Pick a Tag and Explore the Functions of Your Pet Protein.

Protein tags have been essential for advancing our knowledge of the function of proteins, their localization, and the mapping of their interaction partners. Expressing epitope-tagged proteins has become a standard practice in every life science laboratory and, thus, continues to enable new studies. In recent years, several new tagging moieties have entered the limelight, many of them bringing new functionalities, such as targeted protein degradation, accurate quantification, and proximity labeling. Other novel tags aim at tackling research questions in challenging niches. In this review, we elaborate on recently introduced tags and the opportunities they provide for future research endeavors. In addition, we highlight how the genome-engineering revolution may boost the field of protein tags.

A Well-Controlled BioID Design for Endogenous Bait Proteins.

The CRISPR/Cas9 revolution is profoundly changing the way life sciences technologies are used. Many assays now rely on engineered clonal cell lines to eliminate the overexpression of bait proteins. Control cell lines are typically nonengineered cells or engineered clones, implying a considerable risk for artifacts because of clonal variation. Genome engineering can also transform BioID, a proximity labeling method that relies on fusing a bait protein to a promiscuous biotin ligase, BirA*, resulting in the tagging of vicinal proteins. We here propose an innovative design to enable BioID for endogenous proteins wherein we introduce a T2A-BirA* module at the C-terminus of endogenous p53 by genome engineering, leading to bicistronic expression of both p53 and BirA* under control of the endogenous promoter. By targeting a Cas9-cytidine deaminase base editor to the T2A autocleavage site, we can efficiently derive an isogenic population expressing a functional p53-BirA* fusion protein. Using quantitative proteomics we show significant benefits over the classical ectopic expression of p53-BirA*, and we provide a first well-controlled view of the proximal proteins of endogenous p53 in colon carcinoma cells. This novel application for base editors expands the CRISPR/Cas9 toolbox and can be a valuable addition for synthetic biology.

High-Confidence Interactome for RNF41 Built on Multiple Orthogonal Assays.

Ring finger protein 41 (RNF41) is an E3 ubiquitin ligase involved in the ubiquitination and degradation of many proteins including ErbB3 receptors, BIRC6, and parkin. Next to this, RNF41 regulates the intracellular trafficking of certain JAK2-associated cytokine receptors by ubiquitinating and suppressing USP8, which, in turn, destabilizes the ESCRT-0 complex. To further elucidate the function of RNF41 we used different orthogonal approaches to reveal the RNF41 protein complex: affinity purification-mass spectrometry, BioID, and Virotrap. We combined these results with known data sets for RNF41 obtained with microarray MAPPIT and Y2H screens. This way, we establish a comprehensive high-resolution interactome network comprising 175 candidate protein partners. To remove potential methodological artifacts from this network, we distilled the data into a high-confidence interactome map by retaining a total of 19 protein hits identified in two or more of the orthogonal methods. AP2S1, a novel RNF41 interaction partner, was selected from this high-confidence interactome for further functional validation. We reveal a role for AP2S1 in leptin and LIF receptor signaling and show that RNF41 stabilizes and relocates AP2S1.

Robust Generation of Knock-in Cell Lines Using CRISPR-Cas9 and rAAV-assisted Repair Template Delivery.

The programmable Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-associated nuclease 9 (Cas9) technology revolutionized genome editing by providing an efficient way to cut the genome at a desired location (Ledford, 2015). In mammalian cells, DNA lesions trigger the error-prone non-homologous end joining (NHEJ) DNA repair mechanism. However, in presence of a DNA repair template, Homology-Directed Repair (HDR) can occur leading to precise repair of the lesion site. This last process can be exploited to enable precise knock-in changes by introducing the desired genomic alteration on the repair template. In this protocol, we describe the delivery of long repair templates (> 200 nucleotides) using recombinant Adeno Associated Virus (rAAV) for CRISPR-Cas9-based knock-in of a C-terminal tag sequence in a human cell line.

Intelligent Mixing of Proteomes for Elimination of False Positives in Affinity Purification-Mass Spectrometry.

Protein complexes are essential in all organizational and functional aspects of the cell. Different strategies currently exist for analyzing such protein complexes by mass spectrometry, including affinity purification (AP-MS) and proximal labeling-based strategies. However, the high sensitivity of current mass spectrometers typically results in extensive protein lists mainly consisting of nonspecifically copurified proteins. Finding the true positive interactors in these lists remains highly challenging. Here, we report a powerful design based on differential labeling with stable isotopes combined with nonequal mixing of control and experimental samples to discover bona fide interaction partners in AP-MS experiments. We apply this intelligent mixing of proteomes (iMixPro) concept to overexpression experiments for RAF1, RNF41, and TANK and also to engineered cell lines expressing epitope-tagged endogenous PTPN14, JIP3, and IQGAP1. For all baits, we confirmed known interactions and found a number of novel interactions. The results for RNF41 and TANK were compared to a classical affinity purification experiment, which demonstrated the efficiency and specificity of the iMixPro approach.

