Cloning and identification of MYPT3: a prenylatable myosin targetting subunit of protein phosphatase 1

LMTK2 switches on canonical TGF-β1 signaling in human bronchial epithelial cells

Transforming growth factor (TGF-β1) is a critical profibrotic mediator in chronic lung disease, and there are no specific strategies to mitigate its adverse effects. Activation of TGF-β1 signaling is a multipart process involving ligands, transmembrane receptors, and transcription factors. In addition, an intricate network of adaptor proteins fine-tunes the signaling strength, duration, and activity. Namely, Smad7 recruits growth arrest and DNA damage (GADD34) protein that then interacts with the catalytic subunit of phosphoprotein phosphatase 1 (PP1c) to inactivate TGF-β receptor (TβR)-I and downregulate TGF-β1 signaling. Little is known about how TGF-β1 releases TβR-I from the GADD34-PP1c inhibition to activate its signaling. Transmembrane lemur tyrosine kinase 2 (LMTK2) is a PP1c inhibitor, and our published data showed that TGF-β1 recruits LMTK2 to the cell surface. Here, we tested the hypothesis that TGF-β1 recruits LMTK2 to inhibit PP1c, allowing activation of TβR-I. First, LMTK2 interacted with the TGF-β1 pathway in the human bronchial epithelium at multiple checkpoints. Second, TGF-β1 inhibited PP1c by an LMTK2-dependent mechanism. Third, TGF-β1 used LMTK2 to activate canonical Smad3-mediated signaling. We propose a model whereby the LMTK2-PP1c and Smad7-GADD34-PP1c complexes serve as on-and-off switches in the TGF-β1 signaling in human bronchial epithelium.NEW & NOTEWORTHY Activation of the transforming growth factor (TGF)-β1 signaling pathway is complex, involving many ligands, transmembrane receptors, transcription factors, and modulating proteins. The mechanisms of TGF-β1 signaling activation/inactivation are not fully understood. We propose for the first time a model by which transmembrane lemur tyrosine kinase 2 (LMTK2) forms a complex with phosphoprotein phosphatase 1 (PP1c) to activate TGF-β1 signaling and Smad7, growth arrest and DNA damage (GADD34), and PP1C form a complex to inactivate TGF-β1 signaling in human bronchial epithelium.

Smoothelin-Like Protein 1 Regulates the Thyroid Hormone-Induced Homeostasis and Remodeling of C2C12 Cells via the Modulation of Myosin Phosphatase

The pathological elevation of the active thyroid hormone (T3) level results in the manifestation of hyperthyroidism, which is associated with alterations in the differentiation and contractile function of skeletal muscle (SKM). Myosin phosphatase (MP) is a major cellular regulator that hydrolyzes the phosphoserine of phosphorylated myosin II light chain. MP consists of an MYPT1/2 regulatory and a protein phosphatase 1 catalytic subunit. Smoothelin-like protein 1 (SMTNL1) is known to inhibit MP by directly binding to MP as well as by suppressing the expression of MYPT1 at the transcriptional level. Supraphysiological vs. physiological concentration of T3 were applied on C2C12 myoblasts and differentiated myotubes in combination with the overexpression of SMTNL1 to assess the role and regulation of MP under these conditions. In non-differentiated myoblasts, MP included MYPT1 in the holoenzyme complex and its expression and activity was regulated by SMTNL1, affecting the phosphorylation level of MLC20 assessed using semi-quantitative Western blot analysis. SMTNL1 negatively influenced the migration and cytoskeletal remodeling of myoblasts measured by high content screening. In contrast, in myotubes, the expression of MYPT2 but not MYPT1 increased in a T3-dependent and SMTNL1-independent manner. T3 treatment combined with SMTNL1 overexpression impeded the activity of MP. In addition, MP interacted with Na+/K+-ATPase and dephosphorylated its inhibitory phosphorylation sites, identifying this protein as a novel MP substrate. These findings may help us gain a better understanding of myopathy, muscle weakness and the disorder of muscle regeneration in hyperthyroid patients.

