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Here, we present a protocol to enrich endogenous RNA binding sites or "footprints" of RNA:protein (RNP) complexes from mammalian cells. This approach involves two immunoprecipitations of RNP subunits and is therefore dubbed RNA immunoprecipitation in tandem (RIPiT).
RNA immunoprecipitation in tandem (RIPiT) is a method for enriching RNA footprints of a pair of proteins within an RNA:protein (RNP) complex. RIPiT employs two purification steps. First, immunoprecipitation of a tagged RNP subunit is followed by mild RNase digestion and subsequent non-denaturing affinity elution. A second immunoprecipitation of another RNP subunit allows for enrichment of a defined complex. Following a denaturing elution of RNAs and proteins, the RNA footprints are converted into high-throughput DNA sequencing libraries. Unlike the more popular ultraviolet (UV) crosslinking followed by immunoprecipitation (CLIP) approach to enrich RBP binding sites, RIPiT is UV-crosslinking independent. Hence RIPiT can be applied to numerous proteins present in the RNA interactome and beyond that are essential to RNA regulation but do not directly contact the RNA or UV-crosslink poorly to RNA. The two purification steps in RIPiT provide an additional advantage of identifying binding sites where a protein of interest acts in partnership with another cofactor. The double purification strategy also serves to enhance signal by limiting background. Here, we provide a step-wise procedure to perform RIPiT and to generate high-throughput sequencing libraries from isolated RNA footprints. We also outline RIPiT's advantages and applications and discuss some of its limitations.
Within cells, RNA exists in complex with proteins to form RNA:protein complexes (RNPs). RNPs are assembled around RNA binding proteins (RBPs, those that directly bind RNA) but also comprise of non-RBPs (those that bind RBPs but not RNA), and are often dynamic in nature. RBPs and their cofactors function collectively within RNPs to execute regulatory functions. For example, in the nonsense-mediated mRNA decay (NMD) pathway, the UPF proteins (UPF1, UPF2, and UPF3b) recognize the prematurely terminated ribosome. Each of the UPF proteins can bind to RNA, but it is only when they assemble together that an active NMD complex begins to form. Within this complex, UPF1 is further activated by phosphorylation by a non-RBP SMG1, and such UPF1 activation eventually leads to recruitment of mRNA decay inducing factors1,2. In this example, RBPs require non-RBP cofactors for recruitment and activation of the RNP complex that triggers NMD. Yet another property of RNPs is their compositional heterogeneity. Consider the spliceosome, which exists in distinct E, A, B or C complexes. Different spliceosome complexes have overlapping and distinct proteins3. To study RNP functions, it is important to elucidate which RNAs are bound by an RBP and its associated proteins. Many methods exist to accomplish this, with each approach having its distinct advantages and disadvantages4,5,6,7.
The widely popular methods to identify RBP binding sites — crosslinking followed by immunoprecipitation (CLIP) and its various variations - rely on ultraviolet (UV) light to crosslink an RBP to RNA8. However, this is not an effective approach for non-RBPs within RNPs, which do not contact the RNA directly. Here, we describe an alternative approach that is applicable to RBPs and non-RBPs alike, to isolate and identify their RNA binding sites. This approach termed RNA immunoprecipitation in tandem (RIPiT) consists of two immunoprecipitation steps, which help achieve higher specificity as compared to a single purification (Figure 1)9,10. As the individual immunoprecipitation (IP) steps can be carried out at a lower stringency as compared to CLIP, RIPiT does not depend on availability of antibodies that can withstand presence of strong detergents during immunoprecipitation. The most unique advantage of RIPiT is the ability to target two different proteins in two purification steps; this provides a powerful way to enrich a compositionally distinct RNP complex from other similar complexes11.
Small variations to the RIPiT procedure can further enhance RNP enrichment. For instance, some RNA-protein or protein-protein interactions within RNPs are transient and it may be difficult to efficiently purify footprints of such complexes. To stabilize such interactions, RNPs can be crosslinked within cells with formaldehyde prior to cell lysis and RIPiT. For example, we have observed that a weak interaction between the exon junction complex (EJC) core factor, EIF4AIII and the EJC disassembly factor, PYM12 can be stabilized with formaldehyde treatment such that more RNA footprints are enriched (data not shown). Prior to cell harvesting and RIPiT, cells can also be treated with drugs to stabilize or enrich RNPs in a particular state. For example, when studying proteins that are removed from mRNA during translation (e.g., the EJC13, UPF114), treatment with translation inhibitors such as puromycin, cycloheximide or harringtonine can lead to increased occupancy of proteins on RNAs.
