Tandem Mass Tag Labeling Facilitates Reversed-Phase Liquid Chromatography-Mass Spectrometry Analysis of Hydrophilic Phosphopeptides.


Protein phosphorylation is a critical post-translational modification (PTM). Despite recent technological advances in reversed-phase liquid chromatography (RPLC)-mass spectrometry (MS)-based proteomics, comprehensive phosphoproteomic coverage in complex biological systems remains challenging, especially for hydrophilic phosphopeptides with enriched regions of serines, threonines, and tyrosines that often orchestrate critical biological functions. To address this issue, we developed a simple, easily implemented method to introduce a commonly used tandem mass tag (TMT) to increase peptide hydrophobicity, effectively enhancing RPLC-MS analysis of hydrophilic peptides. Different from conventional TMT labeling, this method capitalizes on using a nonprimary amine buffer and TMT labeling occurring before C18-based solid phase extraction. Through phosphoproteomic analyses of MCF7 cells, we have demonstrated that this method can greatly increase the number of identified hydrophilic phosphopeptides and improve MS detection signals. We applied this method to study the peptide QPSSSR, a very hydrophilic tryptic peptide located on the C-terminus of the G protein-coupled receptor (GPCR) CXCR3. Identification of QPSSSR has never been reported, and we were unable to detect it by traditional methods. We validated our TMT labeling strategy by comparative RPLC-MS analyses of both a hydrophilic QPSSSR peptide library as well as common phosphopeptides. We further confirmed the utility of this method by quantifying QPSSSR phosphorylation abundances in HEK 293 cells under different treatment conditions predicted to alter QPSSSR phosphorylation. We anticipate that this simple TMT labeling method can be broadly used not only for decoding GPCR phosphoproteome but also for effective RPLC-MS analysis of other highly hydrophilic analytes.






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Publication Info

Tsai, Chia-Feng, Jeffrey S Smith, Krzysztof Krajewski, Rui Zhao, Ahmed M Moghieb, Carrie D Nicora, Xinyu Xiong, Ronald J Moore, et al. (2019). Tandem Mass Tag Labeling Facilitates Reversed-Phase Liquid Chromatography-Mass Spectrometry Analysis of Hydrophilic Phosphopeptides. Analytical chemistry, 91(18). pp. 11606–11613. 10.1021/acs.analchem.9b01814 Retrieved from https://hdl.handle.net/10161/19379.

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Sudarshan Rajagopal

Associate Professor of Medicine

I am a physician-scientist with a research focus on G protein-coupled receptor signaling in inflammation and vascular disease and a clinical focus on pulmonary vascular disease, as I serve as Co-Director of the Duke Pulmonary Vascular Disease Center. My research spans the spectrum from clinical research in pulmonary vascular disease, to translational research in cardiovascular disease, to the basic science of receptor signaling. 

Our basic science resesarch focuses on understanding and untapping the signaling potential of G protein-coupled receptors (GPCRs) to regulate inflammation in vascular disease. GPCRs are the most common transmembrane receptors in the human genome (over 800 members) and are some of the most successful targets for drug therapies. While it has been known for some time that these receptors signal through multiple downstream effectors (such as heterotrimeric G proteins and multifunctional beta arrestin adapter proteins), over the past decade it has been better appreciated that these receptors are capable of signaling with different efficacies to these effectors, a phenomenon referred to as “biased agonism”. Ligands can be biased, by activating different pathways from one another, and receptors can be biased, by signaling to a limited number of pathways that are normally available to them. Moreover, this phenomenon also appears to be common to other transmembrane and nuclear receptors. While a growing number of biased agonists acting at multiple receptors have been identified, there is still little known regarding the mechanisms underlying biased signaling and its physiologic impact.

Much of our research focuses on the chemokine system, which consists of approximately twenty receptors and fifty ligands that display considerable promiscuity with each other in the regulation of immune cell function in inflammatory diseases. Research from our group and others have shown that many of these ligands act as biased agonists when signaling through the same receptor. We use models of inflammation such as contact hypersensitivity and pulmonary arterial hypertension (PAH). PAH is a disease of the pulmonary arterioles that results in right heart failure and most of its treatments target signaling by GPCRs. We use multiple approaches to probe these signaling mechanisms, including in-house pharmacological assays, advanced phosphoproteomics and single cell RNA sequencing.

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