TIMAP inhibits endothelial myosin light chain phosphatase by competing with MYPT1 for the catalytic protein phosphatase 1 subunit PP1cβ

Transforming growth factor-β membrane associated protein (TIMAP) is an endothelial cell (EC)-predominant PP1 regulatory subunit and a member of the myosin phosphatase target (MYPT) protein family. The MYPTs preferentially bind the catalytic protein phosphatase 1 subunit PP1cβ, forming myosin phosphatase holoenzymes. We investigated whether TIMAP/PP1cβ could also function as a myosin phosphatase. Endogenous PP1cβ, myosin light chain 2 (MLC2), and myosin IIA heavy chain coimmunoprecipitated from EC lysates with endogenous TIMAP, and endogenous MLC2 colocalized with TIMAP in EC projections. Purified recombinant GST-TIMAP interacted directly with purified recombinant His-MLC2. However, TIMAP overexpression in EC enhanced MLC2 phosphorylation, an effect not observed with a TIMAP mutant that does not bind PP1cβ. Conversely, MLC2 phosphorylation was reduced in lung lysates from TIMAP-deficient mice and upon silencing of endogenous TIMAP expression in ECs. Ectopically expressed TIMAP slowed the rate of MLC2 dephosphorylation, an effect requiring TIMAP-PP1cβ interaction. The association of MYPT1 with PP1cβ was profoundly reduced in the presence of excess TIMAP, leading to proteasomal MYPT1 degradation. In the absence of TIMAP, MYPT1-associated PP1cβ readily bound immobilized microcystin-LR, an active-site inhibitor of PP1c. By contrast, TIMAP-associated PP1cβ did not interact with microcystin-LR, indicating that the active site of PP1cβ is blocked when it is bound to TIMAP. Thus, TIMAP inhibits myosin phosphatase activity in ECs by competing with MYPT1 for PP1cβ and blocking the PP1cβ active site.

Role of myosin light chain phosphatase in cardiac physiology and pathophysiology

Maintenance of contractile performance of the heart is achieved in part by the constitutive 40% phosphorylation of myosin regulatory light chain (RLC) in sarcomeres. The importance of this extent of RLC phosphorylation for optimal cardiac performance becomes apparent when various mouse models and resultant phenotypes are compared. The absence or attenuation of RLC phosphorylation results in poor performance leading to heart failure, whereas increased RLC phosphorylation is associated with cardiac protection from stresses. Although information is limited, RLC phosphorylation appears compromised in human heart failure which is consistent with data from mouse studies. The extent of cardiac RLC phosphorylation is determined by the balanced activities of cardiac myosin light chain kinases and phosphatases, the regulatory mechanisms of which are now emerging. This review thusly focuses on kinases that may participate in phosphorylating RLC to make the substrate for cardiac myosin light chain phosphatases, in addition to providing perspectives on the family of myosin light chain phosphatases and involved signaling mechanisms. Because biochemical and physiological information about cardiac myosin light chain phosphatase is sparse, such studies represent an emerging area of investigation in health and disease.

Protein phosphatase 1 catalytic isoforms: specificity toward interacting proteins

The coordinated and reciprocal action of serine-threonine protein kinases and protein phosphatases produces transitory phosphorylation, a fundamental regulatory mechanism for many biological processes. Phosphoprotein phosphatase 1 (PPP1), a major serine-threonine phosphatase, in particular, is ubiquitously distributed and regulates a broad range of cellular functions, including glycogen metabolism, cell cycle progression, and muscle relaxation. PPP1 has evolved effective catalytic machinery but in vitro lacks substrate specificity. In vivo, its specificity is achieved not only by the existence of different PPP1 catalytic isoforms, but also by binding of the catalytic moiety to a large number of regulatory or targeting subunits. Here, we will address exhaustively the existence of diverse PPP1 catalytic isoforms and the relevance of their specific partners and consequent functions.

