Identification of Microprotein-Protein Interactions via APEX Tagging

Exploring the world of small proteins in plant biology and bioengineering

Small proteins are ubiquitous in all kingdoms of life. MicroProteins, initially characterized as small proteins with protein interaction domains that enable them to interact with larger multidomain proteins, frequently modulate the function of these proteins. The study of these small proteins has contributed to a greater comprehension of protein regulation. In addition to sequence homology, sequence-divergent small proteins have the potential to function as microProtein mimics, binding to structurally related proteins. Moreover, a multitude of other small proteins encoded by short open reading frames (sORFs) and peptides, derived from diverse sources such as long noncoding RNAs (lncRNAs) and miRNAs, contribute to a variety of biological processes. The potential of small proteins is evident, offering promising avenues for bioengineering that could revolutionize crop performance and reduce reliance on agrochemicals in future agriculture.

Microproteins unveiling new dimensions in cancer

In the complex landscape of cancer biology, the discovery of microproteins has triggered a paradigm shift, thereby, challenging the conventional conceptions of gene regulation. Though overlooked for years, these entities encoded by the small open reading frames (100-150 codons), have a significant impact on various cellular processes. As precision medicine pioneers delve deeper into the genome and proteome, microproteins have come into the limelight. Typically characterized by a single protein domain that directly binds to the target protein complex and regulates their assembly, these microproteins have been shown to play a key role in fundamental biological processes such as RNA processing, DNA repair, and metabolism regulation. Techniques for identification and characterization, such as ribosome profiling and proteogenomic approaches, have unraveled unique mechanisms by which these microproteins regulate cell signaling or pathological processes in most diseases including cancer. However, the functional relevance of these microproteins in cancer remains unclear. In this context, the current review aims to "rethink the essence of these genes" and explore "how these hidden players-microproteins orchestrate the signaling cascades of cancer, both as accelerators and brakes.".

Leveraging a self-cleaving peptide for tailored control in proximity labeling proteomics

Protein-protein interactions play an important biological role in every aspect of cellular homeostasis and functioning. Proximity labeling mass spectrometry-based proteomics overcomes challenges typically associated with other methods and has quickly become the current state of the art in the field. Nevertheless, tight control of proximity-labeling enzymatic activity and expression levels is crucial to accurately identify protein interactors. Here, we leverage a T2A self-cleaving peptide and a non-cleaving mutant to accommodate the protein of interest in the experimental and control TurboID setup. To allow easy and streamlined plasmid assembly, we built a Golden Gate modular cloning system to generate plasmids for transient expression and stable integration. To highlight our T2A Split/link design, we applied it to identify protein interactions of the glucocorticoid receptor and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid and non-structural protein 7 (NSP7) proteins by TurboID proximity labeling. Our results demonstrate that our T2A split/link provides an opportune control that builds upon previously established control requirements in the field.

Micropeptides: potential treatment strategies for cancer

Some noncoding RNAs (ncRNAs) carry open reading frames (ORFs) that can be translated into micropeptides, although noncoding RNAs (ncRNAs) have been previously assumed to constitute a class of RNA transcripts without coding capacity. Furthermore, recent studies have revealed that ncRNA-derived micropeptides exhibit regulatory functions in the development of many tumours. Although some of these micropeptides inhibit tumour growth, others promote it. Understanding the role of ncRNA-encoded micropeptides in cancer poses new challenges for cancer research, but also offers promising prospects for cancer therapy. In this review, we summarize the types of ncRNAs that can encode micropeptides, highlighting recent technical developments that have made it easier to research micropeptides, such as ribosome analysis, mass spectrometry, bioinformatics methods, and CRISPR/Cas9. Furthermore, based on the distribution of micropeptides in different subcellular locations, we explain the biological functions of micropeptides in different human cancers and discuss their underestimated potential as diagnostic biomarkers and anticancer therapeutic targets in clinical applications, information that may contribute to the discovery and development of new micropeptide-based tools for early diagnosis and anticancer drug development.

A naturally occurring variant of SHLP2 is a protective factor in Parkinson's disease

Mitochondrial DNA single nucleotide polymorphisms (mtSNPs) have been associated with a reduced risk of developing Parkinson's disease (PD), yet the underlying mechanisms remain elusive. In this study, we investigate the functional role of a PD-associated mtSNP that impacts the mitochondrial-derived peptide (MDP) Small Humanin-like Peptide 2 (SHLP2). We identify m.2158 T > C, a mtSNP associated with reduced PD risk, within the small open reading frame encoding SHLP2. This mtSNP results in an alternative form of SHLP2 (lysine 4 replaced with arginine; K4R). Using targeted mass spectrometry, we detect specific tryptic fragments of SHLP2 in neuronal cells and demonstrate its binding to mitochondrial complex 1. Notably, we observe that the K4R variant, associated with reduced PD risk, exhibits increased stability compared to WT SHLP2. Additionally, both WT and K4R SHLP2 show enhanced protection against mitochondrial dysfunction in in vitro experiments and confer protection against a PD-inducing toxin, a mitochondrial complex 1 inhibitor, in a mouse model. This study sheds light on the functional consequences of the m.2158 T > C mtSNP on SHLP2 and provides insights into the potential mechanisms by which this mtSNP may reduce the risk of PD.

