Expressed glycosylphosphatidylinositol-anchored horseradish peroxidase identifies co-clustering molecules in individual lipid raft domains

Extracellular proximal interaction profiling by cell surface-targeted TurboID reveals LDLR as a partner of liganded EGFR

Plasma membrane proteins play pivotal roles in receiving and transducing signals from other cells and from the environment and are vital for cellular functionality. Enzyme-based, proximity-dependent approaches, such as biotin identification (BioID), combined with mass spectrometry have begun to illuminate the landscape of proximal protein interactions within intracellular compartments. To extend the potential of these approaches to study the extracellular environment, we developed extracellular TurboID (ecTurboID), a method designed to profile the interactions between proteins on the surfaces of living cells over short timescales using the fast-acting biotin ligase TurboID. After optimizing our experimental and data analysis strategies to capture extracellular proximity interactions, we used ecTurboID to reveal the proximal interactomes of several plasma membrane proteins, including the epidermal growth factor receptor (EGFR). We found that EGF stimulation induced an association between EGFR and the low-density lipoprotein receptor (LDLR) and changed the interactome of LDLR by increasing its proximity with proteins that regulate EGFR signaling. The identification of this interaction between two well-studied and clinically relevant receptors illustrates the utility of our modified proximity labeling methodology for identifying dynamic extracellular associations between plasma membrane proteins.

Recent Advancements in Subcellular Proteomics: Growing Impact of Organellar Protein Niches on the Understanding of Cell Biology

The mammalian cell is a complex entity, with membrane-bound and membrane-less organelles playing vital roles in regulating cellular homeostasis. Organellar protein niches drive discrete biological processes and cell functions, thus maintaining cell equilibrium. Cellular processes such as signaling, growth, proliferation, motility, and programmed cell death require dynamic protein movements between cell compartments. Aberrant protein localization is associated with a wide range of diseases. Therefore, analyzing the subcellular proteome of the cell can provide a comprehensive overview of cellular biology. With recent advancements in mass spectrometry, imaging technology, computational tools, and deep machine learning algorithms, studies pertaining to subcellular protein localization and their dynamic distributions are gaining momentum. These studies reveal changing interaction networks because of "moonlighting proteins" and serve as a discovery tool for disease network mechanisms. Consequently, this review aims to provide a comprehensive repository for recent advancements in subcellular proteomics subcontexting methods, challenges, and future perspectives for method developers. In summary, subcellular proteomics is crucial to the understanding of the fundamental cellular mechanisms and the associated diseases.

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.

Proximity Labeling and Proteomics: Get to Know Neighbors

Protein-protein interactions are essential in biological reactions and fundamental to cell-cell communication (e.g., the binding of secreted proteins, such as hormones, to cell membrane receptors) and the subsequent intracellular signal transduction cascade. Several studies have been extensively carried out on protein-protein interactions because they have the potential to resolve various problems in molecular biology. Biochemical methods, such as chemical cross-linking and immunoprecipitation, have long been used to analyze which proteins interact with each other. However, there are some problems, such as unphysiological states and non-specific binding, that require the development of more useful experimental methods. This chapter discusses the "proximity labeling (Proteomics)" analysis technique, which has been attracting attention in protein-protein interaction analysis in recent years and is used in many biological studies. "Membrane proximity labeling (proteomics)," which analyzes the interaction of cell membrane proteins, and "intracellular proximity labeling (proteomics)" will be explained in-depth.

Mass spectrometry and the cellular surfaceome

The collection of exposed plasma membrane proteins, collectively termed the surfaceome, is involved in multiple vital cellular processes, such as the communication of cells with their surroundings and the regulation of transport across the lipid bilayer. The surfaceome also plays key roles in the immune system by recognizing and presenting antigens, with its possible malfunctioning linked to disease. Surface proteins have long been explored as potential cell markers, disease biomarkers, and therapeutic drug targets. Despite its importance, a detailed study of the surfaceome continues to pose major challenges for mass spectrometry-driven proteomics due to the inherent biophysical characteristics of surface proteins. Their inefficient extraction from hydrophobic membranes to an aqueous medium and their lower abundance compared to intracellular proteins hamper the analysis of surface proteins, which are therefore usually underrepresented in proteomic datasets. To tackle such problems, several innovative analytical methodologies have been developed. This review aims at providing an extensive overview of the different methods for surfaceome analysis, with respective considerations for downstream mass spectrometry-based proteomics.