Multi-directional function of the protein phosphatase 1 regulatory subunit TIMAP

TIMAP is an endothelial-cell predominant member of the MYPT family of PP1c regulatory subunits. This study explored the TIMAP-PP1c interaction and substrate specificity in vitro. TIMAP associated with all three PP1c isoforms, but endogenous endothelial cell TIMAP preferentially co-immunoprecipitated with PP1cβ. Structural modeling of the TIMAP/PP1c complex predicts that the PP1c C-terminus is buried in the TIMAP ankyrin cluster, and that the PP1c active site remains accessible. Consistent with this model, C-terminal PP1c phosphorylation by cdk2-cyclinA was masked by TIMAP, and PP1c bound TIMAP when the active site was occupied by the inhibitor microcystin. TIMAP inhibited PP1c activity toward phosphorylase a in a concentration-dependent manner, with half-maximal inhibition in the 0.4-1.2 nM range, an effect modulated by the length, and by Ser333/Ser337 phosphomimic mutations of the TIMAP C-terminus. TIMAP-bound PP1cβ effectively dephosphorylated MLC2 and TIMAP itself. By contrast, TIMAP inhibited the PP1cβ activity toward the putative substrate LAMR1, and instead masked LAMR1 PKA- and PKC-phosphorylation sites. This is direct evidence that MLC2 is a TIMAP/PP1c substrate. The data also indicate that TIMAP can modify protein phosphorylation independent of its function as a PP1c regulatory subunit, namely by masking phosphorylation sites of binding partners like PP1c and LAMR1.

Putative protein partners for the human CPI-17 protein revealed by bacterial two-hybrid screening

We have previously demonstrated that PKC-potentiated inhibitory protein of protein phosphatase-1 (CPI-17) is expressed in lung endothelium. CPI-17, a specific inhibitor of myosin light chain phosphatase (MLCP), is involved in the endothelial cytoskeletal and barrier regulation. In this paper, we report the identification of fourteen putative CPI-17 interacting proteins in the lung using BacterioMatch Two-Hybrid System. Five of them: plectin 1 isoform 1, alpha II spectrin, OK/SW-CL.16, gelsolin isoform a, and junction plakoglobin are involved in actin cytoskeleton organization and cell adhesion, suggesting possible significance of these binding partners in CPI-17-mediated cytoskeletal reorganization of endothelial cells. Furthermore, we confirmed the specific interaction between plakoglobin and CPI-17, which is affected by the phosphorylation status of CPI-17 in human lung microvascular endothelial cells.

Myosin phosphatase is inactivated by caspase-3 cleavage and phosphorylation of myosin phosphatase targeting subunit 1 during apoptosis

This study indicates that myosin phosphatase in apoptotic cells is inactivated by the cleavage and the phosphorylation of myosin phosphatase targeting subunit 1 (MYPT1). The results suggest that down-regulation of MYPT1 promotes hyperphosphorylation of myosin II during apoptosis.

In nonapoptotic cells, the phosphorylation level of myosin II is constantly maintained by myosin kinases and myosin phosphatase. During apoptosis, caspase-3–activated Rho-associated protein kinase I triggers hyperphosphorylation of myosin II, leading to membrane blebbing. Although inhibition of myosin phosphatase could also contribute to myosin II phosphorylation, little is known about the regulation of myosin phosphatase in apoptosis. In this study, we have demonstrated that, in apoptotic cells, the myosin-binding domain of myosin phosphatase targeting subunit 1 (MYPT1) is cleaved by caspase-3 at Asp-884, and the cleaved MYPT1 is strongly phosphorylated at Thr-696 and Thr-853, phosphorylation of which is known to inhibit myosin II binding. Expression of the caspase-3 cleaved form of MYPT1 that lacked the C-terminal end in HeLa cells caused the dissociation of MYPT1 from actin stress fibers. The dephosphorylation activity of myosin phosphatase immunoprecipitated from the apoptotic cells was lower than that from the nonapoptotic control cells. These results suggest that down-regulation of MYPT1 may play a role in promoting hyperphosphorylation of myosin II by inhibiting the dephosphorylation of myosin II during apoptosis.

