Engineering of ultraID, a compact and hyperactive enzyme for proximity-dependent biotinylation in living cells

Pupylation-based proximity-tagging of FERONIA-interacting proteins in Arabidopsis

The plasma membrane-localized receptor kinase FERONIA (FER) plays critical roles in a remarkable variety of biological processes throughout the life cycle of Arabidopsis thaliana. Revealing the molecular connections of FER that underlie these processes starts with identifying the proteins that interact with FER. We applied pupylation-based interaction tagging (PUP-IT) to survey cellular proteins in proximity to FER, encompassing weak and transient interactions that can be difficult to capture for membrane proteins. We reproducibly identified 581, 115, and 736 specific FER-interacting protein candidates in protoplasts, seedlings, and flowers, respectively. We also confirmed fourteen previously characterized FER-interacting proteins. Protoplast transient gene expression expedited the testing of new gene constructs for PUP-IT analyses and the validation of candidate proteins. We verified the proximity labeling of five selected candidates that were not previously characterized as FER-interacting proteins. The PUP-IT method could be a valuable tool to survey and validate protein-protein interactions for targets of interest in diverse subcellular compartments in plants.

A proximity proteomics pipeline with improved reproducibility and throughput

Proximity labeling (PL) via biotinylation coupled with mass spectrometry (MS) captures spatial proteomes in cells. Large-scale processing requires a workflow minimizing hands-on time and enhancing quantitative reproducibility. We introduced a scalable PL pipeline integrating automated enrichment of biotinylated proteins in a 96-well plate format. Combining this with optimized quantitative MS based on data-independent acquisition (DIA), we increased sample throughput and improved protein identification and quantification reproducibility. We applied this pipeline to delineate subcellular proteomes across various compartments. Using the 5HT2A serotonin receptor as a model, we studied temporal changes of proximal interaction networks induced by receptor activation. In addition, we modified the pipeline for reduced sample input to accommodate CRISPR-based gene knockout, assessing dynamics of the 5HT2A network in response to perturbation of selected interactors. This PL approach is universally applicable to PL proteomics using biotinylation-based PL enzymes, enhancing throughput and reproducibility of standard protocols.

PDZ-Binding Kinase, a Novel Regulator of Vascular Remodeling in Pulmonary Arterial Hypertension

BACKGROUND

Pulmonary arterial hypertension (PAH) is high blood pressure in the lungs that originates from structural changes in small resistance arteries. A defining feature of PAH is the inappropriate remodeling of pulmonary arteries (PA) leading to right ventricle failure and death. Although treatment of PAH has improved, the long-term prognosis for patients remains poor, and more effective targets are needed.

METHODS

Gene expression was analyzed by microarray, RNA sequencing, quantitative polymerase chain reaction, Western blotting, and immunostaining of lung and isolated PA in multiple mouse and rat models of pulmonary hypertension (PH) and human PAH. PH was assessed by digital ultrasound, hemodynamic measurements, and morphometry.

RESULTS

Microarray analysis of the transcriptome of hypertensive rat PA identified a novel candidate, PBK (PDZ-binding kinase), that was upregulated in multiple models and species including humans. PBK is a serine/threonine kinase with important roles in cell proliferation that is minimally expressed in normal tissues but significantly increased in highly proliferative tissues. PBK was robustly upregulated in the medial layer of PA, where it overlaps with markers of smooth muscle cells. Gain-of-function approaches show that active forms of PBK increase PA smooth muscle cell proliferation, whereas silencing PBK, dominant negative PBK, and pharmacological inhibitors of PBK all reduce proliferation. Pharmacological inhibitors of PBK were effective in PH reversal strategies in both mouse and rat models, providing translational significance. In a complementary genetic approach, PBK was knocked out in rats using CRISPR/Cas9 editing, and loss of PBK prevented the development of PH. We found that PBK bound to PRC1 (protein regulator of cytokinesis 1) in PA smooth muscle cells and that multiple genes involved in cytokinesis were upregulated in experimental models of PH and human PAH. Active PBK increased PRC1 phosphorylation and supported cytokinesis in PA smooth muscle cells, whereas silencing or dominant negative PBK reduced cytokinesis and the number of cells in the G2/M phase of the cell cycle.

CONCLUSIONS

PBK is a newly described target for PAH that is upregulated in proliferating PA smooth muscle cells, where it contributes to proliferation through changes in cytokinesis and cell cycle dynamics to promote medial thickening, fibrosis, increased PA resistance, elevated right ventricular systolic pressure, right ventricular remodeling, and PH.

