Activity of PP2Cβ is Increased by Divalent Cations and Lipophilic Compounds Depending on the Substrate

The role of PP5 and PP2C in cardiac health and disease

Protein phosphatases are important, for example, as functional antagonists of β-adrenergic stimulation of the mammalian heart. While β-adrenergic stimulations increase the phosphorylation state of regulatory proteins and therefore force of contraction in the heart, these phosphorylations are reversed and thus force is reduced by the activity of protein phosphatases. In this context the role of PP5 and PP2C is starting to unravel. They do not belong to the same family of phosphatases with regard to sequence homology, many similarities with regard to location, activation by lipids and putative substrates have been worked out over the years. We also suggest which pathways for regulation of PP5 and/or PP2C described in other tissues and not yet in the heart might be useful to look for in cardiac tissue. Both phosphatases might play a role in signal transduction of sarcolemmal receptors in the heart. Expression of PP5 and PP2C can be increased by extracellular stimuli in the heart. Because PP5 is overexpressed in failing animal and human hearts, and because overexpression of PP5 or PP2C leads to cardiac hypertrophy and KO of PP5 leads to cardiac hypotrophy, one might argue for a role of PP5 and PP2C in heart failure. Because PP5 and PP2C can reduce, at least in vitro, the phosphorylation state of proteins thought to be relevant for cardiac arrhythmias, a role of these phosphatases for cardiac arrhythmias is also probable. Thus, PP5 and PP2C might be druggable targets to treat important cardiac diseases like heart failure, cardiac hypertrophy and cardiac arrhythmias.

Biochemical characterization of the phosphatase domain of the tumor suppressor PH domain leucine-rich repeat protein phosphatase

PH domain leucine-rich repeat protein phosphatase (PHLPP) directly dephosphorylates and inactivates Akt and protein kinase C and is therefore a prime target for pharmacological intervention of two key signaling pathways, the phosphatidylinositol 3-kinase and diacylglycerol signaling pathways. Here we report on the first biochemical characterization of the phosphatase domain of a PHLPP family member. The human PHLPP1 and PHLPP2 phosphatase domains were expressed and purified from bacteria or insect cells and their activities compared to that of full-length proteins immunoprecipitated from mammalian cells. Biochemical analyses reveal that the PHLPP phosphatase domain effectively dephosphorylates synthetic and peptidic substrates, that its activity is modulated by metals and lipophilic compounds, and that it has relatively high thermal stability. Mutational analysis of PHLPP2 reveals an unusual active site architecture compared to the canonical architecture of PP2C phosphatases and identifies key acidic residues (Asp 806, Glu 989, and Asp 1024) and bulky aromatic residues (Phe 783 and Phe 808) whose mutation impairs activity. Consistent with a unique active site architecture, we identify inhibitors that discriminate between PHLPP2 and PP2Cα. These data establish PHLPP as a member of the PP2C family of phosphatases with a unique active site architecture.

Binding of a third metal ion by the human phosphatases PP2Cα and Wip1 is required for phosphatase activity

The PPM phosphatases require millimolar concentrations of Mg²⁺ or Mn²⁺ to activate phosphatase activity in vitro. The human phosphatases PP2Cα (PPM1A) and Wip1 (PPM1D) differ in their physiological function, substrate specificity, and apparent metal affinity. A crystallographic structure of PP2Cα shows only two metal ions in the active site. However, recent structural studies of several bacterial PP2C phosphatases have indicated three metal ions in the active site. Two residues that coordinate the third metal ion are highly conserved, suggesting that human PP2C phosphatases may also bind a third ion. Here, isothermal titration calorimetry analysis of Mg²⁺ binding to PP2Cα distinguished binding of two ions to high affinity sites from the binding of a third ion with a millimolar affinity, similar to the apparent metal affinity required for catalytic activity. Mutational analysis indicated that Asp239 and either Asp146 or Asp243 was required for low-affinity binding of Mg²⁺, but that both Asp146 and Asp239 were required for catalysis. Phosphatase activity assays in the presence of MgCl₂, MnCl₂, or mixtures of the two, demonstrate high phosphatase activity toward a phosphopeptide substrate when Mg²⁺ was bound to the low-affinity site, whether Mg²⁺ or Mn²⁺ ions were bound to the high affinity sites. Mutation of the corresponding putative third metal ion-coordinating residues of Wip1 affected catalytic activity similarly both in vitro and in human cells. These results suggest that phosphatase activity toward phosphopeptide substrates by PP2Cα and Wip1 requires the binding of a Mg²⁺ ion to the low-affinity site.

Substrate-dependent metal preference of PPM1H, a cancer-associated protein phosphatase 2C: comparison with other family members

Protein phosphatase 2C (PP2C) family is characterized by requirement of metal cation for phosphatase activity. We previously established that PPM1H is a cancer-associated member of the PP2C family. Here we further characterized the phosphatase activity of PPM1H, focusing on its dependence on metal cation. PPM1H possesses the potential to dephosphorylate p-nitrophenyl phosphate (pNPP), casein and phosphopeptides. Interestingly, PPM1H shows the metal preference that is varied depending on the substrate (substrate-dependent metal preference); PPM1H prefers Mn(2+) when pNPP or phosphopeptides is used as a substrate. Meanwhile, a preference for Mg(2+) is displayed by PPM1H with casein as a substrate. When both cations are added to the reaction, the degree of the effect is always closer to that with Mn(2+) alone, irrespective of the substrate. This preponderance of Mn(2+) is explained by its greater affinity for PPM1H than Mg(2+). From the literature the substrate-dependent metal preference appears to be shared by other PP2Cs. According to the crystal structure, a binuclear metal center of PP2C plays an important role for coordinating the substrate and nucleophilic waters in the active site. Therefore, the differences in the size, preferred geometry and coordination requirements between two metals, in relation to the substrate, may be responsible for this intriguing property.

Protein phosphatases types 2Cα and 2Cβ in apoptosis

This mini-review highlights the involvement of PP2C (protein phosphatase type 2C) family members α and β in apoptosis. The activity of these isoenzymes can be stimulated by unsaturated fatty acids with special structural features, e.g. oleic acid. Those fatty acids capable of activating PP2Cα and PP2Cβ in vitro induce apoptosis in various cell types as shown here for neurons and endothelial cells. Using RNA interference to reduce the amount of PP2Cα and PP2Cβ results in cells significantly less susceptible to the apoptotic effect of oleic acid. Increased endothelial cell death is considered to be an initial step of atherogenesis. Thus activation of PP2C by physiological unbound (‘free’) unsaturated fatty acids (liberated from lipoproteins) could represent a crucial mechanism in the development of atherosclerosis.