ER entry pathway and glycosylation of GPI-anchored proteins are determined by N-terminal signal sequence and C-terminal GPI-attachment sequence

Newly synthesized proteins in the secretory pathway, including glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs), need to be correctly targeted and imported into the endoplasmic reticulum (ER) lumen. GPI-APs are synthesized in the cytosol as preproproteins, which contain an N-terminal signal sequence (SS), mature protein part, and C-terminal GPI-attachment sequence (GPI-AS), and translocated into the ER lumen where SS and GPI-AS are removed, generating mature GPI-APs. However, how various GPI-APs are translocated into the ER lumen in mammalian cells is unclear. Here, we investigated the ER entry pathways of GPI-APs using a panel of KO cells defective in each signal recognition particle–independent ER entry pathway—namely, Sec62, GET, or SND pathway. We found GPI-AP CD59 largely depends on the SND pathway for ER entry, whereas prion protein (Prion) and LY6K depend on both Sec62 and GET pathways. Using chimeric Prion and LY6K constructs in which the N-terminal SS or C-terminal GPI-AS was replaced with that of CD59, we revealed that the hydrophobicity of the SSs and GPI-ASs contributes to the dependence on Sec62 and GET pathways, respectively. Moreover, the ER entry route of chimeric Prion constructs with the C-terminal GPI-ASs replaced with that of CD59 was changed to the SND pathway. Simultaneously, their GPI structures and which oligosaccharyltransferase isoforms modify the constructs were altered without any amino acid change in the mature protein part. Taking these findings together, this study revealed N- and C-terminal sequences of GPI-APs determine the selective ER entry route, which in turn regulates subsequent maturation processes of GPI-APs.

Proximity-dependent biotinylation approaches to study apicomplexan biology

In the last 10 years, proximity-dependent biotinylation (PDB) techniques greatly expanded the ability to study protein environments in the living cell that range from specific protein complexes to entire compartments. This is achieved by using enzymes such as BirA* and APEX that are fused to proteins of interest and biotinylate proteins in their proximity. PDB techniques are now also increasingly used in apicomplexan parasites. In this review, we first give an overview of the main PDB approaches and how they compare with other techniques that address similar questions. PDB is particularly valuable to detect weak or transient protein associations under physiological conditions and to study cellular structures that are difficult to purify or have a poorly understood protein composition. We also highlight new developments such as novel smaller or faster-acting enzyme variants and conditional PDB approaches, providing improvements in both temporal and spatial resolution which may offer broader application possibilities useful in apicomplexan research. In the second part, we review work using PDB techniques in apicomplexan parasites and how this expanded our knowledge about these medically important parasites.

Proximity labeling approaches to study protein complexes during virus infection

Cellular compartmentalization of proteins and protein complex formation allow cells to tightly control biological processes. Therefore, understanding the subcellular localization and interactions of a specific protein is crucial to uncover its biological function. The advent of proximity labeling (PL) has reshaped cellular proteomics in infection biology. PL utilizes a genetically modified enzyme that generates a "labeling cloud" by covalently labeling proteins in close proximity to the enzyme. Fusion of a PL enzyme to a specific antibody or a "bait" protein of interest in combination with affinity enrichment mass spectrometry (AE-MS) enables the isolation and identification of the cellular proximity proteome, or proxisome. This powerful methodology has been paramount for the mapping of membrane or membraneless organelles as well as for the understanding of hard-to-purify protein complexes, such as those of transmembrane proteins. Unsurprisingly, more and more infection biology research groups have recognized the potential of PL for the identification of host-pathogen interactions. In this chapter, we introduce the enzymes commonly used for PL labeling as well as recent promising advancements and summarize the major achievements in organelle mapping and nucleic acid PL. Moreover, we comprehensively describe the research on host-pathogen interactions using PL, giving special attention to studies in the field of virology.