Protein phosphatase 1α interacting proteins in the human brain

Protein Phosphatase 1 (PP1) is a major serine/threonine-phosphatase whose activity is dependent on its binding to regulatory subunits known as PP1 interacting proteins (PIPs), responsible for targeting PP1 to a specific cellular location, specifying its substrate or regulating its action. Today, more than 200 PIPs have been described involving PP1 in panoply of cellular mechanisms. Moreover, several PIPs have been identified that are tissue and event specific. In addition, the diversity of PP1/PIP complexes can further be achieved by the existence of several PP1 isoforms that can bind preferentially to a certain PIP. Thus, PP1/PIP complexes are highly specific for a particular function in the cell, and as such, they are excellent pharmacological targets. Hence, an in-depth survey was taken to identify specific PP1α PIPs in human brain by a high-throughput Yeast Two-Hybrid approach. Sixty-six proteins were recognized to bind PP1α, 39 being novel PIPs. A large protein interaction databases search was also performed to integrate with the results of the PP1α Human Brain Yeast Two-Hybrid and a total of 246 interactions were retrieved.

Ornithine decarboxylase antizyme Oaz3 modulates protein phosphatase activity

Ornithine decarboxylase antizyme 3 (Oaz3) is expressed in spermatids, makes up the antizyme family of Oaz genes with Oaz1 and Oaz2, and was proposed to encode a 22 kDa antizyme protein involved in polyamine regulation similar to the 22 kDa OAZ1 and OAZ2 proteins. Here we demonstrate however that the major product encoded by Oaz3 is a 12 kDa protein, p12, which lacks the antizyme domain that interacts with ornithine decarboxylase. We show that p12 does not affect ornithine decarboxylase levels, providing an explanation for the surprising observation made in Oaz3 knock-out male mice, which do not display altered testis polyamine metabolism. This suggested a novel activity for Oaz3 p12. Using immuno-electron microscopy we localized p12 to two structures in the mammalian sperm tail, viz. the outer dense fibers and fibrous sheath, as well as to the connecting piece linking head and tail. We identified myosin phosphatase targeting subunit 3 (MYPT3), a regulator of protein phosphatase PP1β, as a major p12-interacting protein, and show that MYPT3 is present in sperm tails and that its ankyrin repeat binds p12. We show that MYPT3 can also bind protein phosphatase PP1γ2, the only protein phosphatase present in sperm tails, and that p12- MYPT3 interaction modulates the activity of both PP1β and PP1γ2. This is, to our knowledge, the first demonstration of a novel activity for an Oaz-encoded protein.

Proteomic analysis reveals Hrs ubiquitin-interacting motif-mediated ubiquitin signaling in multiple cellular processes

Despite the critical importance of protein ubiquitination in the regulation of diverse cellular processes, the molecular mechanisms by which cells recognize and transmit ubiquitin signals remain poorly understood. The endosomal sorting machinery component hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs) contains a ubiquitin-interacting motif (UIM), which is believed to bind ubiquitinated membrane cargo proteins and mediate their sorting to the lysosomal degradation pathway. To gain insight into the role of Hrs UIM-mediated ubiquitin signaling in cells, we performed a proteomic screen for Hrs UIM-interacting ubiquitinated proteins in human brain by using an in vitro expression cloning screening approach. We have identified 48 ubiquitinated proteins that are specifically recognized by the UIM domain of Hrs. Among them, 12 are membrane proteins that are likely to be Hrs cargo proteins, and four are membrane protein-associated adaptor proteins whose ubiquitination may act as a signal to target their associated membrane cargo for Hrs-mediated endosomal sorting. Other classes of the identified proteins include components of the vesicular trafficking machinery, cell signaling molecules, proteins associated with the cytoskeleton and cytoskeleton-dependent transport, and enzymes involved in ubiquitination and metabolism, suggesting the involvement of Hrs UIM-mediated ubiquitin signaling in the regulation of multiple cellular processes. We have characterized the ubiquitination of two identified proteins, Munc18-1 and Hsc70, and their interaction with Hrs UIM, and provided functional evidence supporting a role for Hsc70 in the regulation of Hrs-mediated endosome-to-lysosome trafficking.

