Activation of myosin phosphatase targeting subunit by mitosis-specific phosphorylation

From stress fiber to focal adhesion: a role of actin crosslinkers in force transmission

The contractile apparatus, stress fiber (SF), is connected to the cell adhesion machinery, focal adhesion (FA), at the termini of SF. The SF-FA complex is essential for various mechanical activities of cells, including cell adhesion to the extracellular matrix (ECM), ECM rigidity sensing, and cell migration. This mini-review highlights the importance of SF mechanics in these cellular activities. Actin-crosslinking proteins solidify SFs by attenuating myosin-driven flows of actin and myosin filaments within the SF. In the solidified SFs, viscous slippage between actin filaments in SFs and between the filaments and the surrounding cytosol is reduced, leading to efficient transmission of myosin-generated contractile force along the SFs. Hence, SF solidification via actin crosslinking ensures exertion of a large force to FAs, enabling FA maturation, ECM rigidity sensing and cell migration. We further discuss intracellular mechanisms for tuning crosslinker-modulated SF mechanics and the potential relationship between the aberrance of SF mechanics and pathology including cancer.

Myosin phosphatase targeting subunit1 controls localization and motility of Rab7-containing vesicles: Is myosin phosphatase a cytoplasmic dynein regulator?

Myosin phosphatase targeting subunit1 (MYPT1) is a critical subunit of myosin phosphatase (MP), which brings PP1Cδ phosphatase and its substrate together. We previously showed that MYPT1 depletion resulted in oblique chromatid segregation. Therefore, we hypothesized that MYPT1 may control microtubule-dependent motor activity. Dynein, a minus-end microtubule motor, is known to be involved in mitotic spindle assembly. We thus examined whether MYPT1 and dynein may interact. Proximity ligation assay and co-immunoprecipitation revealed that MYPT1 and dynein intermediate chain (DIC) were associated. We found that DIC phosphorylation is increased in MYPT1-depleted cells in vivo, and that MP was able to dephosphorylate DIC in vitro. MYPT1 depletion also altered the localization and motility of Rab7-containing vesicles. MYPT1-depletion dispersed the perinuclear Rab7 localization to the peripheral in interphase cells. The dispersed Rab7 localization was rescued by microinjection of a constitutively active, truncated MYPT1 mutant, supporting that MP is responsible for the altered Rab7 localization. Analyses of Rab7 vesicle trafficking also revealed that minus-end transport was reduced in MYPT1-depleted cells. These results suggest an unexpected role of MP: MP controls dynein activity in both mitotic and interphase cells, possibly by dephosphorylating dynein subunits including DIC.

Nonmuscle Myosin II Regulation Directs Its Multiple Roles in Cell Migration and Division

Nonmuscle myosin II (NMII) is a multimeric protein complex that generates most mechanical force in eukaryotic cells. NMII function is controlled at three main levels. The first level includes events that trigger conformational changes that extend the complex to enable its assembly into filaments. The second level controls the ATPase activity of the complex and its binding to microfilaments in extended NMII filaments. The third level includes events that modulate the stability and contractility of the filaments. They all work in concert to finely control force generation inside cells. NMII is a common endpoint of mechanochemical signaling pathways that control cellular responses to physical and chemical extracellular cues. Specific phosphorylations modulate NMII activation in a context-dependent manner. A few kinases control these phosphorylations in a spatially, temporally, and lineage-restricted fashion, enabling functional adaptability to the cellular microenvironment. Here, we review mechanisms that control NMII activity in the context of cell migration and division. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

p116Rip promotes myosin phosphatase activity in airway smooth muscle cells

Myosin phosphatase-Rho interacting protein (p116^Rip ) was originally found as a RhoA-binding protein. Subsequent studies by us and others revealed that p116^Rip facilitates myosin light chain phosphatase (MLCP) activity through direct and indirect manners. However, it is unclear how p116^Rip regulates myosin phosphatase activity in cells. To elucidate the role of p116^Rip in cellular contractile processes, we suppressed the expression of p116^Rip by RNA interference in human airway smooth muscle cells (HASMCs). We found that knockdown of p116^Rip in HASMCs led to increased di-phosphorylated MLC (pMLC), that is phosphorylation at both Ser19 and Thr18. This was because of a change in the interaction between MLCP and myosin, but not an alteration of RhoA/ROCK signaling. Attenuation of Zipper-interacting protein kinase (ZIPK) abolished the increase in di-pMLC, suggesting that ZIPK is involved in this process. Moreover, suppression of p116^Rip expression in HASMCs substantially increased the histamine-induced collagen gel contraction. We also found that expression of the p116^Rip was decreased in the airway smooth muscle tissue from asthmatic patients compared with that from non-asthmatic patients, suggesting a potential role of p116^Rip expression in asthma pathogenesis.