Biochemistry and Protein Interactions of the CYREN Microprotein

Over the past decade, advances in genomics have identified thousands of additional protein-coding small open reading frames (smORFs) missed by traditional gene finding approaches. These smORFs encode peptides and small proteins, commonly termed micropeptides or microproteins. Several of these newly discovered microproteins have biological functions and operate through interactions with proteins and protein complexes within the cell. CYREN1 is a characterized microprotein that regulates double-strand break repair in mammalian cells through interaction with Ku70/80 heterodimer. Ku70/80 binds to and stabilizes double-strand breaks and recruits the machinery needed for nonhomologous end join repair. In this study, we examined the biochemical properties of CYREN1 to better understand and explain its cellular protein interactions. Our findings support that CYREN1 is an intrinsically disordered microprotein and this disordered structure allows it to enriches several proteins, including a newly discovered interaction with SF3B1 via a distinct short linear motif (SLiMs) on CYREN1. Since many microproteins are predicted to be disordered, CYREN1 is an exemplar of how microproteins interact with other proteins and reveals an unknown scaffolding function of this microprotein that may link NHEJ and splicing.

Chemical and Biological Strategies for Profiling Protein-Protein Interactions in Living Cells

Protein-protein interactions (PPIs) play critical roles in almost all cellular signal transduction events. Characterization of PPIs without interfering with the functions of intact cells is very important for basic biology study and drug developments. However, the ability to profile PPIs especially those weak/transient interactions in their native states remains quite challenging. To this end, many endeavors are being made in developing new methods with high efficiency and strong operability. By coupling with advanced fluorescent microscopy and mass spectroscopy techniques, these strategies not only allow us to visualize the subcellular locations and monitor the functions of protein of interest (POI) in real time, but also enable the profiling and identification of potential unknown interacting partners in high-throughput manner, which greatly facilitates the elucidation of molecular mechanisms underlying numerous pathophysiological processes. In this review, we will summarize the typical methods for PPIs identification in living cells and their principles, advantages and limitations will also be discussed in detail.

Chemical labeling and proteomics for characterization of unannotated small and alternative open reading frame-encoded polypeptides

Thousands of unannotated small and alternative open reading frames (smORFs and alt-ORFs, respectively) have recently been revealed in mammalian genomes. While hundreds of mammalian smORF- and alt-ORF-encoded proteins (SEPs and alt-proteins, respectively) affect cell proliferation, the overwhelming majority of smORFs and alt-ORFs remain uncharacterized at the molecular level. Complicating the task of identifying the biological roles of smORFs and alt-ORFs, the SEPs and alt-proteins that they encode exhibit limited sequence homology to protein domains of known function. Experimental techniques for the functionalization of these gene classes are therefore required. Approaches combining chemical labeling and quantitative proteomics have greatly advanced our ability to identify and characterize functional SEPs and alt-proteins in high throughput. In this review, we briefly describe the principles of proteomic discovery of SEPs and alt-proteins, then summarize how these technologies interface with chemical labeling for identification of SEPs and alt-proteins with specific properties, as well as in defining the interactome of SEPs and alt-proteins.

Small Open Reading Frame-Encoded Micro-Peptides: An Emerging Protein World

Small open reading frames (sORFs) are often overlooked features in genomes. In the past, they were labeled as noncoding or "transcriptional noise". However, accumulating evidence from recent years suggests that sORFs may be transcribed and translated to produce sORF-encoded polypeptides (SEPs) with less than 100 amino acids. The vigorous development of computational algorithms, ribosome profiling, and peptidome has facilitated the prediction and identification of many new SEPs. These SEPs were revealed to be involved in a wide range of basic biological processes, such as gene expression regulation, embryonic development, cellular metabolism, inflammation, and even carcinogenesis. To effectively understand the potential biological functions of SEPs, we discuss the history and development of the newly emerging research on sORFs and SEPs. In particular, we review a range of recently discovered bioinformatics tools for identifying, predicting, and validating SEPs as well as a variety of biochemical experiments for characterizing SEP functions. Lastly, this review underlines the challenges and future directions in identifying and validating sORFs and their encoded micropeptides, providing a significant reference for upcoming research on sORF-encoded peptides.