TIMAP is a positive regulator of pulmonary endothelial barrier function

TGF-beta-inhibited membrane-associated protein, TIMAP, is expressed at high levels in endothelial cells (EC). It is regarded as a member of the MYPT (myosin phosphatase target subunit) family of protein phosphatase 1 (PP1) regulatory subunits; however, its function in EC is not clear. In our pull-down experiments, recombinant TIMAP binds preferentially the beta-isoform of the catalytic subunit of PP1 (PP1cbeta) from pulmonary artery EC. As PP1cbeta, but not PP1calpha, binds with MYPT1 into functional complex, these results suggest that TIMAP is a novel regulatory subunit of myosin phosphatase in EC. TIMAP depletion by small interfering RNA (siRNA) technique attenuates increases in transendothelial electrical resistance induced by EC barrier-protective agents (sphingosine-1-phosphate, ATP) and enhances the effect of barrier-compromising agents (thrombin, nocodazole) demonstrating a barrier-protective role of TIMAP in EC. Immunofluorescent staining revealed colocalization of TIMAP with membrane/cytoskeletal protein, moesin. Moreover, TIMAP coimmunoprecipitates with moesin suggesting the involvement of TIMAP/moesin interaction in TIMAP-mediated EC barrier enhancement. Activation of cAMP/PKA cascade by forskolin, which has a barrier-protective effect against thrombin-induced EC permeability, attenuates thrombin-induced phosphorylation of moesin at the cell periphery of control siRNA-treated EC. On the contrary, in TIMAP-depleted EC, forskolin failed to affect the level of moesin phosphorylation at the cell edges. These results suggest the involvement of TIMAP in PKA-mediated moesin dephosphorylation and the importance of this dephosphorylation in TIMAP-mediated EC barrier protection.

PPP1R16A, the membrane subunit of protein phosphatase 1beta, signals nuclear translocation of the nuclear receptor constitutive active/androstane receptor

Constitutive active/androstane receptor (CAR), a member of the nuclear steroid/thyroid hormone receptor family, activates transcription of numerous hepatic genes upon exposure to therapeutic drugs and environmental pollutants. Sequestered in the cytoplasm, this receptor signals xenobiotic exposure, such as phenobarbital (PB), by translocating into the nucleus. Unlike other hormone receptors, translocation can be triggered indirectly without binding to xenobiotics. We have now identified a membrane-associated subunit of protein phosphatase 1 (PPP1R16A, or abbreviated as R16A) as a novel CAR-binding protein. When CAR and R16A are coexpressed in mouse liver, CAR translocates into the nucleus. Close association of R16A and CAR molecule on liver membrane was shown by fluorescence resonance energy transfer (FRET) analysis using expressed yellow fluorescent protein (YFP)-CAR and CFP-R16A fusion proteins. R16A can form dimer through its middle region, where protein kinase A phosphorylation sites are recently identified. Translocation of CAR by R16A correlates with the ability of R16A to form an intermolecular interaction via the middle region. Moreover, this interaction is enhanced by PB treatment in mouse liver. R16A specifically interacted with PP1beta in HepG2 cells despite the highly conserved structure of PP1 family molecules. PP1beta activity was inhibited by R16A in vitro and coexpression of PP1beta in liver can prevent YFP-CAR translocation into mouse liver. Taken together, R16A at the membrane may mediate the PB signal to initiate CAR nuclear translocation, through a mechanism including its dimerization and inhibition of PP1beta activity, providing a novel model for the translocation of nuclear receptors in which direct interaction of ligands and the receptors may not be crucial.