Phosphatases in Mitosis: Roles and Regulation

Mitosis requires extensive rearrangement of cellular architecture and of subcellular structures so that replicated chromosomes can bind correctly to spindle microtubules and segregate towards opposite poles. This process originates two new daughter nuclei with equal genetic content and relies on highly-dynamic and tightly regulated phosphorylation of numerous cell cycle proteins. A burst in protein phosphorylation orchestrated by several conserved kinases occurs as cells go into and progress through mitosis. The opposing dephosphorylation events are catalyzed by a small set of protein phosphatases, whose importance for the accuracy of mitosis is becoming increasingly appreciated. This review will focus on the established and emerging roles of mitotic phosphatases, describe their structural and biochemical properties, and discuss recent advances in understanding the regulation of phosphatase activity and function.

Myosin phosphatase: Unexpected functions of a long-known enzyme

Myosin phosphatase (MP) holoenzyme is a Ser/Thr specific enzyme, which is the member of protein phosphatase type 1 (PP1) family and composed of a PP1 catalytic subunit (PP1c/PPP1CB) and a myosin phosphatase targeting subunit (MYPT1/PPP1R12A). PP1c is required for the catalytic activity of the holoenzyme, while MYPT1 regulates MP through targeting the holoenzyme to its substrates. Above the well-characterized function of MP, as the major regulator of smooth muscle contractility mediating the dephosphorylation of 20 kDa myosin light chain, accumulating data support its role in other, non-contractile functions. In this review, we summarize the scaffold function of MP holoenzyme and its roles in processes such as cell cycle, development, gene expression regulation and neurotransmitter release. In particular, we highlight novel interacting proteins of MYPT1 and pathophysiological functions of MP relevant to tumorigenesis, insulin resistance and neurodegenerative disorders. This article is part of a Special Issue entitled: Protein Phosphatases as Critical Regulators for Cellular Homeostasis edited by Prof. Peter Ruvolo and Dr. Veerle Janssens.

Cyclin A/Cdk1 modulates Plk1 activity in prometaphase to regulate kinetochore-microtubule attachment stability

The fidelity of chromosome segregation in mitosis is safeguarded by the precise regulation of kinetochore microtubule (k-MT) attachment stability. Previously, we demonstrated that Cyclin A/Cdk1 destabilizes k-MT attachments to promote faithful chromosome segregation. Here, we use quantitative phosphoproteomics to identify 156 Cyclin A/Cdk1 substrates in prometaphase. One Cyclin A/Cdk1 substrate is myosin phosphatase targeting subunit 1 (MYPT1), and we show that MYPT1 localization to kinetochores depends on Cyclin A/Cdk1 activity and that MYPT1 destabilizes k-MT attachments by negatively regulating Plk1 at kinetochores. Thus, Cyclin A/Cdk1 phosphorylation primes MYPT1 for Plk1 binding. Interestingly, priming of PBIP1 by Plk1 itself (self-priming) increased in MYPT1-depleted cells showing that MYPT1 provides a molecular link between the processes of Cdk1-dependent priming and self-priming of Plk1 substrates. These data demonstrate cross-regulation between Cyclin A/Cdk1-dependent and Plk1-dependent phosphorylation of substrates during mitosis to ensure efficient correction of k-MT attachment errors necessary for high mitotic fidelity.

ERK and p38MAPK pathways regulate myosin light chain phosphatase and contribute to Ca2+ sensitization of intestinal smooth muscle contraction

BACKGROUND

Mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated protein kinase (ERK) and p38MAPK, are known regulators of smooth muscle contractility. The contraction of smooth muscle is mainly regulated by the phosphorylation of regulatory light chains of myosin II (LC20), which is driven by the balance between myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP). We hypothesized that one possible mechanism for MAPK-dependent modulation of intestinal smooth muscle contractility is via the regulation of MLCP activity.

METHODS

Contractile responses to carbachol (CCh) and effects of MAPK inhibitors on CCh-induced contractions were assessed with isolated rat ileal longitudinal smooth muscle strips. Biochemical assessments of MLCP activity and myosin phosphatse targeting subunit (MYPT1) and CPI-17 phosphorylations were completed.