The amount of RNA recovered from RIPiT is usually low (0.5-10 pmoles, i.e., 10-250 ng RNA considering an average RNA length of 75 nt). The primary reason for this is that only a small fraction of a given protein is present in complex with other proteins within RNPs (any "free" protein IP'ed in the first step is lost during the second IP). To generate RNA-Seq libraries from this RNA, we also outline here an adaptation of previously published protocol suitable for such low RNA inputs15,16 (Figure 2), which yields high-throughput sequencing ready samples in 3 days.
1. Establishment of Stable HEK293 Cell Lines Expressing Tetracycline-inducible FLAG-tagged Protein of Interest (POI)
2. Culturing Cells for Tetracycline Induction and RIPiT Procedure
3. Cell harvesting, Formaldehyde Treatment, and Cell Lysis
4. FLAG Immunoprecipitation
5. RNase I Digestion
6. Affinity Elution
7. Magnetic Bead-antibody Conjugation
8. Second Immunoprecipitation
9. Denaturing Elution
10. RNA Extraction and End Curing
11. Estimation of RNA Footprint Size and Abundance
12. Adapter Ligation
13. Reverse Transcription
14. Purification of RT Product
15. Circularization of RT Product
16. Test PCR
17. Large-scale PCR
A successful RIPiT will result in the immunoprecipitation of both proteins of interest and other known interacting proteins, and the absence of non-interacting proteins. As seen in Figure 3A, both Magoh and EIF4AIII were detected in the RIPiT elution, but HNRNPA1 was not (lane 6). In parallel, RNA footprints that have co-purified with the RNP complexes was detected via autoradiography (Figure 3B) or bioanalyzer (
We discuss here some key considerations to successfully perform RIPiT. Foremost, individual IPs must be optimized to achieve highest possible efficiency at each step. The amount of FLAG agarose beads for the input number of cells described here has proven to be robust for a wide range of proteins we have tested. As only a small fraction of partner proteins is co-immunoprecipitated with the FLAG protein, the amount of antibody needed for efficient second IP is usually low (less than 10 µg). Small-scale RIPiT (from on...
The authors have nothing to disclose.
This work was supported by the NIH grant GM120209 (GS). The authors thank the OSUCCC Genomics Shared Resources Core for their services (CCC Support Grant NCI P30 CA16058).
Name | Company | Catalog Number | Comments |
Anti-FLAG Affinity Gel | Sigma | A2220 | |
ATP, [γ-32P]- 3000Ci/mmol 10mCi/ml EasyTide, 250µCi | PerkinElmer | BLU502A250UC | |
BD Disposable Syringes with Luer-Lok Tips (200) | Fisher | 14-823-435 | |
Betaine 5M | Sigma | B0300 | |
biotin-dATP | TriLink | N-5002 | |
biotin-dCTP | Perkin Elmer | NEL540001EA | |
Branson Sonifier, Model SSE-1 | Branson | ||
CircLigase I | VWR | 76081-606 | ssDNA ligase I |
DMEM, High Glucose | ThermoFisher | 11995-065 | |
DNA load buffer NEB | NEB | ||
Dynabeads Protein A | LifeTech | 10002D | |
Flp-In-T-REx 293 Cell Line | ThermoFisher | R78007 | |
GeneRuler Low Range DNA Ladder | ThermoScientific | FERSM1203 | |
Hygromycin B | ThermoFisher | 10687010 | |
Mini-PROTEAN TBE Gel 10 well | Bio-Rad | 4565013 | |
Mini-PROTEAN TBE-Urea Gel | Bio-Rad | 4566033 | |
miRCAT-33 adapter 5′-TGGAATTCTCGGGTGCCAAGGddC-3′ | Any | this protocol is only compatible with the Illumina sequencing platform | |
Mirus transIT-X2 transfection reagent | Mirus | MIR 6004 | |
Mth RNA ligase | NEB | E2610S | |
PE1.0 5′-AATGATACGGCGACCACCGAGATCTACACT CTTTCCCTACACGACGCTCTTCCGATC*T-3′ | Any | this protocol is only compatible with the Illumina sequencing platform | |