Presynaptic Rac1 in the hippocampus selectively regulates working memory

One of the most extensively studied members of the Ras superfamily of small GTPases, Rac1 is an intracellular signal transducer that remodels actin and phosphorylation signaling networks. Previous studies have shown that Rac1-mediated signaling is associated with hippocampal-dependent working memory and longer-term forms of learning and memory and that Rac1 can modulate forms of both pre- and postsynaptic plasticity. How these different cognitive functions and forms of plasticity mediated by Rac1 are linked, however, is unclear. Here, we show that spatial working memory in mice is selectively impaired following the expression of a genetically encoded Rac1 inhibitor at presynaptic terminals, while longer-term cognitive processes are affected by Rac1 inhibition at postsynaptic sites. To investigate the regulatory mechanisms of this presynaptic process, we leveraged new advances in mass spectrometry to identify the proteomic and post-translational landscape of presynaptic Rac1 signaling. We identified serine/threonine kinases and phosphorylated cytoskeletal signaling and synaptic vesicle proteins enriched with active Rac1. The phosphorylated sites in these proteins are at positions likely to have regulatory effects on synaptic vesicles. Consistent with this, we also report changes in the distribution and morphology of synaptic vesicles and in postsynaptic ultrastructure following presynaptic Rac1 inhibition. Overall, this study reveals a previously unrecognized presynaptic role of Rac1 signaling in cognitive processes and provides insights into its potential regulatory mechanisms.

Mapping protein-protein interactions by mass spectrometry

Protein-protein interactions (PPIs) are essential for numerous biological activities, including signal transduction, transcription control, and metabolism. They play a pivotal role in the organization and function of the proteome, and their perturbation is associated with various diseases, such as cancer, neurodegeneration, and infectious diseases. Recent advances in mass spectrometry (MS)-based protein interactomics have significantly expanded our understanding of the PPIs in cells, with techniques that continue to improve in terms of sensitivity, and specificity providing new opportunities for the study of PPIs in diverse biological systems. These techniques differ depending on the type of interaction being studied, with each approach having its set of advantages, disadvantages, and applicability. This review highlights recent advances in enrichment methodologies for interactomes before MS analysis and compares their unique features and specifications. It emphasizes prospects for further improvement and their potential applications in advancing our knowledge of PPIs in various biological contexts.

GlycoID Proximity Labeling to Identify O-GlcNAcylated Protein Interactomes in Live Cells

Cells continuously remodel their intracellular proteins with the monosaccharide O-linked N-acetylglucosamine (O-GlcNAc) to regulate metabolism, signaling, and stress. This protocol describes the use of GlycoID tools to capture O-GlcNAc dynamics in live cells. GlycoID constructs contain an O-GlcNAc binding domain linked to a proximity labeling domain and a subcellular localization sequence. When expressed in mammalian cells, GlycoID tracks changes in O-GlcNAc-modified proteins and their interactomes in response to chemical induction with biotin over time. Pairing the subcellular localization of GlycoID with the chemical induction of activity enables spatiotemporal studies of O-GlcNAc biology during cellular events such as insulin signaling. However, optimizing intracellular labeling experiments requires attention to several variables. Here, we describe two protocols to adapt GlycoID methods to a cell line and biological process of interest. Next, we describe how to conduct a semiquantitative proteomic analysis of O-GlcNAcylated proteins and their interactomes using insulin versus glucagon signaling as a sample application. This articles aims to establish baseline GlycoID protocols for new users and set the stage for widespread use over diverse cellular applications for the functional study of O-GlcNAc glycobiology. © 2024 Wiley Periodicals LLC. Basic Protocol 1: Expression of targeted GlycoID constructs to verify subcellular location and labeling activity in mammalian cells Basic Protocol 2: GlycoID labeling in live HeLa cells for O-GlcNAc proteomic comparisons.