KEY RESULTS

Treatment of ileal smooth muscle with PD98059 (10 μM; MEK inhibitor) or SB203580 (10 μM; p38MAPK inhibitor) significantly inhibited CCh-induced contractile force. Decreased MLCP activity was observed during sustained contractions induced by CCh; the MLCP activity was recovered by treatment with PD98059 and SB203580. However, MYPT1 (Thr697 and Thr855) and CPI-17 (Thr38) phosphorylations were not affected. Application of ML-7 (MLCK inhibitor) during CCh-induced sustained contraction elicited an MLCP-dependent relaxation, the rate of which was accelerated by application of PD98059 and SB203580 with proportional changes in LC20 phosphorylation levels but not MYPT1 phosphorylation (Thr697 or Thr855).

CONCLUSIONS & INFERENCES

ERK and p38MAPK contribute to CCh-induced sustained contraction in a LC20 phosphorylation dependent manner. Moreover, both kinases inhibit MLCP activity possibly by a novel mechanism.

In vivo roles for myosin phosphatase targeting subunit-1 phosphorylation sites T694 and T852 in bladder smooth muscle contraction

KEY POINTS

Force production and maintenance in smooth muscle is largely controlled by myosin regulatory light chain (RLC) phosphorylation, which relies on a balance between Ca(2+)/calmodulin-dependent myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP) activities. MYPT1 is the regulatory subunit of MLCP that biochemically inhibits MLCP activity via T694 or T852 phosphorylation in vitro. Here we separately investigated the contribution of these two phosphorylation sites in bladder smooth muscles by establishing two single point mutation mouse lines, T694A and T852A, and found that phosphorylation of MYPT1 T694, but not T852, mediates force maintenance via inhibition of MLCP activity and enhancement of RLC phosphorylation in vivo. Our findings reveal the role of MYPT1 T694/T852 phosphorylation in vivo in regulation of smooth muscle contraction.

ABSTRACT

Force production and maintenance in smooth muscle is largely controlled by different signalling modules that fine tune myosin regulatory light chain (RLC) phosphorylation, which relies on a balance between Ca(2+)/calmodulin-dependent myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP) activities. To investigate the regulation of MLCP activity in vivo, we analysed the role of two phosphorylation sites on MYPT1 (regulatory subunit of MLCP) that biochemically inhibit MLCP activity in vitro. MYPT1 is constitutively phosphorylated at T694 by unidentified kinases in vivo, whereas the T852 site is phosphorylated by RhoA-associated protein kinase (ROCK). We established two mouse lines with alanine substitution of T694 or T852. Isolated bladder smooth muscle from T852A mice displayed no significant changes in RLC phosphorylation or force responses, but force was inhibited with a ROCK inhibitor. In contrast, smooth muscles containing the T694A mutation showed a significant reduction of force along with reduced RLC phosphorylation. The contractile responses of T694A mutant smooth muscle were also independent of ROCK activation. Thus, phosphorylation of MYPT1 T694, but not T852, is a primary mechanism contributing to inhibition of MLCP activity and enhancement of RLC phosphorylation in vivo. The constitutive phosphorylation of MYPT1 T694 may provide a mechanism for regulating force maintenance of smooth muscle.

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.

Role of serine-threonine phosphoprotein phosphatases in smooth muscle contractility

The degree of phosphorylation of myosin light chain 20 (MLC20) is a major determinant of force generation in smooth muscle. Myosin phosphatases (MPs) contain protein phosphatase (PP) 1 as catalytic subunits and are the major enzymes that dephosphorylate MLC20. MP regulatory targeting subunit 1 (MYPT1), the main regulatory subunit of MP in all smooth muscles, is a key convergence point of contractile and relaxatory pathways. Combinations of regulatory mechanisms, including isoform splicing, multiple phosphorylation sites, and scaffolding proteins, modulate MYPT1 activity with tissue and agonist specificities to affect contraction and relaxation. Other members of the PP1 family that do not target myosin, as well as PP2A and PP2B, dephosphorylate a range of proteins that affect smooth muscle contraction. This review discusses the role of phosphatases in smooth muscle contractility with a focus on MYPT1 in uterine smooth muscle. Myometrium shares characteristics of vascular and other visceral smooth muscles yet, during healthy pregnancy, undergoes hypertrophy, hyperplasia, quiescence, and labor as physiological processes. Myometrium presents an accessible model for the study of normal and pathological smooth muscle function, and a better understanding of myometrial physiology may allow the development of novel therapeutics for the many disorders of myometrial physiology from preterm labor to dysmenorrhea.