Epistasis between hyperglycemic QTLs revealed in a double congenic of the OLETF rat

Glucose homeostasis is believed to be regulated by multiple genetic components, in addition to numerous external factors. It is therefore crucial to dissect and understand what roles each causative gene plays in maintaining proper glucose metabolism. In OLETF (Otsuka Long-Evans Tokushima Fatty) rat, a model of polygenic type 2 diabetes, at least 14 quantitative trait loci (QTLs) influencing plasma glucose levels were identified. In congenic strains some of the OLETF allelic variants were shown to increase glucose levels. In this study the focus was on two of the hyperglycemic loci, Nidd1/of and Nidd2/of. Congenic rats possessing OLETF genome fragment at either locus showed similar levels of mild hyperglycemia. A newly established double congenic rat showed a further aggravation of hyperglycemia. The Nidd1/of locus was also shown to function in the reduction of plasma leptin levels and fat weights, while the Nidd2/of locus led to increased plasma insulin and fat weights. Interestingly, both plasma leptin and fat weights reverted to the control levels in the double congenic rat. These results indicate that there is an epistatic interaction between the two loci. However, it is unlikely that the abnormal level of enhanced glucose homeostasis is mediated, at least not directly, by leptin or fat mass.

Phosphorylation of myosin phosphatase targeting subunit 3 (MYPT3) and regulation of protein phosphatase 1 by protein kinase A

Myosin phosphatase targeting subunit 3 (MYPT3) and transforming growth factor-beta-inhibited membrane-associated protein (TIMAP) are two closely related myosin-binding targeting subunits of protein phosphatase 1 (PP1c) with a characteristic CAAX (where AA indicates aliphatic amino acid) box at the C termini. Here we show that MYPT3 can be a substrate for protein kinase A (PKA). We first mapped the multiple phosphorylation sites within a central conserved motif. Deletion or mutations of this motif resulted in enhancement of the associated PP1c activity, suggesting that phosphorylation of MYPT3 may play an important role in regulating PP1c catalytic activity. However, unlike the other known MYPTs, which upon phosphorylation inhibit PP1c, PKA phosphorylation of MYPT3 resulted in PP1c activation, indicating a different mode of action. There is a direct interaction between the central conserved phosphorylated site motif with the N-terminal ankyrin repeat region; this interaction was significantly reduced with MYPT3 phosphorylation or acidic phosphorylation site mutations, with concomitant alterations in biochemical and morphological consequences. We therefore propose a novel mechanism for the phosphorylation of MYPT3 by PKA and activation of the catalytic activity through direct interaction of a central region of MYPT3 with its N-terminal region.

Myosin phosphatase: structure, regulation and function

Phosphorylation of myosin II plays an important role in many cell functions, including smooth muscle contraction. The level of myosin II phosphorylation is determined by activities of myosin light chain kinase and myosin phosphatase (MP). MP is composed of 3 subunits: a catalytic subunit of type 1 phosphatase, PPlc; a targeting subunit, termed myosin phosphatase target subunit, MYPT; and a smaller subunit, M20, of unknown function. Most of the properties of MP are due to MYPT and include binding of PP1c and substrate. Other interactions are discussed. A recent discovery is the existence of an MYPT family and members include, MYPT1, MYPT2, MBS85, MYPT3 and TIMAP. Characteristics of each are outlined. An important discovery was that the activity of MP could be regulated and both activation and inhibition were reported. Activation occurs in response to elevated cyclic nucleotide levels and various mechanisms are presented. Inhibition of MP is a major component of Ca2+-sensitization in smooth muscle and various molecular mechanisms are discussed. Two mechanisms are cited frequently: (1) Phosphorylation of an inhibitory site on MYPT1, Thr696 (human isoform) and resulting inhibition of PP1c activity. Several kinases can phosphorylate Thr696, including Rho-kinase that serves an important role in smooth muscle function; and (2) Inhibition of MP by the protein kinase C-potentiated inhibitor protein of 17 kDa (CPI-17). Examples where these mechanisms are implicated in smooth muscle function are presented. The critical role of RhoA/Rho-kinase signaling in various systems is discussed, in particular those vascular smooth muscle disorders involving hypercontractility.