Deciphering molecular interactions by proximity labeling

Many biological processes are executed and regulated through the molecular interactions of proteins and nucleic acids. Proximity labeling (PL) is a technology for tagging the endogenous interaction partners of specific protein 'baits', via genetic fusion to promiscuous enzymes that catalyze the generation of diffusible reactive species in living cells. Tagged molecules that interact with baits can then be enriched and identified by mass spectrometry or nucleic acid sequencing. Here we review the development of PL technologies and highlight studies that have applied PL to the discovery and analysis of molecular interactions. In particular, we focus on the use of PL for mapping protein-protein, protein-RNA and protein-DNA interactions in living cells and organisms.

Pseudotyping Lentiviral Vectors: When the Clothes Make the Virus

Delivering transgenes to human cells through transduction with viral vectors constitutes one of the most encouraging approaches in gene therapy. Lentivirus-derived vectors are among the most promising vectors for these approaches. When the genetic modification of the cell must be performed in vivo, efficient specific transduction of the cell targets of the therapy in the absence of off-targeting constitutes the Holy Grail of gene therapy. For viral therapy, this is largely determined by the characteristics of the surface proteins carried by the vector. In this regard, an important property of lentiviral vectors is the possibility of being pseudotyped by envelopes of other viruses, widening the panel of proteins with which they can be armed. Here, we discuss how this is achieved at the molecular level and what the properties and the potentialities of the different envelope proteins that can be used for pseudotyping these vectors are.

Proximity-dependent labeling methods for proteomic profiling in living cells: An update

Characterizing the proteome composition of organelles and subcellular regions of living cells can facilitate the understanding of cellular organization as well as protein interactome networks. Proximity labeling-based methods coupled with mass spectrometry (MS) offer a high-throughput approach for systematic analysis of spatially restricted proteomes. Proximity labeling utilizes enzymes that generate reactive radicals to covalently tag neighboring proteins. The tagged endogenous proteins can then be isolated for further analysis by MS. To analyze protein-protein interactions or identify components that localize to discrete subcellular compartments, spatial expression is achieved by fusing the enzyme to specific proteins or signal peptides that target to particular subcellular regions. Although these technologies have only been introduced recently, they have already provided deep insights into a wide range of biological processes. Here, we provide an updated description and comparison of proximity labeling methods, as well as their applications and improvements. As each method has its own unique features, the goal of this review is to describe how different proximity labeling methods can be used to answer different biological questions. This article is categorized under: Technologies > Analysis of Proteins.

Proximity Dependent Biotinylation: Key Enzymes and Adaptation to Proteomics Approaches

The study of protein subcellular distribution, their assembly into complexes and the set of proteins with which they interact with is essential to our understanding of fundamental biological processes. Complementary to traditional assays, proximity-dependent biotinylation (PDB) approaches coupled with mass spectrometry (such as BioID or APEX) have emerged as powerful techniques to study proximal protein interactions and the subcellular proteome in the context of living cells and organisms. Since their introduction in 2012, PDB approaches have been used in an increasing number of studies and the enzymes themselves have been subjected to intensive optimization. How these enzymes have been optimized and considerations for their use in proteomics experiments are important questions. Here, we review the structural diversity and mechanisms of the two main classes of PDB enzymes: the biotin protein ligases (BioID) and the peroxidases (APEX). We describe the engineering of these enzymes for PDB and review emerging applications, including the development of PDB for coincidence detection (split-PDB). Lastly, we briefly review enzyme selection and experimental design guidelines and reflect on the labeling chemistries and their implication for data interpretation.

BioID Combined with Mass Spectrometry to Study Herpesvirus Protein-Protein Interaction Networks

Herpes viruses are important human pathogens that cause a wide range of diseases from skin lesions to malignancies. Protein interactions drive many cellular events and mediate a number of biochemical pathways leading to different physiological outcomes. Protein interactions between viral proteins and host proteins play significant roles in viral entry, replication and suppression of host-immune responses. Therefore, the study of virus-host interactions promises significant advancement in designing therapeutics to control infection and disease. Various approaches are employed in the field to study and identify protein interactions that combine affinity purification along with different detection methods. Advancements in protein purification and high-throughput detection methods have resulted in an unprecedented level of discovery. Here we detail the use of proximity dependent biotinylation (BioID) as a means of affinity purification coupled with the use of LC-MS/MS for the detection and identification of protein-protein interaction networks.