Cross-species genome wide expression analysis during pluripotent cell determination in mouse and rat preimplantation embryos

The transition between morula and blastocyst stage during preimplantation development represents the first differentiation event of embryogenesis. Morula cells undergo the first cellular specialization and produce two well-defined populations of cells, the trophoblast and the inner cell mass (ICM). Embryonic stem cells (ESCs) with unlimited self-renewal capacity are believed to represent the in vitro counterpart of the ICM. Both mouse and rat ESCs can be derived from the ICM cells, but their in vitro stability differs. In this study we performed a microarray analysis in which we compared the transcriptome of mouse and rat morula, blastocyst, and ICM. This cross-species comparison represents a good model for understanding the differences in derivation and cultivation of ESCs observed in the two species. In order to identify alternative regulation of important molecular mechanisms the investigation of differential gene expression between the two species was extended at the level of signaling pathways, gene families, and single selected genes of interest. Some of the genes differentially expressed between the two species are already known to be important factors in the maintenance of pluripotency in ESCs, like for example Sox2 or Stat3, or play a role in reprogramming somatic cells to pluripotency like c-Myc, Klf4 and p53 and therefore represent interesting candidates to further analyze in vitro in the rat ESCs. This is the first study investigating the gene expression changes during the transition from morula to blastocyst in the rat preimplantation development. Our data show that in the pluripotent pool of cells of the rat and mouse preimplantation embryo substantial differential regulation of genes is present, which might explain the difficulties observed for the derivation and culture of rat ESCs using mouse conditions.

LATS1/WARTS phosphorylates MYPT1 to counteract PLK1 and regulate mammalian mitotic progression

In the mitotic exit network of budding yeast, Dbf2 kinase phosphorylates and regulates Cdc14 phosphatase. In contrast, no phosphatase substrates of LATS1/WARTS kinase, the mammalian equivalent of Dbf2, has been reported. To address this discrepancy, we performed phosphoproteomic screening using LATS1 kinase. Screening identified MYPT1 (myosin phosphatase-targeting subunit 1) as a new substrate for LATS1. LATS1 directly and preferentially phosphorylated serine 445 (S445) of MYPT1. An MYPT1 mutant (S445A) failed to dephosphorylate Thr 210 of PLK1 (pololike kinase 1), thereby activating PLK1. This suggests that LATS1 promotes MYPT1 to antagonize PLK1 activity. Consistent with this, LATS1-depleted HeLa cells or fibroblasts from LATS1 knockout mice showed increased PLK1 activity. We also found deoxyribonucleic acid (DNA) damage-induced LATS1 activation caused PLK1 suppression via the phosphorylation of MYPT1 S445. Furthermore, LATS1 knockdown cells showed reduced G2 checkpoint arrest after DNA damage. These results indicate that LATS1 phosphorylates a phosphatase as does the yeast Dbf2 and demonstrate a novel role of LATS1 in controlling PLK1 at the G2 DNA damage checkpoint.

Molecular mechanism of telokin-mediated disinhibition of myosin light chain phosphatase and cAMP/cGMP-induced relaxation of gastrointestinal smooth muscle

Phospho-telokin is a target of elevated cyclic nucleotide concentrations that lead to relaxation of gastrointestinal and some vascular smooth muscles (SM). Here, we demonstrate that in telokin-null SM, both Ca(2+)-activated contraction and Ca(2+) sensitization of force induced by a GST-MYPT1(654-880) fragment inhibiting myosin light chain phosphatase were antagonized by the addition of recombinant S13D telokin, without changing the inhibitory phosphorylation status of endogenous MYPT1 (the regulatory subunit of myosin light chain phosphatase) at Thr-696/Thr-853 or activity of Rho kinase. Cyclic nucleotide-induced relaxation of force in telokin-null ileum muscle was reduced but not correlated with a change in MYPT1 phosphorylation. The 40% inhibited activity of phosphorylated MYPT1 in telokin-null ileum homogenates was restored to nonphosphorylated MYPT1 levels by addition of S13D telokin. Using the GST-MYPT1 fragment as a ligand and SM homogenates from WT and telokin KO mice as a source of endogenous proteins, we found that only in the presence of endogenous telokin, thiophospho-GST-MYPT1 co-precipitated with phospho-20-kDa myosin regulatory light chain 20 and PP1. Surface plasmon resonance studies showed that S13D telokin bound to full-length phospho-MYPT1. Results of a protein ligation assay also supported interaction of endogenous phosphorylated MYPT1 with telokin in SM cells. We conclude that the mechanism of action of phospho-telokin is not through modulation of the MYPT1 phosphorylation status but rather it contributes to cyclic nucleotide-induced relaxation of SM by interacting with and activating the inhibited full-length phospho-MYPT1/PP1 through facilitating its binding to phosphomyosin and thus accelerating 20-kDa myosin regulatory light chain dephosphorylation.