Microproteins: Overlooked regulators of physiology and disease

Ongoing efforts to generate a complete and accurate annotation of the genome have revealed a significant blind spot for small proteins (<100 amino acids) originating from short open reading frames (sORFs). The recent discovery of numerous sORF-encoded proteins, termed microproteins, that play diverse roles in critical cellular processes has ignited the field of microprotein biology. Large-scale efforts are currently underway to identify sORF-encoded microproteins in diverse cell-types and tissues and specialized methods and tools have been developed to aid in their discovery, validation, and functional characterization. Microproteins that have been identified thus far play important roles in fundamental processes including ion transport, oxidative phosphorylation, and stress signaling. In this review, we discuss the optimized tools available for microprotein discovery and validation, summarize the biological functions of numerous microproteins, outline the promise for developing microproteins as therapeutic targets, and look forward to the future of the field of microprotein biology.

Uncharacterized Proteins CxORFx: Subinteractome Analysis and Prognostic Significance in Cancers

Functions of about 10% of all the proteins and their associations with diseases are poorly annotated or not annotated at all. Among these proteins, there is a group of uncharacterized chromosome-specific open-reading frame genes (CxORFx) from the 'Tdark' category. The aim of the work was to reveal associations of CxORFx gene expression and ORF proteins' subinteractomes with cancer-driven cellular processes and molecular pathways. We performed systems biology and bioinformatic analysis of 219 differentially expressed CxORFx genes in cancers, an estimation of prognostic significance of novel transcriptomic signatures and analysis of subinteractome composition using several web servers (GEPIA2, KMplotter, ROC-plotter, TIMER, cBioPortal, DepMap, EnrichR, PepPSy, cProSite, WebGestalt, CancerGeneNet, PathwAX II and FunCoup). The subinteractome of each ORF protein was revealed using ten different data sources on physical protein-protein interactions (PPIs) to obtain representative datasets for the exploration of possible cellular functions of ORF proteins through a spectrum of neighboring annotated protein partners. A total of 42 out of 219 presumably cancer-associated ORF proteins and 30 cancer-dependent binary PPIs were found. Additionally, a bibliometric analysis of 204 publications allowed us to retrieve biomedical terms related to ORF genes. In spite of recent progress in functional studies of ORF genes, the current investigations aim at finding out the prognostic value of CxORFx expression patterns in cancers. The results obtained expand the understanding of the possible functions of the poorly annotated CxORFx in the cancer context.

Plant microProteins: Small but powerful modulators of plant development

MicroProteins (miPs) are small and single-domain containing proteins of less than 20 kDa. This domain allows microProteins to interact with compatible domains of evolutionary-related proteins and fine-tuning the key physiological pathways in several organisms. Since the first report of a microProtein in mice, numerous microProteins have been identified in plants by computational approaches. However, only a few candidates have been functionally characterized, primarily in Arabidopsis. The recent success of synthetic microProteins in modulating physiological activities in crops makes these proteins interesting candidates for crop engineering. Here, we comprehensively summarise the synthesis, mode of action, and functional roles of microProteins in plants. We also discuss different approaches used to identify plant microProteins. Additionally, we discuss novel approaches to design synthetic microProteins that can be used to target proteins regulating plant growth and development. We finally highlight the prospects and challenges of utilizing microProteins in future crop improvement programs.

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MicroProteins (miPs) are small-sized proteins with a molecular weight of 5–20 kDa

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MiPs can be detected through multiomics and computational approaches

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MiPs are crucial regulators of plant growth and development

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MiPs as condensates, synthetic miPs, and limitations

Novel insights into noncanonical open reading frames in cancer

With technological advances, previously neglected noncanonical open reading frames (nORFs) are drawing ever-increasing attention. However, the translation potential of numerous putative nORFs remains elusive, and the functions of noncanonical peptides have not been systemically summarized. Moreover, the relationship between noncanonical peptides and their counterpart protein or RNA products remains elusive and the clinical implementation of noncanonical peptides has not been explored. In this review, we highlight how recent technological advances such as ribosome profiling, bioinformatics approaches and CRISPR/Cas9 facilitate the research of noncanonical peptides. We delineate the features of each nORF category and the evolutionary process underneath the nORFs. Most importantly, we summarize the diversified functions of noncanonical peptides in cancer based on their subcellular location, which reflect their extensive participation in key pathways and essential cellular activities in cancer cells. Meanwhile, the equilibrium between noncanonical peptides and their corresponding transcripts or counterpart products may be dysregulated under pathological states, which is essential for their roles in cancer. Lastly, we explore their underestimated potential in clinical application as diagnostic biomarkers and treatment targets against cancer.

Nascent alt-protein chemoproteomics reveals a pre-60S assembly checkpoint inhibitor

Many unannotated microproteins and alternative proteins (alt-proteins) are coencoded with canonical proteins, but few of their functions are known. Motivated by the hypothesis that alt-proteins undergoing regulated synthesis could play important cellular roles, we developed a chemoproteomic pipeline to identify nascent alt-proteins in human cells. We identified 22 actively translated alt-proteins or N-terminal extensions, one of which is post-transcriptionally upregulated by DNA damage stress. We further defined a nucleolar, cell-cycle-regulated alt-protein that negatively regulates assembly of the pre-60S ribosomal subunit (MINAS-60). Depletion of MINAS-60 increases the amount of cytoplasmic 60S ribosomal subunit, upregulating global protein synthesis and cell proliferation. Mechanistically, MINAS-60 represses the rate of late-stage pre-60S assembly and export to the cytoplasm. Together, these results implicate MINAS-60 as a potential checkpoint inhibitor of pre-60S assembly and demonstrate that chemoproteomics enables hypothesis generation for uncharacterized alt-proteins.