The EMARS Reaction for Proximity Labeling

To understand cellular processes at molecular levels, elucidation of protein-protein interactions occurring at a specific location in living cells is required. We have developed a proximity labeling method mediated by the enzyme-mediated activation of radical source (EMARS) reaction, which features a radical formation from labeling reagents by horseradish peroxidase (HRP) set on a molecule of interest (probed molecule). Proximal molecules are covalently labeled with a tag conjugated with the labeling reagent. Here we describe protocols for preparation of a labeling reagent, labeling of neighboring proteins of the probed molecule in living cells, and identification of the labeled proteins.

Discovering cellular protein-protein interactions: Technological strategies and opportunities

The analysis of protein interaction networks is one of the key challenges in the study of biology. It connects genotypes to phenotypes, and disruption often leads to diseases. Hence, many technologies have been developed to study protein-protein interactions (PPIs) in a cellular context. The expansion of the PPI technology toolbox however complicates the selection of optimal approaches for diverse biological questions. This review gives an overview of the binary and co-complex technologies, with the former evaluating the interaction of two co-expressed genetically tagged proteins, and the latter only needing the expression of a single tagged protein or no tagged proteins at all. Mass spectrometry is crucial for some binary and all co-complex technologies. After the detailed description of the different technologies, the review compares their unique specifications, advantages, disadvantages, and applicability, while highlighting opportunities for further advancements.

Biological functions of sulfoglycolipids and the EMARS method for identification of co-clustered molecules in the membrane microdomains

Two major sulfoglycolipids, sulfatide (SO3-3Gal-ceramide) and seminolipid (SO3-3Gal-alkylacylglycerol) exist in mammals. Sulfatide is abundant in the myelin sheath and seminolipid is unique to the spermatogenic cells. The carbohydrate moiety of sulfatide and seminolipid is identical and synthesized by common enzymes: ceramide galactosyltransferase (CGT) and cerebroside sulfotransferase (CST). We have purified CST homogenously, cloned the CST gene and generated CST-knockout mice. CST-null mice completely lack sulfoglycolipids all over the body. Analysis of CST-null mice has revealed that sulfatide is an essential component for the axo-glial junction at the paranode region and regulates terminal differentiation of oligodendrocytes, and that seminolipid is responsible for the formation of a functional lactate transporter assembly to take up the critical energy source for spermatocytes. We have developed a new analytical method termed EMARS to identify co-clustered molecules in the membrane microdomains in order to elucidate the functional molecules that collaborate with sulfoglycolipids.

Research Techniques Made Simple: Emerging Methods to Elucidate Protein Interactions through Spatial Proximity

Interactions between proteins are essential for fundamental cellular processes, and the diversity of such interactions enables the vast variety of functions essential for life. A persistent goal in biological research is to develop assays that can faithfully capture different types of protein interactions to allow their study. A major step forward in this direction came with a family of methods that delineates spatial proximity of proteins as an indirect measure of protein-protein interaction. A variety of enzyme- and DNA ligation-based methods measure protein co-localization in space, capturing novel interactions that were previously too transient or low affinity to be identified. Here we review some of the methods that have been successfully used to measure spatially proximal protein-protein interactions.

Thy-1 Regulates VEGF-Mediated Choroidal Endothelial Cell Activation and Migration: Implications in Neovascular Age-Related Macular Degeneration

Purpose

This study addresses the hypothesis that age-related stresses upregulate Thy-1 in choroidal endothelial cells (CECs) and contribute to CEC activation and migration, processes important in choroidal neovascularization (CNV).

