Biochemistry

Chromatin immunoprecipitation (ChIP) is a powerful technology for analyzing protein-DNA interactions in cells. Robust ChIP procedures have been established for investigating direct interactions between protein and DNA. However, detecting indirect protein-DNA interactions in vivo is challenging. Recently, we used ChIP to analyze an indirect protein-DNA interaction between a putative histone demethylase, MoJmjC, and the promoter of the superoxide dismutase 1-encoding gene MoSOD1 in the rice blast fungus Magnaporthe oryzae (M. oryzae) (Fernandez et al., 2014). We tagged MoJmjC with the 3x FLAG epitope (Fernandez et al., 2014), instead of the larger and more commonly used GFP epitope, to mitigate against steric hindrance. We also employed a two-step cross-linking strategy using DSG and formaldehyde-rather than the one-step formaldehyde cross-linking procedure more frequently employed for analyzing direct protein-DNA interactions - in order to better capture the indirect MoJmjC-MoSOD1 DNA interactions in vivo. In addition, we have shown that two-step cross-linking is suitable for ChIP analysis of direct protein-DNA interactions between a GATA transcription factor, Asd4, and its cognate binding site (Marroquin-Guzman and Wilson, 2015). Here, we provide a detailed protocol for chromatin immunoprecipitation, with versatile two-step cross-linking, in M. oryzae.

The RNA chromatin immunoprecipitation assay (RNA-ChIP) allows detection and quantification of RNA–protein interactions using in vivo cross-linking with formaldehyde followed by immunoprecipitation of the RNA–protein complexes. Here we describe the RNA–ChIP protocol that we have adapted for Caenorhabditis elegans (C. elegans) to detect interaction between the nuclear Argonaute CSR-1 (chromosome segregation and RNAi deficient) protein and its target nascent RNAs. We have used a transgenic strain expressing a recombinant long isoform of CSR-1 protein fused with N-terminal 3x FLAG epitope.

Systems biology approaches can be used to study the regulatory interactions occurring between many components of the biological system at the whole-genome level and decipher the circuitries implicated in the regulation of cellular processes, including those imparting virulence to opportunistic fungi. Candida albicans (C. albicans) is a leading human fungal pathogen. It undergoes morphological switching between a budding yeast form and an elongated multicellular hyphal form. This transition is required for C. albicans’ ability to cause disease and is regulated through highly interconnected regulatory interactions between transcription factors (TFs) and target genes. The chromatin immunoprecipitation (ChIP)-High-throughput sequencing (Seq) technology (ChIP-Seq) is a powerful approach for decoding transcriptional regulatory networks. This protocol was optimized for the preparation of ChIP DNA from filamenting C. albicans cells followed by high-throughput sequencing to identify the targets of TFs that regulate the yeast-to-hyphae transition.

Elucidation of molecular mechanisms of genome functions requires identification of molecules interacting with genomic regions of interest in vivo. To this end, it is useful to isolate the target regions retaining molecular interactions. We established locus-specific chromatin immunoprecipitation (ChIP) technologies consisting of insertional ChIP (iChIP) and engineered DNA-binding molecule-mediated ChIP (enChIP) for isolation of target genomic regions (Hoshino and Fujii, 2009; Fujita and Fujii, 2011; Fujita and Fujii, 2012; Fujita and Fujii, 2013a; Fujita and Fujii, 2013b; Fujita et al., 2013). Identification and characterization of molecules interacting with the isolated genomic regions facilitates understanding of molecular mechanisms of functions of the target genome regions.

Here, we describe enChIP, in which engineered DNA-binding molecules, such as zinc-finger proteins, transcription activator-like (TAL) proteins, and a catalytically inactive Cas9 (dCas9) plus small guide RNA (gRNA), are utilized for affinity purification of target genomic regions. The scheme of enChIP is as follows:
1. A zinc-finger protein, TAL or dCas9 plus gRNA is generated to recognize DNA sequence in a genomic region of interest.
2. The engineered DNA-binding molecule is fused with a tag(s) and the nuclear localization signal (NLS), and expressed in the cell to be analyzed.
3. The resultant cell is crosslinked, if necessary, and lysed, and DNA is fragmented.
4. The complexes including the engineered DNA-binding molecule are subjected to affinity purification such as mmunoprecipitation. The isolated complexes retain molecules interacting with the genomic region of interest.
5. Reverse crosslinking and subsequent purification of DNA, RNA, or proteins allow identification and characterization of these molecules.
In this protocol, we describe enChIP with a TAL protein to isolate a genomic region of interest and analyze the interacting proteins by mass spectrometry (Fujita et al., 2013).

Epstein-Barr virus (EBV) nuclear antigen 2 (EBNA2) induces expression of both viral and cellular genes in virus infected B cells by mimicking activated Notch receptors (Notch-IC) that mediate transcription activation through binding to the repressing domain of the recombining binding protein suppressor of hairless (RBP-Jκ). In general, chromatin immunoprecipitation (ChIP) assays, electrophoresis mobility shift assays (EMSA), streptavidin-agarose mediated DNA pull-down assays, together with cell-based transcription reporter assays were conducted to verify whether the query protein is involved in EBNA2-dependent transcription. The ATP-bound state of nuclear chaperone nucleophosmin (NPM1) has been implicated in pleiotropic biological processes. An ATP-agarose-mediated pull-down protocol was developed to monitor the formation of the pre-initiation complex that is induced by ATP-bound NPM1. According to EBNA2 and Notch-IC have been shown to be partially interchangeable with respect to activation of target genes in B cell lines, it is conceivable that EBNA2 is a biological equivalent of an activated Notch IC.

Steroid hormone receptors, for example estradiol receptor, act like transcription factors. In the cell, steroids bind to a specific receptor. Upon ligand binding, many steroid receptors dimerize and enter nuclei where they bind specific DNA sequences called Hormone Responsive Elements (HRE) and regulate gene transcription. ER is able to bind DNA sites that are not Estrogen Responsive Elements (ERE) so regulating also the transcription of genes that are not classically controlled by estrogens.

Chromatin Immunoprecipitation (ChIP) is an important procedure that allows you to verify if a certain protein is physically located at a regulatory region. This information, taken together with other procedures such as luciferase assays and EMSAs, will give definitive proof that the query protein is involved in the transcription of a protein. This procedure for p65 ChIP can be adapted to investigate other proteins; just a change of the antibody will suffice.

The transcription factor known as NF-κB is a homo- or hetero-dimer consisting of members of the Rel/NFKB family. The most abundant NF-κB complexes are made of two different proteins, p65 (Rel-A) and p50 (NFKB1). The NF-κB complex is initially inhibited by IκB by direct binding, thus trapping NF-κB in the cytoplasm. After a stimulatory signal, IκB kinase (IKK) phosphorylates IκB, allowing IκB to undergo proteasome-mediated degradation. The degradation of IκB and phosphorylation of p65 by multiple kinases activates NF-κB, allowing it to transport to the nucleus and cause the transcriptional activation of many of its target genes containing κB sites (consensus sequence: gggRNNYYcc, R = purine Y = pyrimidine), such as PUMA, IL-6, and TNF.

The Illumina sequencing platform is very popular among next-generation sequencing platforms. However, the DNA sequencing library construction kit provided by Illumina is considerably expensive. The protocol described here can be used to construct high-quality sequencing libraries from chromatin immunoprecipitated DNA. It uses key reagents from third-party vendors and greatly reduces the cost in library construction for Illumina sequencing.