Quantitative nanoproteomics for protein complexes (QNanoPX) related to estrogen transcriptional action

We developed an integrated proteomics approach using a chemically functionalized gold nanoparticle (AuNP) as a novel probe for affinity purification to analyze a large protein complex in vivo. We then applied this approach to globally map the transcriptional activation complex of the estrogen response element (ERE). This approach was designated as quantitative nanoproteomics for protein complexes (QNanoPX). In this approach, the positive AuNP-ERE probes were functionalized with polyethylene glycol (PEG), and the consensus sequence of ERE and negative AuNP-PEG probes were functionalized with PEG without the ERE via a thiolated self-assembly monolayer technique. The AuNP-ERE probe had substantially low nonspecific binding and high solubility, which resulted in a 20-fold enrichment of the factor compared with gel beads. In addition, the surface-only binding allows the probe to capture a large protein complex without any restrictions due to pore size. The affinity purification method was combined with MS-based quantitative proteomics and statistical methods to reveal the components of the ERE complex in MCF-7 cells and to identify those components within the complex that were altered by the presence of 17beta-estradiol (E2). Results indicated that a majority of proteins pulled down by the positive probe exhibited significant binding, and approximately one-half of the proteins, including estrogen receptor alpha (ERalpha), were slightly but significantly affected by a 24-h treatment with E2. Based on a combination of bioinformatics and pathway analysis, most of the affected proteins, however, appeared to be related to the transcriptional regulation of not only ERalpha but also c-Myc. Further confirmation indicated that E2 enhanced the ERE binding of c-Myc by 14-fold, indicating that c-Myc may play a major role, along with ERalpha, in E2-mediated transcription. Taken together, our results demonstrated a successful QNanoPX approach toward new pathway discovery and further revealed the importance of cross-interactions among transcription factors.