Abstract
Proteins seldom function in isolation and thus protein-protein interactions are critical in understanding the molecular basis of diseases and health (1, 2). There are several well established techniques that are used to investigate protein-protein interactions(3). Most of the methods require some form of genetic modification of the target protein and thus always adds extra steps. However, Proximity Ligation Assay(4-6) (PLA) aka Duolink® is one such method that requires no genetic modification of the target protein and probes protein-protein interactions in fixed live cells and tissues. Briefly, PLA requires the use of primary antibodies specific to the proteins of interest. Once the sample (fixed cells or tissues) is incubated with species specific primary antibodies, secondary antibodies that are conjugated with oligonucleotides (also known as PLUS and MINUS probes respectively) and connecter oligonucleotides are added. This complex is ligated if the two PLUS and MINUS probes are within 40nm of each other. The resulting nucleic acid is amplified using rolling circle amplification and then probed with appropriate fluorescent probes. If the two proteins are interacting, one could visualize the interaction as a single red foci (for example Far Red Detection) using fluorescent microscopy. Here, we used PLA to probe protein-protein interactions between Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and seven in absentia homolog -1 (Siah-1) – an E3 ubiquitin ligase. We first use PLA to show that GAPDH and Siah-1 proteins exist endogenously in the cytosol of multiple mammalian cell lines. Our data suggest the use of DU145 and T98G cell lines to show translocation of the GAPDH-Siah- 1 complex. Next, we used common nitrosylation agents(7, 8) (S-nitrosoglutathione-GSNO and S-Nitroso-N-acetyl-DL-pencillamine–SNAP) in different concentrations and observed that GAPDH and Siah-1 interact presumably due to the nitrosylation of the former, which is consistent with previous studies(9, 10). Interestingly, no interactions were observed between the two proteins in the absence of GSNO or SNAP indicating that nitrosylation might be critical for GAPDH-Siah1 interactions. Our results suggest that GAPDH-Siah-1 interactions originate in the cytosol and migrate to the nucleus under the conditions tested. We quantify the PLA signal using Duolink® Image Tool and observe a clear enhancement of GAPDH-Siah-1 PLA signal upon treating the cells with GSNO or SNAP. Next, we used R-(-)-Deprenyl (deprenyl), a known inhibitor of GAPDH4, and show that it abrogates GAPDH-Siah-1 PLA complex under the conditions tested. Finally, our data suggest that PLA can detect and quantify the GAPDH-Siah1 complex; a well-known protein-protein interaction implicated in neurodegeneration(9-11) and thus could be a method of choice for similar applications.