A proximity proteomics pipeline with improved reproducibility and throughput

Proximity labeling (PL) through biotinylation coupled with mass spectrometry (MS) has emerged as a powerful technique for capturing spatial proteomes within living cells. Large-scale sample processing for proximity proteomics requires a workflow that minimizes hands-on time while enhancing quantitative reproducibility. Here, we present a scalable PL pipeline integrating automated enrichment of biotinylated proteins in a 96-well plate format. By combining this pipeline with an optimized quantitative MS acquisition method based on data-independent acquisition (DIA), we not only significantly increased sample throughput but also improved the reproducibility of protein identification and quantification. We applied this pipeline to delineate subcellular proteomes across various cellular compartments, including endosomes, late endosomes/lysosomes, the Golgi apparatus, and the plasma membrane. Moreover, employing 5HT2A serotonin receptor as a model, we investigated temporal changes of proximal interaction networks induced by the receptor's activation with serotonin. Finally, to demonstrate the applicability of our PL pipeline across multiple experimental conditions, we further modified the PL pipeline for reduced sample input amounts to accommodate CRISPR-based gene knockout, and assessed the dynamics of the 5HT2A network in response to the perturbation of selected proximal interactors. Importantly, the presented PL approach is universally applicable to PL proteomics using biotinylation-based PL enzymes, increasing both throughput and reproducibility of standard protocols.

Synaptic proteomics decode novel molecular landscape in the brain

Synapses play a pivotal role in forming neural circuits, with critical implications for brain functions such as learning, memory, and emotions. Several advances in synaptic research have demonstrated the diversity of synaptic structure and function, which can form thousands of connections depending on the neuronal cell types. Moreover, synapses not only interconnect neurons but also establish connections with glial cells such as astrocytes, which play a key role in the architecture and function of neuronal circuits in the brain. Emerging evidence suggests that dysfunction of synaptic proteins contributes to a variety of neurological and psychiatric disorders. Therefore, it is crucial to determine the molecular networks within synapses in various neuronal cell types to gain a deeper understanding of how the nervous system regulates brain function. Recent advances in synaptic proteome approaches, such as fluorescence-activated synaptosome sorting (FASS) and proximity labeling, have allowed for a detailed and spatial analysis of many cell-type-specific synaptic molecules in vivo. In this brief review, we highlight these novel spatial proteomic approaches and discuss the regulation of synaptic formation and function in the brain. This knowledge of molecular networks provides new insight into the understanding of many neurological and psychiatric disorders.

Climbing into their Skin to Understand Contextual Protein-Protein Associations and Localizations: Functional Investigations in Transgenic Live Model Organisms

Borrowing some quotes from Harper Lee's novel "To Kill A Mockingbird" to help frame our manuscript, we discuss methods to profile local proteomes. We initially focus on chemical biology regimens that function in live organisms and use reactive biotin species for this purpose. We then consider ways to add new dimensions to these experimental regimens, principally by releasing less reactive (i. e., more selective) (preter)natural electrophiles. Although electrophile release methods may have lower resolution and label fewer proteins than biotinylation methods, their ability to probe simultaneously protein function and locale raises new and interesting possibilities for the field.

Biotin protein ligase as you like it: Either extraordinarily specific or promiscuous protein biotinylation

Biotin (vitamin H or B7) is a coenzyme essential for all forms of life. Biotin has biological activity only when covalently attached to a few key metabolic enzyme proteins. Most organisms have only one attachment enzyme, biotin protein ligase (BPL), which attaches biotin to all target proteins. The sequences of these proteins and their substrate proteins are strongly conserved throughout biology. Structures of both the biotin ligase- and biotin-acceptor domains of mammals, plants, several bacterial species, and archaea have been determined. These, together with mutational analyses of ligases and their protein substrates, illustrate the exceptional specificity of this protein modification. For example, the Escherichia coli BPL biotinylates only one of the >4000 cellular proteins. Several bifunctional bacterial biotin ligases transcriptionally regulate biotin synthesis and/or transport in concert with biotinylation. The human BPL has been demonstrated to play an important role in that mutations in the BPL encoding gene cause one form of the disease, biotin-responsive multiple carboxylase deficiency. Promiscuous mutant versions of several BPL enzymes release biotinoyl-AMP, the active intermediate of the ligase reaction, to solvent. The released biotinoyl-AMP acts as a chemical biotinylation reagent that modifies lysine residues of neighboring proteins in vivo. This proximity-dependent biotinylation (called BioID) approach has been heavily utilized in cell biology.

