Phosphoprotein Phosphatases and Proteomics
Regulation of phosphorylation signaling by phosphoprotein phosphatases
Reversible protein phosphorylation is critical for the regulation of mitosis. A ‘tug of war’ between kinases and phosphatases controls the phosphorylation state of proteins. The precise coordination of both activities is essential for executing mitosis by regulating protein activity, interactions, localization, and stability. Even a minor imbalance in kinase and phosphatase activities can disrupt mitotic progression, leading to developmental defects and cancer. Despite substantial progress in deciphering kinase-mediated phosphorylation and its functional consequences, much less is known about phosphatase regulation and substrates.
Serine/threonine (S/T) phosphorylation accounts for ~98% of all phosphorylation in mammalian cells. Although ~400 protein kinases catalyze S/T phosphorylation, only seven phosphoprotein phosphatase (PPP) catalytic subunits dephosphorylate most S/T sites. PPP catalytic subunits are among the most conserved proteins from yeast to humans. They contain two metal ions in their active sites, which are essential for catalysis. The PPP family includes PP1, PP2A, calcineurin (CN, also known as PP2B or PP3), PP4, PP5, PP6, and PP7. Despite the apparent simplicity suggested by the small number of catalytic subunits, complexity and specificity arise through the formation of dimeric (PP1 and CN) or trimeric (PP2A, PP4, and PP6) holoenzyme complexes. Each holoenzyme functions as a distinct entity. The non-catalytic subunits modulate the activity and specificity of the catalytic subunits, recruit substrates, dictate subcellular localization, and ensure appropriate regulation.
The long-term goals of our research program are to uncover the mechanisms that coordinate and integrate PPPs, kinases, and their shared substrates, and to investigate the impact of dephosphorylation on cellular signaling, with an emphasis on cell division.
Proteomic approaches to investigate signaling in breast cancer
Breast cancer (BC) is a devastating disease, with an estimated 300,000 new cases diagnosed this year in the United States. One in 8 women in the US will develop invasive breast cancer in their lifetime. BC is subdivided into four molecularly defined groups based on hormone receptor expression. Triple-negative breast cancer (TNBC) accounts for 15-20% of all BC and lacks hormone receptor expression. We and others have identified clinically actionable TNBC subtypes.
Of the approximately 43,000 breast cancer deaths this year, more than 90% are due to metastatic (Stage IV) BC. TNBC represents nearly 50% of patients with metastatic disease, with an overall survival (OS) of 1 – 2 years.
The long-term goal of our research is to improve the survival of high-risk TNBC patients by identifying new treatment strategies. To do so, we have established a proteomic platform to efficiently quantify protein abundances in patient tumors and identify TNBC subtypes and actionable pathways for anti-cancer treatment.