PP1 and PP2A phosphatases--cooperating partners in modulating retinoblastoma protein activation

The retinoblastoma/pocket protein family is one of the master regulators of the eukaryotic cell cycle. It includes the retinoblastoma protein (Rb) and the related p107 and p130 proteins. The importance of the Rb pathway for homeostasis and tumour suppression is evident from the fact that inactivating mutations in Rb are frequently associated with many cancers. Rbs regulate the cell cycle by controlling the activity of the E2F family of transcription factors. The activity of Rb proteins themselves is modulated by their phosphorylation status at several Ser/Thr residues: phosphorylation by cyclin-dependent kinases inactivates Rb proteins and positively influences the transcription of genes necessary for cell cycle progression. Although the mechanisms of cyclin-dependent kinase-mediated inactivation of Rb proteins are understood in great detail, our knowledge of the process that counteracts Rb phosphorylation is still quite limited. The present review focuses on the Ser/Thr phosphatases that are responsible for the dephosphorylation and thus activation of Rb proteins. Two major scenarios are considered: (a) when pocket proteins are dephosphorylated during regular cell cycle progression and (b) when rapid dephosphorylation is dictated by external stress or growth inhibitory conditions, such as oxidative stress, UV radiation or other DNA-damaging stimuli, and cell differentiation factors. It transpires that protein phosphatase 1 and protein phosphatase 2A can efficiently modulate pocket protein activity in a highly context-dependent manner and both are tightly regulated by the presence of different regulatory subunits or interacting proteins.

Determinants of myosin II cortical localization during cytokinesis

Myosin II is an essential component of the contractile ring that divides the cell during cytokinesis. Previous work showed that regulatory light chain (RLC) phosphorylation is required for localization of myosin at the cellular equator. However, the molecular mechanisms that concentrate myosin at the site of furrow formation remain unclear. By analyzing the spatiotemporal dynamics of mutant myosin subunits in Drosophila S2 cells, we show that myosin accumulates at the equator through stabilization of interactions between the cortex and myosin filaments and that the motor domain is dispensable for localization. Filament stabilization is tightly controlled by RLC phosphorylation. However, we show that regulatory mechanisms other than RLC phosphorylation contribute to myosin accumulation at three different stages: (1) turnover of thick filaments throughout the cell cycle, (2) myosin heavy chain-based control of myosin assembly at the metaphase-anaphase transition, and (3) redistribution and/or activation of myosin binding sites at the equator during anaphase. Surprisingly, the third event can occur to a degree in a Rho-independent fashion, gathering preassembled filaments to the equatorial zone via cortical flow. We conclude that multiple regulatory pathways cooperate to control myosin localization during mitosis and cytokinesis to ensure that this essential biological process is as robust as possible.

Phosphorylation-dependent autoinhibition of myosin light chain phosphatase accounts for Ca2+ sensitization force of smooth muscle contraction

The reversible regulation of myosin light chain phosphatase (MLCP) in response to agonist stimulation and cAMP/cGMP signals plays an important role in the regulation of smooth muscle (SM) tone. Here, we investigated the mechanism underlying the inhibition of MLCP induced by the phosphorylation of myosin phosphatase targeting subunit (MYPT1), a regulatory subunit of MLCP, at Thr-696 and Thr-853 using glutathione S-transferase (GST)-MYPT1 fragments having the inhibitory phosphorylation sites. GST-MYPT1 fragments, including only Thr-696 and only Thr-853, inhibited purified MLCP (IC(50) = 1.6 and 60 nm, respectively) when they were phosphorylated with RhoA-dependent kinase (ROCK). The activities of isolated catalytic subunits of type 1 and type 2A phosphatases (PP1 and PP2A) were insensitive to either fragment. Phospho-GST-MYPT1 fragments docked directly at the active site of MLCP, and this was blocked by a PP1/PP2A inhibitor microcystin (MC)-LR or by mutation of the active sites in PP1. GST-MYPT1 fragments induced a contraction of beta-escin-permeabilized ileum SM at constant pCa 6.3 (EC(50) = 2 microm), which was eliminated by Ala substitution of the fragment at Thr-696 or by ROCK inhibitors or 8Br-cGMP. GST-MYPT1-(697-880) was 5-times less potent than fragments including Thr-696. Relaxation induced by 8Br-cGMP was not affected by Ala substitution at Ser-695, a known phosphorylation site for protein kinase A/G. Thus, GST-MYPT1 fragments are phosphorylated by ROCK in permeabilized SM and mimic agonist-induced inhibition and cGMP-induced activation of MLCP. We propose a model in which MYPT1 phosphorylation at Thr-696 and Thr-853 causes an autoinhibition of MLCP that accounts for Ca(2+) sensitization of smooth muscle force.

