and metabolism and is considered a therapeutic target for cancer. Regulation of Akt by membrane recruitment and post-translational modifications (PTMs) has been extensively studied. The most well-established mechanism for cellular Akt activation involves phosphorylation on its activation loop on Thr308 by PDK1 and on its C-terminal tail on Ser473 by mTORC2. In addition, dual phosphorylation on Ser477 and Thr479 has been shown to activate Akt. Other C-terminal tail PTMs have been identified, but their functional impacts have not been well-characterized. Here, we investigate the regulatory effects of phosphorylation of Tyr474 and O-GlcNAcylation of Ser473 on Akt. We use expressed protein ligation as a tool to produce semisynthetic Akt proteins containing phosphoTyr474 and O-GlcNAcSer473 to dissect the enzymatic functions of these PTMs. We find that O-GlcNAcylation at Ser473 and phosphorylation at Tyr474 can also partially increase Akt's kinase activity toward both peptide and protein substrates. Additionally, we performed kinase assays employing human protein microarrays to investigate global substrate specificity of Akt, comparing phosphorylated versus O-GlcNAcylated Ser473 forms. We observed a high similarity in the protein substrates phosphorylated by phosphoSer473 Akt and O-GlcNAcSer473 Akt. Two Akt substrates identified using microarrays, PPM1H, a protein phosphatase, and NEDD4L, an E3 ubiquitin ligase, were validated in solution-phase assays and cell transfection experiments.

Protein immobilization on liposomes and lipid-coated nanoparticles by protein trans-splicing.

A plethora of methods exist to link proteins to surfaces in order to generate functionalized materials. However, general tools that lead to functional immobilization of recombinantly expressed proteins on membranes such as liposomes or lipid-coated nanoparticles are rare. Here we present an approach that takes advantage of a double-palmitoylated peptide that mediates stable membrane anchoring in combination with protein trans-splicing for efficient immobilization of recombinant proteins fused to split intein segments. Two different DnaE split inteins from Synechocystis and Nostoc punctiforme are tested and compared to immobilization via direct native chemical ligation using a protein thioester. Protein trans-splicing proceeds at low protein concentrations and leads to functionalized vesicles and membrane-coated silica nanoparticles.

The structural determinants of PH domain-mediated regulation of Akt revealed by segmental labeling.

Akt is a critical protein kinase that governs cancer cell growth and metabolism. Akt appears to be autoinhibited by an intramolecular interaction between its N-terminal pleckstrin homology (PH) domain and kinase domain, which is relieved by C-tail phosphorylation, but the precise molecular mechanisms remain elusive. Here, we use a combination of protein semisynthesis, NMR, and enzymological analysis to characterize structural features of the PH domain in its autoinhibited and activated states. We find that Akt autoinhibition depends on the length/flexibility of the PH-kinase linker. We identify a role for a dynamic short segment in the PH domain that appears to regulate autoinhibition and PDK1-catalyzed phosphorylation of Thr308 in the activation loop. We determine that Akt allosteric inhibitor MK2206 drives distinct PH domain structural changes compared to baseline autoinhibited Akt. These results highlight how the conformational plasticity of Akt governs the delicate control of its catalytic properties.

The protein kinase Akt acts as a coat adaptor in endocytic recycling.

Coat proteins have a central role in vesicular transport by binding to cargoes for their sorting into intracellular pathways. Cargo recognition is mediated by components of the coat complex known as adaptor proteins1-3. We previously showed that Arf-GAP with coil-coil, ANK repeat and PH domain-containing protein 1 (ACAP1) functions as an adaptor for a clathrin coat complex that has a function in endocytic recycling4-6. Here, we show that the protein kinase Akt acts as a co-adaptor in this complex, and is needed in conjunction with ACAP1 to bind to cargo proteins to promote their recycling. In addition to advancing the understanding of endocytic recycling, we uncover a fundamentally different function in which a kinase acts, as Akt in this case is an effector rather than a regulator in a cellular event.

Elucidation of remdesivir cytotoxicity pathways through genome-wide CRISPR-Cas9 screening and transcriptomics.

The adenosine analogue remdesivir has emerged as a front-line antiviral treatment for SARS-CoV-2, with preliminary evidence that it reduces the duration and severity of illness1.Prior clinical studies have identified adverse events1,2, and remdesivir has been shown to inhibit mitochondrial RNA polymerase in biochemical experiments7, yet little is known about the specific genetic pathways involved in cellular remdesivir metabolism and cytotoxicity. Through genome-wide CRISPR-Cas9 screening and RNA sequencing, we show that remdesivir treatment leads to a repression of mitochondrial respiratory activity, and we identify five genes whose loss significantly reduces remdesivir cytotoxicity. In particular, we show that loss of the mitochondrial nucleoside transporter SLC29A3 mitigates remdesivir toxicity without a commensurate decrease in SARS-CoV-2 antiviral potency and that the mitochondrial adenylate kinase AK2 is a remdesivir kinase required for remdesivir efficacy and toxicity. This work elucidates the cellular mechanisms of remdesivir metabolism and provides a candidate gene target to reduce remdesivir cytotoxicity.

