Serine/Threonine Phosphatases: Mechanism through Structure Yigong Shi Center for Structural Biology, School of Life Sciences, and School of Medicine, Tsinghua University, Beijing 100084, China Correspondence: shi-lab@tsinghua.edu.cn DOI 10.1016/j.cell.2009.10.006 Protein phosphorylation is regulated by opposing activities of kinases and phosphatases. Protein serine/threonine phosphatases (PSPs) control the specific dephosphorylation of thousands of phosphoprotein substrates. Many PSPs, such as protein phosphatase 1 (PP1) and PP2A, achieve substrate specificity and regulation through combinatorial interactions between conserved catalytic subunits and regulatory subunits. Other PSPs, such as PP2C and FCP/SCP, contain both catalytic and regulatory domains within the same polypeptide chain. This review discusses biochemical and structural investigations that advance the mechanistic understanding of the three major classes of PSPs, with a focus on PP2A. The concept of protein phosphorylation was discovered by Edmond Fischer and Edwin Krebs. This process involves the transfer of a phosphate group from ATP to phosphorylase b, resulting in phosphorylase a, a phosphoprotein. The enzyme that converts phosphorylase a back to b, called the "PR enzyme" (phosphorylase phosphatase), was reported a decade earlier, although the chemical nature of the reaction remained enigmatic until inorganic phosphate was found to be a product of the reaction. Decades of research have shown that reversible phosphorylation of proteins, executed by kinases and phosphatases, constitutes a major form of signaling and an essential mechanism of regulation in all living organisms. In eukaryotic cells, phosphorylation mainly occurs on three hydroxyl-containing amino acids, serine, threonine, and tyrosine, of which serine is the predominant target. Proteomic analysis of 6600 phosphorylation sites on 2244 human proteins revealed that phosphoserine (pSer), phosphothreonine (pThr), and phosphotyrosine (pTyr) account for 86.4%, 11.8%, and 1.8%, respectively, of the phosphorylated amino acids. The human genome is thought to contain 518 putative protein kinases, which can be classified into two families: 90 tyrosine (Tyr) kinases (PTKs) and 428 serine/threonine (Ser/Thr) kinases (PSKs). The exquisite specificity of signaling and the reversible nature of phosphorylation suggest that there would be similar numbers of protein phosphatases in the human genome.Serine/Threonine Phosphatases: Mechanism through Structure Yigong Shi Center for Structural Biology, School of Life Sciences, and School of Medicine, Tsinghua University, Beijing 100084, China Correspondence: shi-lab@tsinghua.edu.cn DOI 10.1016/j.cell.2009.10.006 Protein phosphorylation is regulated by opposing activities of kinases and phosphatases. Protein serine/threonine phosphatases (PSPs) control the specific dephosphorylation of thousands of phosphoprotein substrates. Many PSPs, such as protein phosphatase 1 (PP1) and PP2A, achieve substrate specificity and regulation through combinatorial interactions between conserved catalytic subunits and regulatory subunits. Other PSPs, such as PP2C and FCP/SCP, contain both catalytic and regulatory domains within the same polypeptide chain. This review discusses biochemical and structural investigations that advance the mechanistic understanding of the three major classes of PSPs, with a focus on PP2A. The concept of protein phosphorylation was discovered by Edmond Fischer and Edwin Krebs. This process involves the transfer of a phosphate group from ATP to phosphorylase b, resulting in phosphorylase a, a phosphoprotein. The enzyme that converts phosphorylase a back to b, called the "PR enzyme" (phosphorylase phosphatase), was reported a decade earlier, although the chemical nature of the reaction remained enigmatic until inorganic phosphate was found to be a product of the reaction. Decades of research have shown that reversible phosphorylation of proteins, executed by kinases and phosphatases, constitutes a major form of signaling and an essential mechanism of regulation in all living organisms. In eukaryotic cells, phosphorylation mainly occurs on three hydroxyl-containing amino acids, serine, threonine, and tyrosine, of which serine is the predominant target. Proteomic analysis of 6600 phosphorylation sites on 2244 human proteins revealed that phosphoserine (pSer), phosphothreonine (pThr), and phosphotyrosine (pTyr) account for 86.4%, 11.8%, and 1.8%, respectively, of the phosphorylated amino acids. The human genome is thought to contain 518 putative protein kinases, which can be classified into two families: 90 tyrosine (Tyr) kinases (PTKs) and 428 serine/threonine (Ser/Thr) kinases (PSKs). The exquisite specificity of signaling and the reversible nature of phosphorylation suggest that there would be similar numbers of protein phosphatases in the human genome.