Proteomic-driven identification of short open reading frame-encoded peptides

Accumulating evidence has shown that a large number of short open reading frames (sORFs) also have the ability to encode proteins. The discovery of sORFs opens up a new research area, leading to the identification and functional study of sORF encoded peptides (SEPs) at the omics level. Besides bioinformatics prediction and ribosomal profiling, mass spectrometry (MS) has become a significant tool as it directly detects the sequence of SEPs. Though MS-based proteomics methods have proved to be effective for qualitative and quantitative analysis of SEPs, the detection of SEPs is still a great challenge due to their low abundance and short sequence. To illustrate the progress in method development, we described and discussed the main steps of large-scale proteomics identification of SEPs, including SEP extraction and enrichment, MS detection, data processing and quality control, quantification, and function prediction and validation methods. This article is protected by copyright. All rights reserved.

Short open reading frames (sORFs) and microproteins: an update on their identification and validation measures

A short open reading frame (sORFs) constitutes ≤ 300 bases, encoding a microprotein or sORF-encoded protein (SEP) which comprises ≤ 100 amino acids. Traditionally dismissed by genome annotation pipelines as meaningless noise, sORFs were found to possess coding potential with ribosome profiling (RIBO-Seq), which unveiled sORF-based transcripts at various genome locations. Nonetheless, the existence of corresponding microproteins that are stable and functional was little substantiated by experimental evidence initially. With recent advancements in multi-omics, the identification, validation, and functional characterisation of sORFs and microproteins have become feasible. In this review, we discuss the history and development of an emerging research field of sORFs and microproteins. In particular, we focus on an array of bioinformatics and OMICS approaches used for predicting, sequencing, validating, and characterizing these recently discovered entities. These strategies include RIBO-Seq which detects sORF transcripts via ribosome footprints, and mass spectrometry (MS)-based proteomics for sequencing the resultant microproteins. Subsequently, our discussion extends to the functional characterisation of microproteins by incorporating CRISPR/Cas9 screen and protein–protein interaction (PPI) studies. Our review discusses not only detection methodologies, but we also highlight on the challenges and potential solutions in identifying and validating sORFs and their microproteins. The novelty of this review lies within its validation for the functional role of microproteins, which could contribute towards the future landscape of microproteomics.

Subcellular Transcriptomics and Proteomics: A Comparative Methods Review

The internal environment of cells is molecularly crowded, which requires spatial organization via subcellular compartmentalization. These compartments harbor specific conditions for molecules to perform their biological functions, such as coordination of the cell cycle, cell survival, and growth. This compartmentalization is also not static, with molecules trafficking between these subcellular neighborhoods to carry out their functions. For example, some biomolecules are multifunctional, requiring an environment with differing conditions or interacting partners, and others traffic to export such molecules. Aberrant localization of proteins or RNA species has been linked to many pathological conditions, such as neurological, cancer, and pulmonary diseases. Differential expression studies in transcriptomics and proteomics are relatively common, but the majority have overlooked the importance of subcellular information. In addition, subcellular transcriptomics and proteomics data do not always colocate because of the biochemical processes that occur during and after translation, highlighting the complementary nature of these fields. In this review, we discuss and directly compare the current methods in spatial proteomics and transcriptomics, which include sequencing- and imaging-based strategies, to give the reader an overview of the current tools available. We also discuss current limitations of these strategies as well as future developments in the field of spatial -omics.

Small Open Reading Frames, How to Find Them and Determine Their Function

Advances in genomics and molecular biology have revealed an abundance of small open reading frames (sORFs) across all types of transcripts. While these sORFs are often assumed to be non-functional, many have been implicated in physiological functions and a significant number of sORFs have been described in human diseases. Thus, sORFs may represent a hidden repository of functional elements that could serve as therapeutic targets. Unlike protein-coding genes, it is not necessarily the encoded peptide of an sORF that enacts its function, sometimes simply the act of translating an sORF might have a regulatory role. Indeed, the most studied sORFs are located in the 5′UTRs of coding transcripts and can have a regulatory impact on the translation of the downstream protein-coding sequence. However, sORFs have also been abundantly identified in non-coding RNAs including lncRNAs, circular RNAs and ribosomal RNAs suggesting that sORFs may be diverse in function. Of the many different experimental methods used to discover sORFs, the most commonly used are ribosome profiling and mass spectrometry. These can confirm interactions between transcripts and ribosomes and the production of a peptide, respectively. Extensions to ribosome profiling, which also capture scanning ribosomes, have further made it possible to see how sORFs impact the translation initiation of mRNAs. While high-throughput techniques have made the identification of sORFs less difficult, defining their function, if any, is typically more challenging. Together, the abundance and potential function of many of these sORFs argues for the necessity of including sORFs in gene annotations and systematically characterizing these to understand their potential functional roles. In this review, we will focus on the high-throughput methods used in the detection and characterization of sORFs and discuss techniques for validation and functional characterization.