The essential role of PP1beta in Drosophila is to regulate nonmuscle myosin

Reversible phosphorylation of myosin regulatory light chain (MRLC) is a key regulatory mechanism controlling myosin activity and thus regulating the actin/myosin cytoskeleton. We show that Drosophila PP1beta, a specific isoform of serine/threonine protein phosphatase 1 (PP1), regulates nonmuscle myosin and that this is the essential role of PP1beta. Loss of PP1beta leads to increased levels of phosphorylated nonmuscle MRLC (Sqh) and actin disorganisation; these phenotypes can be suppressed by reducing the amount of active myosin. Drosophila has two nonmuscle myosin targeting subunits, one of which (MYPT-75D) resembles MYPT3, binds specifically to PP1beta, and activates PP1beta's Sqh phosphatase activity. Expression of a mutant form of MYPT-75D that is unable to bind PP1 results in elevation of Sqh phosphorylation in vivo and leads to phenotypes that can also be suppressed by reducing the amount of active myosin. The similarity between fly and human PP1beta and MYPT genes suggests this role may be conserved.

Cerebellar long-term synaptic depression requires PKC-mediated activation of CPI-17, a myosin/moesin phosphatase inhibitor

Cerebellar LTD requires brief activation of PKC and is expressed as a functional downregulation of AMPA receptors. Modulation of vascular smooth-muscle contraction by G protein-coupled receptors (called Ca(2+) sensitization) also involves PKC phosphorylation and activation of a specific inhibitor of myosin/moesin phosphatase (MMP). This inhibitor, called CPI-17, is also expressed in brain. Here, we tested the hypothesis that LTD, like Ca(2+) sensitization, employs a PKC/CPI-17 cascade. Introduction of activated recombinant CPI-17 into cells produced a use-dependent attenuation of glutamate-evoked responses and occluded subsequent LTD. Moreover, the requirement for endogenous CPI-17 in LTD was demonstrated with neutralizing antibodies plus gene silencing by siRNA. These interventions had no effect on basal synaptic strength but blocked LTD induction. Thus, a biochemical circuit that involves PKC-mediated activation of CPI-17 modulates the distinct physiological processes of vascular contractility and cerebellar LTD.

mPPP1R16B is a novel mouse protein phosphatase 1 targeting subunit whose mRNA is located in cell bodies and dendrites of neurons in four distinct regions of the brain

We cloned a cDNA encoding a novel mouse protein whose human homolog has been annotated in GenBank as a regulatory subunit of protein phosphatase 1, PPP1R16B. Both the primary protein sequence and the domain structure are highly conserved between PPP1R16B and proteins of unknown function from other species, such as Caenorhabditis elegans and Drosphila melanogaster. Besides a protein phosphatase 1 interaction motif, mouse PPP1R16B (mPPP1R16B) and the related proteins contain ankyrin repeats that may constitute binding sites for other proteins and C-terminal prenylation signals that are likely to target the proteins to the plasma membrane. In the adult mouse, Ppp1r16b mRNA is expressed in most tissues examined, with highest expression levels in kidney and brain. In the brain, Ppp1r16b message is particularly enriched in the olfactory bulb, striatum, dentate gyrus, and cerebellum. During postnatal cerebellar development, Ppp1r16b mRNA expression levels increase gradually and are maximal around postnatal day 30. In situ hybridization revealed that Ppp1r16b message is found in both the cell bodies and the dendrites in Purkinje cells of the cerebellum and granule neurons of the dentate gyrus.