Methods

Measurements were made of Thy-1 protein (Western blot) in CECs and Thy-1 mRNA (real time quantitative PCR) in CECs treated with VEGF, CCL11, or PBS or in RPE/choroids from young or old donors or lasered or nonlasered mice. Immunolabeled Thy-1 in ocular sections was compared from young versus old human donor eyes or those with or without neovascular AMD or from lasered versus nonlasered mice. Choroidal endothelial cells transfected with Thy-1 or control siRNA or pretreated with Thy-1 blocking peptide or control were stimulated with VEGF or 7-ketocholesterol (7-KC). Choroidal endothelial cell migration, proliferation, cytoskeletal stress fibers, Rac1 activation, and phosphorylated VEGF receptor 2 (VEGFR2), integrin β3, and Src were measured. Statistics were performed using ANOVA.

Results

Thy-1 was expressed in retinal ganglion cells and in vascular endothelial-cadherin–labeled choroid and localized to human or mouse laser-induced CNV lesions. Thy-1 protein and mRNA were significantly increased in CECs treated with VEGF or CCL11 and in RPE/choroids from aged versus young donor eyes or from lasered mice versus nonlasered controls. Knockdown or inhibition of Thy-1 in CECs significantly reduced VEGF-induced CEC migration and proliferation, stress fiber formation and VEGFR2, Src, integrin β3 and Rac1 activation, and 7-KC–induced Rac1 and Src activation.

Conclusions

Thy-1 in CECs regulates VEGF-induced CEC activation and migration and links extracellular 7-KC to intracellular signaling. Future studies elucidating Thy-1 mechanisms in neovascular AMD are warranted.

Proximity-dependent labeling methods for proteomic profiling in living cells

Characterizing the proteome composition of organelles and subcellular regions of living cells can facilitate the understanding of cellular organization as well as protein interactome networks. Proximity labeling-based methods coupled with mass spectrometry (MS) offer a high-throughput approach for systematic analysis of spatially restricted proteomes. Proximity labeling utilizes enzymes that generate reactive radicals to covalently tag neighboring proteins with biotin. The biotinylated endogenous proteins can then be isolated for further analysis by MS. To analyze protein-protein interactions or identify components that localize to discrete subcellular compartments, spatial expression is achieved by fusing the enzyme to specific proteins or signal peptides that target to particular subcellular regions. Although these technologies have only been introduced recently, they have already provided deep insights into a wide range of biological processes. Here, we describe and compare current methods of proximity labeling as well as their applications. As each method has its own unique features, the goal of this review is to describe how different proximity labeling methods can be used to answer different biological questions. WIREs Dev Biol 2017, 6:e272. doi: 10.1002/wdev.272 For further resources related to this article, please visit the WIREs website.

Filling the Void: Proximity-Based Labeling of Proteins in Living Cells

There are inherent limitations with traditional methods to study protein behavior or to determine the constituency of proteins in discrete subcellular compartments. In response to these limitations, several methods have recently been developed that use proximity-dependent labeling. By fusing proteins to enzymes that generate reactive molecules, most commonly biotin, proximate proteins are covalently labeled to enable their isolation and identification. In this review we describe current methods for proximity-dependent labeling in living cells and discuss their applications and future use in the study of protein behavior.

Protein Neighbors and Proximity Proteomics

Within cells, proteins can co-assemble into functionally integrated and spatially restricted multicomponent complexes. Often, the affinities between individual proteins are relatively weak, and proteins within such clusters may interact only indirectly with many of their other protein neighbors. This makes proteomic characterization difficult using methods such as immunoprecipitation or cross-linking. Recently, several groups have described the use of enzyme-catalyzed proximity labeling reagents that covalently tag the neighbors of a targeted protein with a small molecule such as fluorescein or biotin. The modified proteins can then be isolated by standard pulldown methods and identified by mass spectrometry. Here we will describe the techniques as well as their similarities and differences. We discuss their applications both to study protein assemblies and to provide a new way for characterizing organelle proteomes. We stress the importance of proteomic quantitation and independent target validation in such experiments. Furthermore, we suggest that there are biophysical and cell-biological principles that dictate the appropriateness of enzyme-catalyzed proximity labeling methods to address particular biological questions of interest.