Phosphorylation of a Human Microprotein Promotes Dissociation of Biomolecular Condensates

Proteogenomic identification of translated small open reading frames in humans has revealed thousands of microproteins, or polypeptides of fewer than 100 amino acids, that were previously invisible to geneticists. Hundreds of microproteins have been shown to be essential for cell growth and proliferation, and many regulate macromolecular complexes. However, the vast majority of microproteins remain functionally uncharacterized, and many lack secondary structure and exhibit limited evolutionary conservation. One such intrinsically disordered microprotein is NBDY, a 68-amino acid component of membraneless organelles known as P-bodies. In this work, we show that NBDY can undergo liquid-liquid phase separation, a biophysical process thought to underlie the formation of membraneless organelles, in the presence of RNA in vitro. Phosphorylation of NBDY drives liquid phase remixing in vitro and macroscopic P-body dissociation in cells undergoing growth factor signaling and cell division. These results suggest that NBDY phosphorylation enables regulation of P-body dynamics during cell proliferation and, more broadly, that intrinsically disordered microproteins may contribute to liquid-liquid phase separation and remixing behavior to affect cellular processes.

Bottom-up and top-down proteomic approaches for the identification, characterization, and quantification of the low molecular weight proteome with focus on short open reading frame-encoded peptides

The recent discovery of alternative open reading frames creates a need for suitable analytical approaches to verify their translation and to characterize the corresponding gene products at the molecular level. As the analysis of small proteins within a background proteome by means of classical bottom-up proteomics is challenging, method development for the analysis of small open reading frame encoded peptides (SEPs) have become a focal point for research. Here, we highlight bottom-up and top-down proteomics approaches established for the analysis of SEPs in both pro- and eukaryotes. Major steps of analysis, including sample preparation and (small) proteome isolation, separation and mass spectrometry, data interpretation and quality control, quantification, the analysis of post-translational modifications, and exploration of functional aspects of the SEPs by means of proteomics technologies are described. These methods do not exclusively cover the analytics of SEPs but simultaneously include the low molecular weight proteome, and moreover, can also be used for the proteome-wide analysis of proteolytic processing events.

A short ORF-encoded transcriptional regulator

Recent technological advances have expanded the annotated protein coding content of mammalian genomes, as hundreds of previously unidentified, short open reading frame (ORF)-encoded peptides (SEPs) have now been found to be translated. Although several studies have identified important physiological roles for this emerging protein class, a general method to define their interactomes is lacking. Here, we demonstrate that genetic incorporation of the photo-crosslinking noncanonical amino acid AbK into SEP transgenes allows for the facile identification of SEP cellular interaction partners using affinity-based methods. From a survey of seven SEPs, we report the discovery of short ORF-encoded histone binding protein (SEHBP), a conserved microprotein that interacts with chromatin-associated proteins, localizes to discrete genomic loci, and induces a robust transcriptional program when overexpressed in human cells. This work affords a straightforward method to help define the physiological roles of SEPs and demonstrates its utility by identifying SEHBP as a short ORF-encoded transcription factor.

Revisiting sORFs: overcoming challenges to identify and characterize functional microproteins

Short ORFs (sORFs), that is, occurrences of a start and stop codon within 100 codons or less, can be found in organisms of all domains of life, outnumbering annotated protein-coding ORFs by orders of magnitude. Even though functional proteins smaller than 100 amino acids are known, the coding potential of sORFs has often been overlooked, as it is not trivial to predict and test for functionality within the large number of sORFs. Recent advances in ribosome profiling and mass spectrometry approaches, together with refined bioinformatic predictions, have enabled a huge leap forward in this field and identified thousands of likely coding sORFs. A relatively low number of small proteins or microproteins produced from these sORFs have been characterized so far on the molecular, structural, and/or mechanistic level. These however display versatile and, in some cases, essential cellular functions, allowing for the exciting possibility that many more, previously unknown small proteins might be encoded in the genome, waiting to be discovered. This review will give an overview of the steadily growing microprotein field, focusing on eukaryotic small proteins. We will discuss emerging themes in the molecular action of microproteins, as well as advances and challenges in microprotein identification and characterization.

