Protein phosphatase‐1 (PP1) and protein phosphatase‐2A (PP2A) are responsible for the dephosphorylation of the majority of phosphoserine/threonine residues in cells. In this study, we show that (–)‐epigallocatechin‐3‐gallate (EGCG) and 1,2,3,4,6‐penta‐O‐galloyl‐β‐d‐glucose (PGG), polyphenolic constituents of green tea and tannins, inhibit the activity of the PP1 recombinant δ‐isoform of the PP1 catalytic subunit and the native PP1 catalytic subunit (PP1c) with IC₅₀ values of 0.47–1.35 μm and 0.26–0.4 μm, respectively. EGCG and PGG inhibit PP2Ac less potently, with IC₅₀ values of 15 and 6.6 μm, respectively. The structure–inhibitory potency relationships of catechin derivatives suggests that the galloyl group may play a major role in phosphatase inhibition. The interaction of EGCG and PGG with PP1c was characterized by NMR and surface plasmon resonance‐based binding techniques. Competitive binding assays and molecular modeling suggest that EGCG docks at the hydrophobic groove close to the catalytic center of PP1c, partially overlapping with the binding surface of microcystin‐LR or okadaic acid. This hydrophobic interaction is further stabilized by hydrogen bonding via hydroxyl/oxo groups of EGCG to PP1c residues. Comparative docking shows that EGCG binds to PP2Ac in a similar manner, but in a distinct pose. Long‐term treatment (24 h) with these compounds and other catechins suppresses the viability of HeLa cells with a relative effectiveness reminiscent of their in vitro PP1c‐inhibitory potencies. The above data imply that the phosphatase‐inhibitory features of these polyphenols may be implicated in the wide spectrum of their physiological influence.

Calcineurin (CN) influences myosin phosphorylation and alters endothelial barrier function; however, the molecular mechanism is still obscure. Here we examine whether CN controls myosin phosphorylation via mediating the phosphorylation state of Thr696 in myosin phosphatase (MP) target subunit 1 (MYPT1), the phosphorylation site inhibitory to the catalytic activity of MP. Exposure of bovine or human pulmonary artery endothelial cells (BPAECs or HPAECs) to the CN inhibitor cyclosporin A (CsA) induces a rise in intracellular Ca²⁺ and increases the phosphorylation level of cofilin^Ser3 and MYPT1^Thr696 in a Ca²⁺-and Rho-kinase-dependent manner. An active catalytic fragment of CN overexpressed in tsA201 cells decreases endogenous MYPT-phospho-Thr696 (MYPT1^pThr696) levels. Purified CN dephosphorylates ³²P-labelled MYPT1, suggesting direct action of CN on this substrate. Interaction of MYPT1 with CN is revealed by MYPT1 pull-down experiments and colocalization in both BPAECs and HPAECs as well as by surface plasmon resonance (SPR)-based binding studies. Stabilization of the MYPT1–CN complex occurs via the MYPT1^300PLIEST305 sequence similar to the CN substrate-docking PxIxIT-motif. Thrombin induces a transient increase of MYPT1^pThr696 in BPAECs, whereas its combination with CsA results in maintained phosphorylation levels of both MYPT1^pThr696 and myosin. These phosphorylation events might correlate with changes in endothelial permeability since CsA slows down the recovery from the thrombin-induced decrease of the transendothelial electrical resistance of the BPAEC monolayer. CN may improve endothelial barrier function via inducing dephosphorylation of cofilin^pSer3 and by interaction with MYPT1 and activating MP through MYPT1^pThr696 dephosphorylation, thereby affecting actin polymerization and decreasing myosin phosphorylation.

The opportunistic pathogen Candida is one of the most common causes of nosocomial bloodstream infections. Because candidemia is associated with high mortality rates and because the incidences of multidrug-resistant Candida are increasing, efforts to identify novel targets for the development of potent antifungals are warranted. Here, we describe the structure and function of the first member of a family of protein phosphatases that is specific to fungi, protein phosphatase Z1 (PPZ1) from Candida albicans . We show that PPZ1 not only is active but also is as susceptible to inhibition by the cyclic peptide inhibitor microcystin-LR as its most similar human homolog, protein phosphatase 1α (PP1α [GLC7 in the yeast Saccharomyces cerevisiae ]). Unexpectedly, we also discovered that, despite its 66% sequence identity to PP1α, the catalytic domain of PPZ1 contains novel structural elements that are not present in PP1α. We then used activity and pulldown assays to show that these structural differences block a large subset of PP1/GLC7 regulatory proteins from effectively binding PPZ1, demonstrating that PPZ1 does not compete with GLC7 for its regulatory proteins. Equally important, these unique structural elements provide new pockets suitable for the development of PPZ1-specific inhibitors. Together, these studies not only reveal why PPZ1 does not negatively impact GLC7 activity in vivo but also demonstrate that the family of fungus-specific phosphatases—especially PPZ1 from C. albicans —are highly suitable targets for the development of novel drugs that specifically target C. albicans without cross-reacting with human phosphatases. IMPORTANCE Candida albicans is a medically important human pathogen that is the most common cause of fungal infections in humans. In particular, approximately 46,000 cases of health care-associated candidiasis occur each year in the United States. Because these infections are associated with high mortality rates and because multiple species of Candida are becoming increasingly resistant to antifungals, there are increasing efforts to identify novel targets that are essential for C. albicans virulence. Here we use structural and biochemical approaches to elucidate how a member of a fungus-specific family of enzymes, serine/threonine phosphatase PPZ1, functions in C. albicans . We discovered multiple unique features of PPZ1 that explain why it does not cross-react with, and in turn compete for, PP1-specific regulators, a long-standing question in the field. Most importantly, however, these unique features identified PPZ1 as a potential target for the development of novel antifungal therapeutics that will provide new, safe, and potent treatments for candidiasis in humans.