Presynaptic Rac1 in the hippocampus selectively regulates working memory

One of the most extensively studied members of the Ras superfamily of small GTPases, Rac1 is an intracellular signal transducer that remodels actin and phosphorylation signaling networks. Previous studies have shown that Rac1-mediated signaling is associated with hippocampal-dependent working memory and longer-term forms of learning and memory and that Rac1 can modulate forms of both pre- and postsynaptic plasticity. How these different cognitive functions and forms of plasticity mediated by Rac1 are linked, however, is unclear. Here, we show that spatial working memory is selectively impaired following the expression of a genetically encoded Rac1-inhibitor at presynaptic terminals, while longer-term cognitive processes are affected by Rac1 inhibition at postsynaptic sites. To investigate the regulatory mechanisms of this presynaptic process, we leveraged new advances in mass spectrometry to identify the proteomic and post-translational landscape of presynaptic Rac1 signaling. We identified serine/threonine kinases and phosphorylated cytoskeletal signaling and synaptic vesicle proteins enriched with active Rac1. The phosphorylated sites in these proteins are at positions likely to have regulatory effects on synaptic vesicles. Consistent with this, we also report changes in the distribution and morphology of synaptic vesicles and in postsynaptic ultrastructure following presynaptic Rac1 inhibition. Overall, this study reveals a previously unrecognized presynaptic role of Rac1 signaling in cognitive processes and provides insights into its potential regulatory mechanisms.

Biotin-Based Strategies to Explore the World of Ubiquitin and Ubiquitin-Like Modifiers

A complex code of cellular signals is mediated by ubiquitin and ubiquitin-like (Ub/UbL) modifications on substrate proteins. The so-called Ubiquitin Code specifies protein fates, such as stability, subcellular localization, functional activation or suppression, and interactions. Hundreds of enzymes are involved in placing and removing Ub/UbL on thousands of substrates, while the consequences of modifications and the mechanisms of specificity are still poorly defined. Challenges include rapid and transient engagement of enzymes and Ub/UbL interactors, low stoichiometry of modified versus non-modified cellular substrates, and protease-mediated loss of Ub/UbL in lysates. To decipher this complexity and confront the challenges, many tools have been created to trap and identify substrates and interactors linked to Ub/UbL modification. This review focuses on an assortment of biotin-based tools developed for this purpose (for example BioUbLs, UbL-ID, BioE3, BioID), taking advantage of the strong affinity of biotin-streptavidin and the stringent lysis/washing approach allowed by it, paired with sensitive mass-spectrometry-based proteomic methods. Knowing how substrates change during development and disease, the consequences of substrate modification, and matching substrates to particular UbL-ligating enzymes will contribute new insights into how Ub/UbL signaling works and how it can be exploited for therapies.

Protein subcellular relocalization and function of duplicated flagellar calcium binding protein genes in honey bee trypanosomatid parasite

The honey bee trypanosomatid parasite, Lotmaria passim, contains two genes that encode the flagellar calcium binding protein (FCaBP) through tandem duplication in its genome. FCaBPs localize in the flagellum and entire body membrane of L. passim through specific N-terminal sorting sequences. This finding suggests that this is an example of protein subcellular relocalization resulting from gene duplication, altering the intracellular localization of FCaBP. However, this phenomenon may not have occurred in Leishmania, as one or both of the duplicated genes have become pseudogenes. Multiple copies of the FCaBP gene are present in several Trypanosoma species and Leptomonas pyrrhocoris, indicating rapid evolution of this gene in trypanosomatid parasites. The N-terminal flagellar sorting sequence of L. passim FCaBP1 is in close proximity to the BBSome complex, while that of Trypanosoma brucei FCaBP does not direct GFP to the flagellum in L. passim. Deletion of the two FCaBP genes in L. passim affected growth and impaired flagellar morphogenesis and motility, but it did not impact host infection. Therefore, FCaBP represents a duplicated gene with a rapid evolutionary history that is essential for flagellar structure and function in a trypanosomatid parasite.

Screening for molecular glues - Challenges and opportunities

Molecular glues are small molecules, typically smaller than PROTACs, and usually with improved physicochemical properties that aim to stabilise the interaction between two proteins. Most often this approach is used to improve or induce an interaction between the target and an E3 ligase, but other interactions which stabilise interactions to increase activity or to inhibit binding to a natural effector have also been demonstrated. This review will describe the effects of induced proximity, discuss current methods used to identify molecular glues and introduce approaches that could be adapted for molecular glue screening.