Alt-RPL36 downregulates the PI3K-AKT-mTOR signaling pathway by interacting with TMEM24

Thousands of human small and alternative open reading frames (smORFs and alt-ORFs, respectively) have recently been annotated. Many alt-ORFs are co-encoded with canonical proteins in multicistronic configurations, but few of their functions are known. Here, we report the detection of alt-RPL36, a protein co-encoded with human RPL36. Alt-RPL36 partially localizes to the endoplasmic reticulum, where it interacts with TMEM24, which transports the phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) precursor phosphatidylinositol from the endoplasmic reticulum to the plasma membrane. Knock-out of alt-RPL36 increases plasma membrane PI(4,5)P₂ levels, upregulates PI3K-AKT-mTOR signaling, and increases cell size. Alt-RPL36 contains four phosphoserine residues, point mutations of which abolish interaction with TMEM24 and, consequently, alt-RPL36 effects on PI3K signaling and cell size. These results implicate alt-RPL36 as an upstream regulator of PI3K-AKT-mTOR signaling. More broadly, the RPL36 transcript encodes two sequence-independent polypeptides that co-regulate translation via different molecular mechanisms, expanding our knowledge of multicistronic human gene functions.

Many alternative ORFs are co-encoded with characterized proteins, but their function is often not understood. Here, the authors discover that ribosomal protein L36 is co-encoded with alternative protein, which they identify as an upstream regulator of PI3K-AKT-mTOR signaling.

Proteomic Analysis of Human Esophageal Cancer Using Tandem Mass Tag Quantifications

Esophageal cancer (EC) is a type of extremely aggressive gastrointestinal cancer with high incidences in China and other Asian countries. EC does not have specific symptoms and is relatively easy to metastasize, which makes it difficult in early diagnosis. Thus, novel noninvasive diagnostic method is urgently needed in clinical practice. In this study, mass spectrometry with tandem mass tags and differential protein analysis were applied for identifying esophageal cancer-related proteins. The identified proteins were annotated based on their enrichment in Gene Ontology (GO) terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. In addition, hierarchical clustering was applied based on differentially expressed proteins. As a result, a total of 5131 quantifiable proteins were identified from our liquid chromatography-tandem mass spectrometry with tandem mass tags (LC-MS/MS-TMT) method with 63 upregulated and 97 downregulated differential proteins between esophageal cancer and controlled normal samples. The differentially expressed proteins were highly enriched in GO terms associated with mitochondrial dissemble and apoptosis, and blood vessel regulation, and the upregulated differentially expressed proteins in EC samples were significantly enriched in major histocompatibility complex MHC-class I/II pathway of immune system. The functional clustering analysis revealed potential protein-protein interactions among tetraspanin, myosin, and S-100. In summary, our study provided a practical technological procedure of proteomic analysis for discovering novel biomarkers of a specific cancer type.

Strategies to Investigate Ubiquitination in Huntington's Disease

Many neurodegenerative disorders including Huntington's Disease are hallmarked by intracellular protein aggregates that are decorated by ubiquitin and different ubiquitin ligases and deubiquitinating enzymes. The protein aggregates observed in Huntington's Disease are caused by a polyglutamine expansion in the N-terminus of the huntingtin protein (Htt). Improving the degradation of mutant Htt via the Ubiquitin Proteasome System prior to aggregation would be a therapeutic strategy to delay or prevent the onset of Huntington's Disease for which there is currently no cure. Here we examine the current approaches used to study the ubiquitination of both soluble Htt as well as insolubilized Htt present in aggregates, and we describe what is known about involved (de)ubiquitinating enzymes. Furthermore, we discuss novel methodologies to study the dynamics of Htt ubiquitination in living cells using fluorescent ubiquitin probes, to identify and quantify Htt ubiquitination by mass spectrometry-based approaches, and various approaches to identify involved ubiquitinating enzymes.

Proximity Labeling Techniques to Study Chromatin

Mammals contain over 200 different cell types, yet nearly all have the same genomic DNA sequence. It is a key question in biology how the genetic instructions in DNA are selectively interpreted by cells to specify various transcriptional programs and therefore cellular identity. The structural and functional organization of chromatin governs the transcriptional state of individual genes. To understand how genomic loci adopt different levels of gene expression, it is critical to characterize all local chromatin factors as well as long-range interactions in the 3D nuclear compartment. Much of our current knowledge regarding protein interactions in a chromatin context is based on affinity purification of chromatin components coupled to mass spectrometry (AP-MS). AP-MS has been invaluable to map strong protein-protein interactions in the nucleus. However, the interaction is detected after cell lysis and biochemical enrichment, allowing for loss or gain of false positive or negative interaction partners. Recently, proximity-dependent labeling methods have emerged as powerful tools for studying chromatin in its native context. These methods take advantage of engineered enzymes that are fused to a chromatin factor of interest and can directly label all factors in proximity. Subsequent pull-down assays followed by mass spectrometry or sequencing approaches provide a comprehensive snapshot of the proximal chromatin interactome. By combining this method with dCas9, this approach can also be extended to study chromatin at specific genomic loci. Here, we review and compare current proximity-labeling approaches available for studying chromatin, with a particular focus on new emerging technologies that can provide important insights into the transcriptional and chromatin interaction networks essential for cellular identity.