Stable and dynamic microtubules coordinately shape the myosin activation zone during cytokinetic furrow formation

The cytokinetic furrow arises from spatial and temporal regulation of cortical contractility. To test the role microtubules play in furrow specification, we studied myosin II activation in echinoderm zygotes by assessing serine19-phosphorylated regulatory light chain (pRLC) localization after precisely timed drug treatments. Cortical pRLC was globally depressed before cytokinesis, then elevated only at the equator. We implicated cell cycle biochemistry (not microtubules) in pRLC depression, and differential microtubule stability in localizing the subsequent myosin activation. With no microtubules, pRLC accumulation occurred globally instead of equatorially, and loss of just dynamic microtubules increased equatorial pRLC recruitment. Nocodazole treatment revealed a population of stable astral microtubules that formed during anaphase; among these, those aimed toward the equator grew longer, and their tips coincided with cortical pRLC accumulation. Shrinking the mitotic apparatus with colchicine revealed pRLC suppression near dynamic microtubule arrays. We conclude that opposite effects of stable versus dynamic microtubules focuses myosin activation to the cell equator during cytokinesis.

O-linked beta-N-acetylglucosaminyltransferase substrate specificity is regulated by myosin phosphatase targeting and other interacting proteins

O-GlcNAc-transferase (OGT) substrate specificity is regulated by transiently interacting proteins. To further examine the regulation of OGT, we have identified 27 putative OGT-interacting proteins through a yeast two-hybrid screen. Two of these proteins, Trak1 (OIP106) and O-GlcNAcase, have been shown previously to interact with and regulate OGT. We demonstrate here that MYPT1 and CARM1 also interact with and target OGT. MYPT1 and CARM1 are substrates of OGT in vitro and in vivo. MYPT1 and CARM1 also function to alter OGT substrate specificity in vitro. Furthermore depletion of MYPT1 in Neuro-2a neuroblastoma cells alters GlcNAcylation of several proteins under basal conditions, suggesting that MYPT1 regulates OGT substrate specificity in vivo.

A bitter PP1 fights the sweet polo

In a recent paper in Developmental Cell, Yamashiro et al. (2008) report that the PP1 regulatory subunit MYPT1 interacts with PLK1 and antagonizes essential mitotic functions of PLK1, at least in part by promoting the dephosphorylation of PLK1 at Thr210.

Assembly of MYPT1 with protein phosphatase-1 in fibroblasts redirects localization and reorganizes the actin cytoskeleton

Dephosphorylation of actin-binding proteins by a specialized form of protein Ser/Thr phosphatase type-1 (PP1) regulates smooth muscle contraction and morphology and motility of nonmuscle cells. This myosin and ezrin/radixin/moesin (ERM)-targeted phosphatase comprises the delta isoform PP1 catalytic subunit plus a primary regulatory subunit called myosin phosphatase targeting (MYPT1). We reconstructed myosin/ERM phosphatase in living rat embryo fibroblasts (REF52 cells) by transient expression of epitope-tagged MYPT1 (myc-MYPT1) plus HA-tagged PP1. Unexpectedly, wild-type myc-MYPT1 expressed alone accumulated predominantly in the nucleus, as visualized by immunofluorescent microscopy, whereas if coexpressed with HA-PP1, it was localized in the cytosol and deposited on cytoskeleton myofilaments. The F38A mutation of MYPT1 that eliminates PP1 binding gave nuclear localization of myc-MYPT1, even when coexpressed with HA-PP1. Thus, expression of both subunits was necessary to form myosin/ERM phosphatase in situ and mediate myofilament localization. The results indicate there is little endogenous PP1 available for interaction or interchange with ectopic regulatory subunits in living cells. We concluded that myosin binding by the C-terminal domain of MYPT1 is not sufficient to override nuclear import in fibroblasts, but the binding of PP1 to myc-MYPT1 neutralizes nuclear import. Full-length myc-MYPT1 plus HA-PP1 induced only subtle changes in organization of the actin cytoskeleton, however coexpression of myc-MYPT1(1-300) with HA-PP1 dispersed stress fibers without major alteration in morphology and myc-MYPT1(1-498) disrupted the cytoskeleton and produced radically extended cells that appeared like neurons. Based on these responses, we conclude that the MYPT1 C-terminus functions as an auto-inhibitory domain, and a central domain in MYPT1 can mediate extensive reorganization of the actin cytoskeleton.