Interaction network of human early embryonic transcription factors

Embryonic genome activation (EGA) occurs during preimplantation development and is characterized by the initiation of de novo transcription from the embryonic genome. Despite its importance, the regulation of EGA and the transcription factors involved in this process are poorly understood. Paired-like homeobox (PRDL) family proteins are implicated as potential transcriptional regulators of EGA, yet the PRDL-mediated gene regulatory networks remain uncharacterized. To investigate the function of PRDL proteins, we are identifying the molecular interactions and the functions of a subset family of the Eutherian Totipotent Cell Homeobox (ETCHbox) proteins, seven PRDL family proteins and six other transcription factors (TFs), all suggested to participate in transcriptional regulation during preimplantation. Using mass spectrometry-based interactomics methods, AP-MS and proximity-dependent biotin labeling, and chromatin immunoprecipitation sequencing we derive the comprehensive regulatory networks of these preimplantation TFs. By these interactomics tools we identify more than a thousand high-confidence interactions for the 21 studied bait proteins with more than 300 interacting proteins. We also establish that TPRX2, currently assigned as pseudogene, is a transcriptional activator.

Evolution of chemistry and selection technology for DNA-encoded library

DNA-encoded chemical library (DEL) links the power of amplifiable genetics and the non-self-replicating chemical phenotypes, generating a diverse chemical world. In analogy with the biological world, the DEL world can evolve by using a chemical central dogma, wherein DNA replicates using the PCR reactions to amplify the genetic codes, DNA sequencing transcripts the genetic information, and DNA-compatible synthesis translates into chemical phenotypes. Importantly, DNA-compatible synthesis is the key to expanding the DEL chemical space. Besides, the evolution-driven selection system pushes the chemicals to evolve under the selective pressure, i.e., desired selection strategies. In this perspective, we summarized recent advances in expanding DEL synthetic toolbox and panning strategies, which will shed light on the drug discovery harnessing in vitro evolution of chemicals via DEL.

TurboID-Based Proximity Labeling: A Method to Decipher Protein-Protein Interactions in Plants

Proteins form complex networks through interaction to drive biological processes. Thus, dissecting protein-protein interactions (PPIs) is essential for interpreting cellular processes. To overcome the drawbacks of traditional approaches for analyzing PPIs, enzyme-catalyzed proximity labeling (PL) techniques based on peroxidases or biotin ligases have been developed and successfully utilized in mammalian systems. However, the use of toxic H2O2 in peroxidase-based PL, the requirement of long incubation time (16-24 h), and higher incubation temperature (37 °C) with biotin in BioID-based PL significantly restricted their applications in plants. TurboID-based PL, a recently developed approach, circumvents the limitations of these methods by providing rapid PL of proteins under room temperature. We recently optimized the use of TurboID-based PL in plants and demonstrated that it performs better than BioID in labeling endogenous proteins. Here, we describe a step-by-step protocol for TurboID-based PL in studying PPIs in planta, including Agrobacterium-based transient expression of proteins, biotin treatment, protein extraction, removal of free biotin, quantification, and enrichment of the biotinylated proteins by affinity purification. We describe the PL using plant viral immune receptor N, which belongs to the nucleotide-binding leucine-rich repeat (NLR) class of immune receptors, as a model. The method described could be easily adapted to study PPI networks of other proteins in Nicotiana benthamiana and provides valuable information for future application of TurboID-based PL in other plant species.

Cell Cycle-Specific Protein Phosphatase 1 (PP1) Substrates Identification Using Genetically Modified Cell Lines

The identification of protein phosphatase 1 (PP1) holoenzyme substrates has proven to be a challenging task. PP1 can form different holoenzyme complexes with a variety of regulatory subunits, and many of those are cell cycle regulated. Although several methods have been used to identify PP1 substrates, their cell cycle specificity is still an unmet need. Here, we present a new strategy to investigate PP1 substrates throughout the cell cycle using clustered regularly interspersed short palindromic repeats (CRISPR)-Cas9 genome editing and generate cell lines with endogenously tagged PP1 regulatory subunit (regulatory interactor of protein phosphatase one, RIPPO). RIPPOs are tagged with the auxin-inducible degron (AID) or ascorbate peroxidase 2 (APEX2) modules, and PP1 substrate identification is conducted by SILAC proteomic-based approaches. Proteins in close proximity to RIPPOs are first identified through mass spectrometry (MS) analyses using the APEX2 system; then a list of differentially phosphorylated proteins upon RIPPOs rapid degradation (achieved via the AID system) is compiled via SILAC phospho-mass spectrometry. The "in silico" overlap between the two proteomes will be enriched for PP1 putative substrates. Several methods including fluorescence resonance energy transfer (FRET), proximity ligation assays (PLA), and in vitro assays can be used as substrate validations approaches.