An extra dimension in protein tagging by quantifying universal proteotypic peptides using targeted proteomics.

The use of protein tagging to facilitate detailed characterization of target proteins has not only revolutionized cell biology, but also enabled biochemical analysis through efficient recovery of the protein complexes wherein the tagged proteins reside. The endogenous use of these tags for detailed protein characterization is widespread in lower organisms that allow for efficient homologous recombination. With the recent advances in genome engineering, tagging of endogenous proteins is now within reach for most experimental systems, including mammalian cell lines cultures. In this work, we describe the selection of peptides with ideal mass spectrometry characteristics for use in quantification of tagged proteins using targeted proteomics. We mined the proteome of the hyperthermophile Pyrococcus furiosus to obtain two peptides that are unique in the proteomes of all known model organisms (proteotypic) and allow sensitive quantification of target proteins in a complex background. By combining these 'Proteotypic peptides for Quantification by SRM' (PQS peptides) with epitope tags, we demonstrate their use in co-immunoprecipitation experiments upon transfection of protein pairs, or after introduction of these tags in the endogenous proteins through genome engineering. Endogenous protein tagging for absolute quantification provides a powerful extra dimension to protein analysis, allowing the detailed characterization of endogenous proteins.

Proteomics in the genome engineering era.

Genome engineering experiments used to be lengthy, inefficient, and often expensive, preventing a widespread adoption of such experiments for the full assessment of endogenous protein functions. With the revolutionary clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 technology, genome engineering became accessible to the broad life sciences community and is now implemented in several research areas. One particular field that can benefit significantly from this evolution is proteomics where a substantial impact on experimental design and general proteome biology can be expected. In this review, we describe the main applications of genome engineering in proteomics, including the use of engineered disease models and endogenous epitope tagging. In addition, we provide an overview on current literature and highlight important considerations when launching genome engineering technologies in proteomics workflows.

Importance of extended protease substrate recognition motifs in steering BNIP-2 cleavage by human and mouse granzymes B.

BACKGROUND: Previous screening of the substrate repertoires and substrate specificity profiles of granzymes resulted in long substrate lists highly likely containing bystander substrates. Here, a recently developed degradomics technology that allows distinguishing efficiently from less efficiently cleaved substrates was applied to study the degradome of mouse granzyme B (mGrB). RESULTS: In vitro kinetic degradome analysis resulted in the identification of 37 mGrB cleavage events, 9 of which could be assigned as efficiently targeted ones. Previously, cleavage at the IEAD75 tetrapeptide motif of Bid was shown to be efficiently and exclusively targeted by human granzyme B (hGrB) and thus not by mGrB. Strikingly, and despite holding an identical P4-P1 human Bid (hBid) cleavage motif, mGrB was shown to efficiently cleave the BCL2/adenovirus E1B 19 kDa protein-interacting protein 2 or BNIP-2 at IEAD28. Like Bid, BNIP-2 represents a pro-apoptotic Bcl-2 protein family member and a potential regulator of GrB induced cell death. Next, in vitro analyses demonstrated the increased efficiency of human and mouse BNIP-2 cleavage by mGrB as compared to hGrB indicative for differing Bid/BNIP-2 substrate traits beyond the P4-P1 IEAD cleavage motif influencing cleavage efficiency. Murinisation of differential primed site residues in hBNIP-2 revealed that, although all contributing, a single mutation at the P3' position was found to significantly increase the mGrB/hGrB cleavage ratio, whereas mutating the P1' position from I29 > T yielded a 4-fold increase in mGrB cleavage efficiency. Finally, mutagenesis analyses revealed the composite BNIP-2 precursor patterns to be the result of alternative translation initiation at near-cognate start sites within the 5' leader sequence (5'UTR) of BNIP-2. CONCLUSIONS: Despite their high sequence similarity, and previously explained by their distinct tetrapeptide specificities observed, the substrate repertoires of mouse and human granzymes B only partially overlap. Here, we show that the substrate sequence context beyond the P4-P1 positions can influence orthologous granzyme B cleavage efficiencies to an unmatched extent. More specifically, in BNIP-2, the identical and hGrB optimal IEAD tetrapeptide substrate motif is targeted highly efficiently by mGrB, while this tetrapeptide motif is refractory towards mGrB cleavage in Bid.