Myosin phosphatase-targeting subunit 1 regulates mitosis by antagonizing polo-like kinase 1

Myosin phosphatase-targeting subunit 1 (MYPT1) binds to the catalytic subunit of protein phosphatase 1 (PP1C). This binding is believed to target PP1C to specific substrates including myosin II, thus controlling cellular contractility. Surprisingly, we found that during mitosis, mammalian MYPT1 binds to polo-like kinase 1 (PLK1). MYPT1 is phosphorylated during mitosis by proline-directed kinases including cdc2, which generates the binding motif for the polo box domain of PLK1. Depletion of PLK1 by small interfering RNAs is known to result in loss of gamma-tubulin recruitment to the centrosomes, blocking centrosome maturation and leading to mitotic arrest. We found that codepletion of MYPT1 and PLK1 reinstates gamma-tubulin at the centrosomes, rescuing the mitotic arrest. MYPT1 depletion increases phosphorylation of PLK1 at its activating site (Thr210) in vivo, explaining, at least in part, the rescue phenotype by codepletion. Taken together, our results identify a previously unrecognized role for MYPT1 in regulating mitosis by antagonizing PLK1.

Caldesmon and the regulation of cytoskeletal functions

Caldesmon (CaD) is an extraordinary actin-binding protein, because in addition to actin, it also bindsmyosin, calmodulin and tropomyosin. As a component of the smoothmuscle and nonmuscle contractile apparatus CaD inhibits the actomyosin ATPase activity and its inhibitory action is modulated by both Ca2+ and phosphorylation. The multiplicity of binding partners and diverse biochemical properties suggest CaD is a potent and versatile regulatory protein both in contractility and cell motility. However, after decades ofinvestigation in numerous laboratories, hard evidence is still lacking to unequivocally identify its in vivo functions, although indirect evidence is mounting to support an important role in connection with the actin cytoskeleton. This chapter reviews the highlights of the past findings and summarizes the current views on this protein, with emphasis of its interaction with tropomyosin.

Myosin phosphatase target subunit: Many roles in cell function

Phosphorylation of myosin II is important in many aspects of cell function and involves a myosin kinase, e.g. myosin light chain kinase, and a myosin phosphatase (MP). MP is regulated by the myosin phosphatase target subunit (MYPT1). The domain structure, properties, and genetic analyses of MYPT1 and its isoforms are outlined. MYPT1 binds the catalytic subunit of type 1 phosphatase, delta isoform, and also acts as an interactive platform for many other proteins. A key reaction for MP is with phosphorylated myosin II and the first process shown to be regulated by MP was contractile activity of smooth muscle. In cell division and cell migration myosin II phosphorylation also plays a critical role and these are discussed. However, based on the wide range of partners for MYPT1 it is likely that MP is implicated with substrates other than myosin II. Open questions are whether the diverse functions of MP reflect different cellular locations and/or specific roles for the MYPT1 isoforms.

Regulation of myosin light chain phosphatase and pulmonary arterial relaxation

Neonatal circulatory transition is dependent upon tightly regulated pulmonary circuit relaxation. Persistent pulmonary hypertension (PPHN), a rapidly progressive disease of pulmonary arterial vasospasm and remodelling, may be characterized by pulmonary arterial myocyte relaxation failure. A key regulator of vascular tone is myocyte calcium sensitivity, determined by the relative stoichiometry of myosin light chain phosphorylation and dephosphorylation. We have recently reported downregulation of myosin light chain phosphatase activity in a hypoxic model of neonatal pulmonary hypertension. This review examines the recognized pathways of regulation governing myosin light chain phosphatase activity, including targeting subunit isoform switching, targeting unit phosphorylation and catalytic site inhibition. In light of the reviewed literature, further speculation is proposed on the potential contributions of these mechanisms to the pathophysiology of the perinatal pulmonary arterial relaxation defect in PPHN.Key words: smooth muscle, pulmonary hypertension, myosin light chain phosphatase, CPI-17, MYPT, review.

Regulation of myosin II during cytokinesis in higher eukaryotes

Cellular myosin II is the principal motor responsible for cytokinesis. In higher eukaryotes, phosphorylation of the regulatory light chain (MLC) of myosin II is a primary means of activating myosin II and is known to be crucial for the execution of cell division. Because signals transmitted by the mitotic spindle coordinate key spatial and temporal aspects of cytokinesis, such signals should ultimately function to activate myosin II. Thus, it follows that identification of regulatory factors involved in MLC phosphorylation should elucidate the nature of spindle-derived regulatory signals and lead to a model for how they control cytokinesis. However, the identity of these upstream molecules remains elusive. This review (which is part of the Cytokinesis series) summarizes current views of the regulatory pathway controlling MLC phosphorylation and features four candidate molecules that are likely immediate upstream myosin regulators. I discuss proposed functions for MLCK, ROCK, citron kinase and myosin phosphatase during cytokinesis and consider the possibility of a link between these molecules and the signals transmitted by the mitotic spindle.