A miniTurbo-based proximity labeling protocol to identify conditional protein interactomes in vivo in Myxococcus xanthus

Protein-protein interactions are foundational for many cellular processes. Such interactions are especially challenging to identify if they are transient or depend on environmental conditions. This protocol details steps to identify stable and transient protein interactomes in the bacterium Myxococcus xanthus using biotin ligase miniTurbo-based proximity labeling. We include instructions for optimizing the expression of control proteins, in vivo biotin labeling of bacteria grown on a surface or in suspension culture, enrichment of biotinylated proteins, and sample processing for proteomic analysis. For complete details on the use and execution of this protocol, please refer to Branon et al. (2018).1.

Bacterial enzymes: powerful tools for protein labeling, cell signaling, and therapeutic discovery

Bacteria have evolved a diverse set of enzymes that enable them to subvert host defense mechanisms as well as to form part of the prokaryotic immune system. Due to their unique and varied biochemical activities, these bacterial enzymes have emerged as key tools for understanding and investigating biological systems. In this review, we summarize and discuss some of the most prominent bacterial enzymes used for the site-specific modification of proteins, in vivo protein labeling, proximity labeling, interactome mapping, signaling pathway manipulation, and therapeutic discovery. Finally, we provide a perspective on the complementary advantages and limitations of using bacterial enzymes compared with chemical probes for exploring biological systems.

Labeling strategies to track protozoan parasite proteome dynamics

Intracellular protozoan parasites are responsible for wide-spread infectious diseases. These unicellular pathogens have complex, multi-host life cycles, which present challenges for investigating their basic biology and for discovering vulnerabilities that could be exploited for disease control. Throughout development, parasite proteomes are dynamic and support stage-specific functions, but detection of these proteins is often technically challenging and complicated by the abundance of host proteins. Thus, to elucidate key parasite processes and host-pathogen interactions, labeling strategies are required to track pathogen proteins during infection. Herein, we discuss the application of bioorthogonal non-canonical amino acid tagging and proximity-dependent labeling to broadly study protozoan parasites and include outlooks for future applications to study Plasmodium, the causative agent of malaria. We highlight the potential of these technologies to provide spatiotemporal labeling with selective parasite protein enrichment, which could enable previously unattainable insight into the biology of elusive developmental stages.

Characterization of Arbuscular Mycorrhizal Effector Proteins

Plants are colonized by various fungi with both pathogenic and beneficial lifestyles. One type of colonization strategy is through the secretion of effector proteins that alter the plant's physiology to accommodate the fungus. The oldest plant symbionts, the arbuscular mycorrhizal fungi (AMF), may exploit effectors to their benefit. Genome analysis coupled with transcriptomic studies in different AMFs has intensified research on the effector function, evolution, and diversification of AMF. However, of the current 338 predicted effector proteins from the AM fungus Rhizophagus irregularis, only five have been characterized, of which merely two have been studied in detail to understand which plant proteins they associate with to affect the host physiology. Here, we review the most recent findings in AMF effector research and discuss the techniques used for the functional characterization of effector proteins, from their in silico prediction to their mode of action, with an emphasis on high-throughput approaches for the identification of plant targets of the effectors through which they manipulate their hosts.

Proximity Labeling in Plants

Proteins are workhorses in the cell; they form stable and more often dynamic, transient protein-protein interactions, assemblies, and networks and have an intimate interplay with DNA and RNA. These network interactions underlie fundamental biological processes and play essential roles in cellular function. The proximity-dependent biotinylation labeling approach combined with mass spectrometry (PL-MS) has recently emerged as a powerful technique to dissect the complex cellular network at the molecular level. In PL-MS, by fusing a genetically encoded proximity-labeling (PL) enzyme to a protein or a localization signal peptide, the enzyme is targeted to a protein complex of interest or to an organelle, allowing labeling of proximity proteins within a zoom radius. These biotinylated proteins can then be captured by streptavidin beads and identified and quantified by mass spectrometry. Recently engineered PL enzymes such as TurboID have a much-improved enzymatic activity, enabling spatiotemporal mapping with a dramatically increased signal-to-noise ratio. PL-MS has revolutionized the way we perform proteomics by overcoming several hurdles imposed by traditional technology, such as biochemical fractionation and affinity purification mass spectrometry. In this review, we focus on biotin ligase-based PL-MS applications that have been, or are likely to be, adopted by the plant field. We discuss the experimental designs and review the different choices for engineered biotin ligases, enrichment, and quantification strategies. Lastly, we review the validation and discuss future perspectives.

