Mechanisms of Phosphorylation Signaling by Phosphoprotein Phosphatases

摘要

Abstract Reversible protein phosphorylation is a major regulatory mechanism that controls most cellular processes, including mitosis. A ‘tug of war’ between kinases and phosphatases controls protein activity and function. The precise coordination of both activities is essential for the accurate execution of cell division. Despite substantial progress in deciphering kinase-mediated phosphorylation and its functional consequences, much less is known about phosphatases and how dephosphorylation is regulated. The long-term goal of our research program is to uncover the mechanisms that coordinate and integrate PPPs, kinases, and their shared substrates in the signaling networks that control cell division. The majority of cellular dephosphorylation is carried out by the 7 catalytic phosphatase subunits of the phosphoprotein phosphatase (PPP) family. Despite the apparent simplicity suggested by the small number of catalytic PPP enzymes, complexity and specificity arise through the formation of holoenzymes. Each holoenzyme functions as a distinct entity. The non-catalytic subunits modulate the activity of the catalytic subunits, enabling substrate specificity, dictating subcellular localization, and ensuring appropriate regulation. Combinatorially, there are several hundred functional PPP holoenzymes. There are key gaps in knowledge and challenges in studying PPP function and mechanisms of dephosphorylation. Although hundreds of thousands of phosphorylated sites on proteins have been identified, only ~450 have been matched to be dephosphorylated by a specific PPP holoenzyme. This is in part due to the lack of holoenzyme-specific inhibitors and the complexity of PPP holoenzymes. To overcome these obstacles and fill these gaps in knowledge, we will (i) identify holoenzyme-specific substrates; (ii) dissect mechanisms of substrate recruitment and site-specific dephosphorylation; and (iii) determine regulatory inputs governing specific PPP holoenzymes. We have developed new cell biological and biochemical approaches to investigate PPP signaling. We plan to map the PPP substrate space comprehensively, providing a foundation for studying PPPs for our lab and the signaling community by sharing our findings broadly and openly via a user-friendly database that we will maintain. The mechanisms that PPPs use to recruit substrates and catalyze site-specific dephosphorylation are still emerging. We will continue to dissect these mechanisms to predict and systematically dissect PPP-substrate relationships in cells. Finally, determining regulatory inputs that govern PPP holoenzyme formation, activity, and function is needed to understand their role in regulating mitosis. We combine quantitative measurements of the proteome and phosphoproteome with the reconstitution of minimal signaling units in vitro and in cells to precisely determine phosphatase function and regulation in cell division.