Genomic Profiling by ALaP-Seq Reveals Transcriptional Regulation by PML Bodies through DNMT3A Exclusion

The promyelocytic leukemia (PML) body is a phase-separated nuclear structure physically associated with chromatin, implying its crucial roles in genome functions. However, its role in transcriptional regulation is largely unknown. We developed APEX-mediated chromatin labeling and purification (ALaP) to identify the genomic regions proximal to PML bodies. We found that PML bodies associate with active regulatory regions across the genome and with ∼300 kb of the short arm of the Y chromosome (YS300) in mouse embryonic stem cells. The PML body association with YS300 is essential for the transcriptional activity of the neighboring Y-linked clustered genes. Mechanistically, PML bodies provide specific nuclear spaces that the de novo DNA methyltransferase DNMT3A cannot access, resulting in the steady maintenance of a hypo-methylated state at Y-linked gene promoters. Our study underscores a new mechanism for gene regulation in the 3D nuclear space and provides insights into the functional properties of nuclear structures for genome function.

APEX Proximity Labeling as a Versatile Tool for Biological Research

Most proteins are specifically localized in membrane-encapsulated organelles or non-membrane-bound compartments. The subcellular localization of proteins facilitates their functions and integration into functional networks; therefore, protein localization is tightly regulated in diverse biological contexts. However, protein localization has been mainly analyzed through immunohistochemistry or the fractionation of subcellular compartments, each of which has major drawbacks. Immunohistochemistry can examine only a handful of proteins at a time, and fractionation inevitably relies on the lysis of cells, which disrupts native cellular conditions. Recently, an engineered ascorbate peroxidase (APEX)-based proximity labeling technique combined with mass spectrometry was developed, which allows for temporally and spatially resolved proteomic mapping. In the presence of H₂O₂, engineered APEX oxidizes biotin-phenols into biotin-phenoxyl radicals, and these short-lived radicals biotinylate electron-rich amino acids within a radius of several nanometers. Biotinylated proteins are subsequently enriched by streptavidin and identified by mass spectrometry. This permits the sensitive and efficient labeling of proximal proteins around locally expressed APEX. Through the targeted expression of APEX in the subcellular region of interest, proteomic profiling of submitochondrial spaces, the outer mitochondrial membrane, the endoplasmic reticulum (ER)-mitochondrial contact, and the ER membrane has been performed. Furthermore, this method has been modified to define interaction networks in the vicinity of target proteins and has also been applied to analyze the spatial transcriptome. In this Perspective, we provide an outline of this newly developed technique and discuss its potential applications to address diverse biological questions.

Co-regulation map of the human proteome enables identification of protein functions

Assigning functions to the vast array of proteins present in eukaryotic cells remains challenging. To identify relationships between proteins, and thereby enable functional annotation of proteins, we determined changes in abundance of 10,323 human proteins in response to 294 biological perturbations using isotope-labeling mass spectrometry. We applied the machine learning algorithm treeClust to reveal functional associations between co-regulated human proteins from ProteomeHD, a compilation of our own data and datasets from the Proteomics Identifications database. This produced a co-regulation map of the human proteome. Co-regulation was able to capture relationships between proteins that do not physically interact or colocalize. For example, co-regulation of the peroxisomal membrane protein PEX11β with mitochondrial respiration factors led us to discover an organelle interface between peroxisomes and mitochondria in mammalian cells. We also predicted the functions of microproteins that are difficult to study with traditional methods. The co-regulation map can be explored at www.proteomeHD.net .

Regulation of the ER stress response by a mitochondrial microprotein

Cellular homeostasis relies on having dedicated and coordinated responses to a variety of stresses. The accumulation of unfolded proteins in the endoplasmic reticulum (ER) is a common stress that triggers a conserved pathway called the unfolded protein response (UPR) that mitigates damage, and dysregulation of UPR underlies several debilitating diseases. Here, we discover that a previously uncharacterized 54-amino acid microprotein PIGBOS regulates UPR. PIGBOS localizes to the mitochondrial outer membrane where it interacts with the ER protein CLCC1 at ER–mitochondria contact sites. Functional studies reveal that the loss of PIGBOS leads to heightened UPR and increased cell death. The characterization of PIGBOS reveals an undiscovered role for a mitochondrial protein, in this case a microprotein, in the regulation of UPR originating in the ER. This study demonstrates microproteins to be an unappreciated class of genes that are critical for inter-organelle communication, homeostasis, and cell survival.

Cells trigger an unfolded protein response (UPR) in the endoplasmic reticulum, but its regulation by mitochondria is unclear. Here, the authors report a 54-residue microprotein PIGBOS that participates in inter-organelle contact between the endoplasmic reticulum and the mitochondria and may regulate UPR.