Dynamics and composition of small heat shock protein condensates and aggregates

Small heat shock proteins (sHSPs) are essential ATP-independent chaperones that protect the cellular proteome. These proteins assemble into polydisperse oligomeric structures, the composition of which dramatically affects their chaperone activity. The biomolecular consequences of variations in sHSP ratios, especially inside living cells, remain elusive. Here, we study the consequences of altering the relative expression levels of HspB2 and HspB3 in HEK293T cells. These chaperones are partners in a hetero-oligomeric complex, and genetic mutations that abolish their mutual interaction are associated with myopathic disorders. HspB2 displays three distinct phenotypes when co-expressed with HspB3 at varying ratios. Expression of HspB2 alone leads to formation of liquid nuclear condensates, while shifting the stoichiometry towards HspB3 resulted in the formation of large solid-like aggregates. Only cells co-expressing HspB2 with a limited amount of HspB3 formed fully soluble complexes that were distributed homogeneously throughout the nucleus. Strikingly, both condensates and aggregates were reversible, as shifting the HspB2:HspB3 balance in situ resulted in dissolution of these structures. To uncover the molecular composition of HspB2 condensates and aggregates, we used APEX-mediated proximity labelling. Most proteins interact transiently with the condensates and were neither enriched nor depleted in these cells. In contrast, we found that HspB2:HspB3 aggregates sequestered several disordered proteins and autophagy factors, suggesting that the cell is actively attempting to clear these aggregates. This study presents a striking example of how changes in the relative expression levels of interacting proteins affects their phase behavior. Our approach could be applied to study the role of protein stoichiometry and the influence of client binding on phase behavior in other biomolecular condensates and aggregates.

Direct Selection of DNA-Encoded Libraries for Biased Agonists of GPCRs on Live Cells

G protein-coupled receptors (GPCRs) are the largest superfamily of human membrane target proteins for approved drugs. GPCR ligands can have a complex array of pharmacological activities. Among these activities, biased agonists have potential to serve as both chemical probes to understand specific aspects of receptor signaling and therapeutic leads with more specific, desired activity. Challenges exist, however, in the development of new biased activators due, in part, to the low throughput of traditional screening approaches. DNA-encoded chemical libraries (DELs) dramatically improve the throughput of drug discovery by allowing a collective selection, rather than discrete screening, of large compound libraries. The use of DELs has been largely limited to affinity-based selections against purified protein targets, which identify binders only. Herein, we report a split protein complementation approach that allows direct identification of DNA-linked molecules that induce the dimerization of two proteins. We used this selection with a DEL against opioid receptor GPCRs on living cells for the identification of small molecules that possess the specific function of activation of either β-arrestin or G protein signaling pathways. This approach was applied to δ-, μ-, and κ-opioid receptors and enabled the discovery of compound [66,66], a selective, G-protein-biased agonist of the κ-opioid receptor (EC₅₀ = 100 nM, E _max = 82%, G_i bias factor = 6.6). This approach should be generally applicable for the direct selection of chemical inducers of dimerization from DELs and expand the utility of DELs to enrich molecules with a specific and desired biochemical function.

TurboID-based proteomic profiling of meiotic chromosome axes in Arabidopsis thaliana

During meiotic prophase I, sister chromatids are arranged in a loop-base array along a proteinaceous structure, called the meiotic chromosome axis. This structure is essential for synapsis and meiotic recombination progression and hence formation of genetically diverse gametes. Proteomic studies in plants aiming to unravel the composition and regulation of meiotic axes are constrained by limited meiotic cells embedded in floral organs. Here we report TurboID (TbID)-based proximity labelling (PL) in meiotic cells of Arabidopsis thaliana. TbID fusion to the two meiotic chromosome axis proteins ASY1 and ASY3 enabled the identification of their proximate 'interactomes' based on affinity purification coupled with mass spectrometry. We identified 39 ASY1 and/or ASY3 proximate candidates covering most known chromosome axis-related proteins. Functional studies of selected candidates demonstrate that not only known meiotic candidates but also new meiotic proteins were uncovered. Hence, TbID-based PL in meiotic cells enables the identification of chromosome axis proximate proteins in A. thaliana.

Proximity biotinylation to define the local environment of the protein kinase A catalytic subunit in adrenal cells

Mutant protein kinase A catalytic subunit (PKAc) drives adrenal Cushing's syndrome, though its signaling interactions remain unclear. This protocol details steps to use live-cell proximity labeling to identify subcellular compartments and proteins closely associated with variants of PKAc in human adrenal cells. We include instructions for clonal cell line generation, live biotin labeling of proximal proteins, isolation of biotinylated proteins, and sample processing for proteomic analysis using the biotin ligase miniTurbo with wild-type and mutant PKAc.1,2 For complete details on the use and execution of this protocol, please refer to Omar et al. (2022).3.