AKTivation mechanisms.

Akt1-3 (Akt) are a subset of the AGC protein Ser/Thr kinase family and play important roles in cell growth, metabolic regulation, cancer, and other diseases. We describe some of the roles of Akt in cell signaling and the biochemical and structural mechanisms of the regulation of Akt catalysis by the phospholipid PIP3 and by phosphorylation. Recent findings highlight a diverse set of strategies to control Akt catalytic activity to ensure its normal biological functions.

Occurrence of metal-rich acidic groundwaters around the Mekong Delta (Vietnam): A phenomenon linked to well installation.

Acidic groundwaters enriched with heavy metals are frequently observed in the coastal plain aquifers. The acidic pHs are observed even in the deep confined aquifers in southern Vietnam. This study geochemically explores the causes of these acidic groundwaters by investigating 41 groundwater samples, 4 soil samples and a 54 m long sediment core and the long-term monitoring data (4189 observations) obtained from 178 wells of the National Groundwater Monitoring Network for the South of Vietnam (NGMNS). The groundwater data show elevated Fe, Mn, Al, Pb, and Zn concentrations as the pH becomes acidic and suggest pyrite oxidation be the major cause for the groundwater acidification. This is further confirmed by pyrite framboids observed in the sediment or soil samples taken from the sites where strongly acidic groundwaters were observed. Results of leaching experiments using sediment and soil samples indicate that high metal concentrations in the acidic pH are associated with the increased metal solubility and mineral dissolution kinetics. The acidification of deep groundwaters is revealed to be associated with well installation, indicating the importance of proper well-installation techniques to protect water quality of deep confined aquifers.

Studying weak and dynamic interactions of posttranslationally modified proteins using expressed protein ligation.

Many cellular processes are regulated by posttranslational modifications that are recognized by specific domains in protein binding partners. These interactions are often weak, thus allowing a highly dynamic and combinatorial regulatory network of protein-protein interactions. We report an efficient strategy that overcomes challenges in structural analysis of such a weak transient interaction between the Tudor domain of the Survival of Motor Neuron (SMN) protein and symmetrically dimethylated arginine (sDMA). The posttranslational modification is chemically introduced and covalently linked to the effector module by a one-pot expressed protein ligation (EPL) procedure also enabling segmental incorporation of NMR-active isotopes for structural analysis. Covalent coupling of the two interacting moieties shifts the equilibrium to the bound state, and stoichiometric interactions are formed even for low affinity interactions. Our approach should enable the structural analysis of weak interactions by NMR or X-ray crystallography to better understand the role of posttranslational modifications in dynamic biological processes.

Recombinant expression of soluble murine prion protein for C-terminal modification.

Membrane attachment of prion protein (PrP) via its glycosylphosphatidylinositol (GPI) anchor plays a key role during conversion of cellular PrP(C) into its pathogenic isoform PrP(Sc). Strategies to access homogenous lipidated PrP via expressed protein ligation (EPL) are required to fully decipher the effect of membrane attachment. Such strategies suffer from insoluble expression of PrP-intein fusion constructs and low folding efficiencies that severely limit the available amount of homogeneous lipidated PrP. Here, we describe an alternative method for expression of soluble PrP-intein fusion proteins in Escherichia coli that provides access to natively folded PrP ready to use in EPL.

Semisynthesis of membrane-attached prion proteins.

Conversion of cellular prion protein (PrP(C)) into the pathological conformer (PrP(Sc)) is the hallmark of prion diseases and has been studied extensively by using recombinantly expressed PrP (rPrP). Because of the inherent difficulties of expressing and purifying posttranslationally modified rPrP variants only a limited amount of data is available for membrane-associated PrP and its behavior in vitro and in vivo. Protein semisynthesis provides two alternative routes to access multimilligram amounts of membrane-attached rPrP, which are described in detail here. In both cases, rPrP fused to a C-terminal extension comprising either the Mycobacterium xenopi GyrA mini-intein or the Synechocystis sp. DnaE N-terminal split intein is expressed in E. coli. Protein purification was followed by reaction with chemically synthesized palmitoylated membrane anchor peptides to yield rPrP(Palm) or with a chemically synthesized glycosylphosphatidylinositol (GPI) anchor to give rPrP(GPI). Solubility problems encountered with synthetic membrane anchors were overcome by either incorporating a polyethylene glycol-based C-terminal tag (removable by specific proteolysis) or by direct incorporation into liposomes. The new rPrP(Palm) variants studied by a variety of in vitro methods exhibited a high affinity to liposomes and an increased lag phase during aggregation when compared to rPrP. Similar results were obtained for rPrP(GPI), in which only one alkyl chain is sufficient for quantitative membrane attachment. In vivo studies demonstrated that double lipidated rPrP(Palm) is efficiently taken up into the membranes of mouse neuronal and human epithelial kidney cells.