Proximity Ligation Assay for the Analysis of Iron-Mediated Protein-Protein Interactions in the Nucleus

Iron forms essential cofactors used by many nuclear enzymes involved in genome maintenance. However, unchaperoned nuclear iron may represent a threat to the surrounding genetic material as it promotes redox toxicity that may affect DNA integrity. Safely handling intracellular iron implies metal transfer and cofactor assembly processes based on protein-protein interactions. Identifying those interactions commonly occurs via high-throughput approaches using affinity purification or proximity labeling coupled with mass spectrometry analysis. However, these methods do not identify the subcellular location of the interactions. The one-on-one confirmation of proposed nuclear interactions is also challenging. Many approaches used to look at protein interactions are not tailored for looking at the nucleus because the methods used to solubilize nuclear content are harsh enough to disrupt those transient interactions. Here, we describe step-by-step the use of Proximity Ligation Assay (PLA) to analyze iron-mediated protein-protein interactions in the nucleus of cultured human cells. PLA allows the subcellular visualization of the interactions via the in situ detection of the two interacting proteins using fluorescence confocal microscopy. Briefly, cells are fixed, blocked, permeabilized, and incubated with primary antibodies directed to target proteins. Primary antibodies are recognized using PLA probes consisting of one PLUS and one MINUS oligonucleotide-labeled secondary antibody. If the two proteins are close enough (<40 nm), the PLA probes are ligated and used as the template for rolling circle amplification (RCA) with fluorescently labeled oligonucleotides that yield a signal detectable using fluorescence confocal microscopy. A fluorescently labeled membrane-specific stain (WGA) and the DNA-specific probe DAPI are used to identify cellular and nuclear boundaries, respectively. Confocal images are then analyzed using the CellProfiler software to confirm the abundance and localization of the studied protein-protein interactions.