Using the Proteomics Toolbox to Resolve Topology and Dynamics of Compartmentalized cAMP Signaling

cAMP is a second messenger that regulates a myriad of cellular functions in response to multiple extracellular stimuli. New developments in the field have provided exciting insights into how cAMP utilizes compartmentalization to ensure specificity when the message conveyed to the cell by an extracellular stimulus is translated into the appropriate functional outcome. cAMP compartmentalization relies on the formation of local signaling domains where the subset of cAMP signaling effectors, regulators and targets involved in a specific cellular response cluster together. These domains are dynamic in nature and underpin the exacting spatiotemporal regulation of cAMP signaling. In this review, we focus on how the proteomics toolbox can be utilized to identify the molecular components of these domains and to define the dynamic cellular cAMP signaling landscape. From a therapeutic perspective, compiling data on compartmentalized cAMP signaling in physiological and pathological conditions will help define the signaling events underlying disease and may reveal domain-specific targets for the development of precision medicine interventions.

Nuclear High Mobility Group A2 (HMGA2) Interactome Revealed by Biotin Proximity Labeling

The non-histone chromatin binding protein High Mobility Group AT-hook protein 2 (HMGA2) has important functions in chromatin remodeling, and genome maintenance and protection. Expression of HMGA2 is highest in embryonic stem cells, declines during cell differentiation and cell aging, but it is re-expressed in some cancers, where high HMGA2 expression frequently coincides with a poor prognosis. The nuclear functions of HMGA2 cannot be explained by binding to chromatin alone but involve complex interactions with other proteins that are incompletely understood. The present study used biotin proximity labeling, followed by proteomic analysis, to identify the nuclear interaction partners of HMGA2. We tested two different biotin ligase HMGA2 constructs (BioID2 and miniTurbo) with similar results, and identified known and new HMGA2 interaction partners, with functionalities mainly in chromatin biology. These HMGA2 biotin ligase fusion constructs offer exciting new possibilities for interactome discovery research, enabling the monitoring of nuclear HMGA2 interactomes during drug treatments.

From prediction to function: Current practices and challenges towards the functional characterization of type III effectors

The type III secretion system (T3SS) is a well-studied pathogenicity determinant of many bacteria through which effectors (T3Es) are translocated into the host cell, where they exercise a wide range of functions to deceive the host cell's immunity and to establish a niche. Here we look at the different approaches that are used to functionally characterize a T3E. Such approaches include host localization studies, virulence screenings, biochemical activity assays, and large-scale omics, such as transcriptomics, interactomics, and metabolomics, among others. By means of the phytopathogenic Ralstonia solanacearum species complex (RSSC) as a case study, the current advances of these methods will be explored, alongside the progress made in understanding effector biology. Data obtained by such complementary methods provide crucial information to comprehend the entire function of the effectome and will eventually lead to a better understanding of the phytopathogen, opening opportunities to tackle it.

Selection methods for proximity-dependent enrichment of ligands from DNA-encoded libraries using enzymatic fusion proteins

Herein, we report a selection approach to enrich ligands from DNA-encoded libraries (DELs) based on proximity to an enzymatic tag on the target protein. This method involves uncaging or installation of a biotin purification tag on the DNA construct either through photodeprotection of a protected biotin group using a light emitting protein tag (nanoluciferase) or by acylation using an engineered biotin ligase (UltraID). This selection does not require purification of the target protein and results in improved recovery and enrichment of DNA-linked ligands. This approach should serve as a general and convenient tool for molecular discovery with DELs.

CRISPR-Guided Proximity Labeling of RNA–Protein Interactions

Proximity labeling employs modified biotin ligases or peroxidases that produce reactive radicals to covalently label proximate proteins with biotin in living cells. The resulting biotinylated proteins can then be isolated and identified. A combination of programmable DNA targeting and proximity labeling that maps proteomic landscape at DNA elements with dCas9-APEX2 has been established in living cells. However, defining interactome at RNA elements has lagged behind. In combination with RNA-targeting CRISPR-Cas13, proximity labeling can also be used to identify proteins that interact with specific RNA elements in living cells. From this viewpoint, we briefly summarize the latest advances in CRISPR-guided proximity labeling in studying RNA–protein interactions, and we propose applying the most recent engineered proximity-labeling enzymes to study RNA-centric interactions in the future.