Proteomics Approaches for Biomarker and Drug Target Discovery in ALS and FTD

Neurodegenerative disorders such as amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are increasing in prevalence but lack targeted therapeutics. Although the pathological mechanisms behind these diseases remain unclear, both ALS and FTD are characterized pathologically by aberrant protein aggregation and inclusion formation within neurons, which correlates with neurodegeneration. Notably, aggregation of several key proteins, including TAR DNA binding protein of 43 kDa (TDP-43), superoxide dismutase 1 (SOD1), and tau, have been implicated in these diseases. Proteomics methods are being increasingly applied to better understand disease-related mechanisms and to identify biomarkers of disease, using model systems as well as human samples. Proteomics-based approaches offer unbiased, high-throughput, and quantitative results with numerous applications for investigating proteins of interest. Here, we review recent advances in the understanding of ALS and FTD pathophysiology obtained using proteomics approaches, and we assess technical and experimental limitations. We compare findings from various mass spectrometry (MS) approaches including quantitative proteomics methods such as stable isotope labeling by amino acids in cell culture (SILAC) and tandem mass tagging (TMT) to approaches such as label-free quantitation (LFQ) and sequential windowed acquisition of all theoretical fragment ion mass spectra (SWATH-MS) in studies of ALS and FTD. Similarly, we describe disease-related protein-protein interaction (PPI) studies using approaches including immunoprecipitation mass spectrometry (IP-MS) and proximity-dependent biotin identification (BioID) and discuss future application of new techniques including proximity-dependent ascorbic acid peroxidase labeling (APEX), and biotinylation by antibody recognition (BAR). Furthermore, we explore the use of MS to detect post-translational modifications (PTMs), such as ubiquitination and phosphorylation, of disease-relevant proteins in ALS and FTD. We also discuss upstream technologies that enable enrichment of proteins of interest, highlighting the contributions of new techniques to isolate disease-relevant protein inclusions including flow cytometric analysis of inclusions and trafficking (FloIT). These recently developed approaches, as well as related advances yet to be applied to studies of these neurodegenerative diseases, offer numerous opportunities for discovery of potential therapeutic targets and biomarkers for ALS and FTD.

In vitro BioID: mapping the CENP-A microenvironment with high temporal and spatial resolution

The centromere is located at the primary constriction of condensed chromosomes where it acts as a platform regulating chromosome segregation. The histone H3 variant CENP-A is the foundation for kinetochore formation. CENP-A directs the formation of a highly dynamic molecular neighborhood whose temporal characterization during mitosis remains a challenge due to limitations in available techniques. BioID is a method that exploits a “promiscuous” biotin ligase (BirA118R or BirA*) to identify proteins within close proximity to a fusion protein of interest. As originally described, cells expressing BirA* fusions were exposed to high biotin concentrations for 24 h during which the ligase transferred activated biotin (BioAmp) to other proteins within the immediate vicinity. The protein neighborhood could then be characterized by streptavidin-based purification and mass spectrometry. Here we describe a further development to this technique, allowing CENP-A interactors to be characterized within only a few minutes, in an in vitro reaction in lysed cells whose physiological progression is “frozen.” This approach, termed in vitro BioID (ivBioID), has the potential to study the molecular neighborhood of any structural protein whose interactions change either during the cell cycle or in response to other changes in cell physiology.

MIEF1 Microprotein Regulates Mitochondrial Translation

Recent technological advances led to the discovery of hundreds to thousands of peptides and small proteins (microproteins) encoded by small open reading frames (smORFs). Characterization of new microproteins demonstrates their role in fundamental biological processes and highlights the value in discovering and characterizing more microproteins. The elucidation of microprotein-protein interactions (MPIs) is useful for determining the biochemical and cellular roles of microproteins. In this study, we characterize the protein interaction partners of mitochondrial elongation factor 1 microprotein (MIEF1-MP) using a proximity labeling strategy that relies on APEX2. MIEF1-MP localizes to the mitochondrial matrix where it interacts with the mitochondrial ribosome (mitoribosome). Functional studies demonstrate that MIEF1-MP regulates mitochondrial translation via its binding to the mitoribosome. Loss of MIEF1-MP decreases the mitochondrial translation rate, while an elevated level of MIEF1-MP increases the translation rate. The identification of MIEF1-MP reveals a new gene involved in this process.

Micropeptides Encoded in Transcripts Previously Identified as Long Noncoding RNAs: A New Chapter in Transcriptomics and Proteomics

Integrative analysis using omics-based technologies results in the identification of a large number of putative short open reading frames (sORFs) with protein-coding capacity within transcripts previously identified as long noncoding RNAs (lncRNAs) or transcripts of unknown function (TUFs). sORFs were previously overlooked because of their diminutive size and the difficulty of identification by bioinformatics analyses. There is now growing evidence of the existence of potentially functional micropeptides produced from sORFs within cells of diverse species. Recent characterization of a few of these revealed their significant divergent roles in many fundamental biological processes, where some also show important relationships with pathogenesis. Recent works therefore provide new insights for exploring the wealth of information that may lie within sORF-encoded short proteins. Here, we summarize the current progress and view of micropeptides encoded in sORFs of protein-coding genes.