Microcystin-LR (MCY-LR) is a cyanobacterial toxin, a specific inhibitor of type 1 and 2A protein phosphatases (PP1 and PP2A) with significant impact on aquatic ecosystems. It has the potential to alter regulation of the plant cell cycle. The aim of this study was improved understanding of the mitotic alterations induced by cyanotoxin in Vicia faba, a model organism for plant cell biology studies. Vicia faba seedlings were treated over the long and short term with MCY-LR purified in our laboratory. Short-term treatments were performed on root meristems synchronized with hydroxylurea. Sections of lateral root tips were labelled for chromatin, phosphorylated histone H3 and β-tubulin via histochemical and immunohistochemical methods. Mitotic activity and the occurrence of mitotic alterations were detected and analysed by fluorescence microscopy. The phosphorylation state of histone H3 was studied by Western blotting. Long-term MCY-LR exposure of lateral root tip meristems increased the percentage of either early or late mitosis in a concentration-dependent manner. We observed hypercondensed chromosomes and altered sister chromatid segregation (lagging chromosomes) leading to the formation of micronuclei, accompanied by the formation of disrupted, multipolar and monopolar spindles, disrupted phragmoplasts and the hyperphosphorylation of histone H3 at Ser10. Short-term MCY-LR treatment of synchronized cells showed that PP1 and PP2A inhibition delayed the onset of anaphase at 1 µg mL⁻¹ MCY-LR, accelerated cell cycle at 10 µg mL⁻¹ MCY-LR and induced the formation of lagging chromosomes. In this case mitotic microtubule alterations were not detected, but histone H3 was hyperphosphorylated. MCY-LR delayed metaphase–anaphase transition. Consequently, it induced aberrant chromatid segregation and micronucleus formation that could be associated with both H3 hyperphosphorylation and altered microtubule organization. However, these two phenomena seemed to be independent. The toxin may be a useful tool in the study of plant cell cycle regulation.

Protein phosphatase-1M (PP1M, myosin phosphatase) consists of a PP1 catalytic subunit (PP1c) and the myosin phosphatase target subunit-1 (MYPT1). RhoA-activated kinase (ROK) regulates PP1M via inhibitory phosphorylation of MYPT1. Using multidisciplinary approaches, we have studied the roles of PP1M and ROK in neurotransmission. Electron microscopy demonstrated the presence of MYPT1 and ROK in both pre- and post-synaptic terminals. Tautomycetin (TMC), a PP1-specific inhibitor, decreased the depolarization-induced exocytosis from cortical synaptosomes. trans-4-[(1R)-1-aminoethyl]-N-4-pyridinylcyclohexanecarboxamide dihydrochloride, a ROK-specific inhibitor, had the opposite effect. Mass spectrometry analysis identified several MYPT1-bound synaptosomal proteins, of which interactions of synapsin-I, syntaxin-1, calcineurin-A subunit, and Ca(2+) /calmodulin-dependent kinase II with MYPT1 were confirmed. In intact synaptosomes, TMC increased, whereas Y27632 decreased the phosphorylation levels of MYPT1(Thr696) , myosin-II light chain(Ser19) , synapsin-I(Ser9) , and syntaxin-1(Ser14) , indicating that PP1M and ROK influence their phosphorylation status. Confocal microscopy indicated that MYPT1 and ROK are present in the rat ventral cochlear nucleus both pre- and post-synaptically. Analysis of the neurotransmission in an auditory glutamatergic giant synapse demonstrated that PP1M and ROK affect neurotransmission via both pre- and post-synaptic mechanisms. Our data suggest that both PP1M and ROK influence synaptic transmission, but further studies are needed to give a full account of their mechanism of action.