PE2.0 5′-CAAGCAGAAGACGGCATACGAGATCGGTCTC GGCATTCCTGCTGAACCGCTCTTCCGATC*T-3′ | Any | this protocol is only compatible with the Illumina sequencing platform | |
Phenol/Chloroform/Isoamyl Alcohol (25:24:1, pH 6.7, 100ml) | Fisher | BP1752I-100 | |
Purple Gel Loading Dye (6x) | NEB | NEB #7025 | |
Q5 DNA Polymerase | NEB | M0491S/L | |
RNase I, E. coli, 1000 units | Eppicenter | N6901K | |
SPIN-X column | Corning | CLS8160-24EA | |
Streptavidin beads | ThermoFisher | 60210 | |
Superscript III (SSIII) | ThermoScientific | 18080044 | reverse transcriptase enzyme |
SybrGold | ThermoFisher | S11494 | gold nucleic acid gel stain |
T4 Polynucleotide Kinase-2500U | NEB | M0201L | |
T4RNL2 Tr. K227Q | NEB | M0351S | |
Tetracycline | Sigma | 87128 | |
Thermostable 5´ App DNA/RNA Ligase | NEB | M0319S | |
TruSeq_SE1 5′-pGGCACTANNNNNAGATCGGAAGA GCGTCGTGTAGGGAAAGAGTGT-SPACER 18-CTCGGCATTCCTGCTGAACCGCTC TTCCGATCTCCTTGGCACCCGAGAATTCCA-3′ | Any | this protocol is only compatible with the Illumina sequencing platform | |
TruSeq_SE10 5′-pGGTGTTCNNNNNAGATCGGAAG AGCGTCGTGTAGGGAAAGAGTGT-SPACER 18-CTCGGCATTCCTGCTGAACCGCT CTTCCGATCTCCTTGGCACCCGAGAATTCCA-3′ | Any | this protocol is only compatible with the Illumina sequencing platform | |
TruSeq_SE11 5′-pGGTAAGTNNNNNAGATCGGAA GAGCGTCGTGTAGGGAAAGAGTGT-SPACER 18-CTCGGCATTCCTGCTGAACCGCTC TTCCGATCTCCTTGGCACCCGAGAATTCCA-3′ | Any | this protocol is only compatible with the Illumina sequencing platform | |
TruSeq_SE12 5′-pGGAGATGNNNNNAGATCGGAAGA GCGTCGTGTAGGGAAAGAGTGT-SPACER 18-CTCGGCATTCCTGCTGAACCGCTC TTCCGATCTCCTTGGCACCCGAGAATTCCA-3′ | Any | this protocol is only compatible with the Illumina sequencing platform | |
TruSeq_SE2 5′-pGGGTAGCNNNNNAGATCGGAAGAG CGTCGTGTAGGGAAAGAGTGT-SPACER 18-CTCGGCATTCCTGCTGAACCGCT CTTCCGATCTCCTTGGCACCCGAGAATTCCA-3′ | Any | this protocol is only compatible with the Illumina sequencing platform | |
TruSeq_SE35′-pGGTCGATNNNNNAGATCGGAAG AGCGTCGTGTAGGGAAAGAGTGT-SPACER 18-CTCGGCATTCCTGCTGAACCGCT CTTCCGATCTCCTTGGCACCCGAGAATTCCA-3′ | Any | this protocol is only compatible with the Illumina sequencing platform | |
TruSeq_SE4 5′-pGGCCTCGNNNNNAGATCGGAAGA GCGTCGTGTAGGGAAAGAGTGT-SPACER 18-CTCGGCATTCCTGCTGAACCGCTC TTCCGATCTCCTTGGCACCCGAGAATTCCA-3′ | Any | this protocol is only compatible with the Illumina sequencing platform | |
TruSeq_SE5 5′-pGGTGACANNNNNAGATCGGAAGA GCGTCGTGTAGGGAAAGAGTGT-SPACER 18-CTCGGCATTCCTGCTGAACCGCTC TTCCGATCTCCTTGGCACCCGAGAATTCCA-3′ | Any | this protocol is only compatible with the Illumina sequencing platform | |
TruSeq_SE6 5′-pGGTAGACNNNNNAGATCGGAAGAG CGTCGTGTAGGGAAAGAGTGT-SPACER 18-CTCGGCATTCCTGCTGAACCGCTCTTC CGATCTCCTTGGCACCCGAGAATTCCA-3′ | Any | this protocol is only compatible with the Illumina sequencing platform | |
TruSeq_SE7 5′-pGGGCCCTNNNNNAGATCGGAAG AGCGTCGTGTAGGGAAAGAGTGT-SPACER 18-CTCGGCATTCCTGCTGAACCGCTCT TCCGATCTCCTTGGCACCCGAGAATTCCA-3′ | Any | this protocol is only compatible with the Illumina sequencing platform | |
TruSeq_SE8 5′-pGGATCGGNNNNNAGATCGGAAGAG CGTCGTGTAGGGAAAGAGTGT-SPACER 18-CTCGGCATTCCTGCTGAACCGCTCTT CCGATCTCCTTGGCACCCGAGAATTCCA-3′ | Any | this protocol is only compatible with the Illumina sequencing platform | |
TruSeq_SE9 5′-pGGACTGANNNNNAGATCGGAAGAG CGTCGTGTAGGGAAAGAGTGT-SPACER 18-CTCGGCATTCCTGCTGAACCGCTCTTC CGATCTCCTTGGCACCCGAGAATTCCA-3′ | Any | this protocol is only compatible with the Illumina sequencing platform | |
Typhoon 5 Bimolecular Imager | GE Healthcare Life Science | 29187191 |
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