Localization of myosin phosphatase target subunit and its mutants

Transient transfection of NIH3T3 cells with various constructs of myosin phosphatase target subunit (MYPT1) and GFP showed distinct cellular localizations. Constructs containing the N-terminal nuclear localization signals (NLS), i.e. full-length MYPT1 and N-terminal MYPT1 fragments, were concentrated in the nucleus. Full-length chicken and human MYPT1-GFP showed discrete nuclear foci. Deletion of the N-terminal NLS or use of central or C-terminal MYPT1 fragments did not show unique nuclear distributions (C-terminal NLS are present). Transient transfection of NIH3T3 cells (in the presence of serum) with full-length MYPT1-GFP caused a marked decrease in number of attached cells, an apparent block in the cell cycle prior to M phase and signs of increased apoptosis. Under conditions of serum starvation the unique nuclear localization of MYPT1-GFP was not found and there was no marked decrease in the number of attached cells (after 48 h). Stable transfection of HEK 293 cells with GFP-MYPT1 was obtained. MYPT1 and its N-terminal mutants bound to retinoblastoma protein (Rb), raising the possibility that Rb is implicated in the effects caused by overexpression of MYPT1.

Signaling pathways involved in adenosine triphosphate-induced endothelial cell barrier enhancement

Endothelial barrier dysfunction caused by inflammatory agonists is a frequent underlying cause of vascular leak and edema. Novel strategies to preserve barrier integrity could have profound clinical impact. Adenosine triphosphate (ATP) released from endothelial cells by shear stress and injury has been shown to protect the endothelial barrier in some settings. We have demonstrated that ATP and its nonhydrolyzed analogues enhanced barrier properties of cultured endothelial cell monolayers and caused remodeling of cell-cell junctions. Increases in cytosolic Ca2+ and Erk activation caused by ATP were irrelevant to barrier enhancement. Experiments using biochemical inhibitors or siRNA indicated that G proteins (specifically Galphaq and Galphai2), protein kinase A (PKA), and the PKA substrate vasodilator-stimulated phosphoprotein were involved in ATP-induced barrier enhancement. ATP treatment decreased phosphorylation of myosin light chain and specifically activated myosin-associated phosphatase. Depletion of Galphaq with siRNA prevented ATP-induced activation of myosin phosphatase. We conclude that the mechanisms of ATP-induced barrier enhancement are independent of intracellular Ca2+, but involve activation of myosin phosphatase via a novel G-protein-coupled mechanism and PKA.

Okadaic acid induces phosphorylation and translocation of myosin phosphatase target subunit 1 influencing myosin phosphorylation, stress fiber assembly and cell migration in HepG2 cells

It was determined that the myosin phosphatase (MP) activity and content of myosin phosphatase target subunit 1 (MYPT1) were correlated in subcellular fractions of human hepatocarcinoma (HepG2) cells. In control cells MYPT1 was localized in the cytoplasm and in the nucleus, as determined by confocal microscopy. Treatment of HepG2 cells with 50 nM okadaic acid (OA), a cell-permeable phosphatase inhibitor, induced several changes: 1) a marked redistribution of MYPT1 to the plasma membrane associated with an increased level of phosphorylation of MYPT1 at Thr695. Both effects showed only a slight influence with the Rho-kinase inhibitor, Y-27632; 2) an increase in phosphorylation of MYPT1 at Thr850 associated with its accumulation in the perinuclear region and nucleus. These effects were markedly reduced by Y-27632; 3) an increased phosphorylation of the 20 kDa myosin II light chain at Ser19 associated with an increased location of myosin II at the cell center. These effects were partially counteracted by Y-27632; 4) an increase in stress fiber formation and a decrease in cell migration, both OA-induced effects were blocked by Y-27632. In HepG2 lysates, OA (5-100 nM) did not affect MP activity but inhibited PP2A activity. These results indicate that OA induces differential phosphorylation and translocation of MYPT1, dependent on PP2A and, to varying extents, on ROK. These changes are associated with an increased level of myosin II phosphorylation and attenuation of hepatic cell migration.

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.

Distinct roles of MLCK and ROCK in the regulation of membrane protrusions and focal adhesion dynamics during cell migration of fibroblasts

We examined the role of regulatory myosin light chain (MLC) phosphorylation of myosin II in cell migration of fibroblasts. Myosin light chain kinase (MLCK) inhibition blocked MLC phosphorylation at the cell periphery, but not in the center. MLCK-inhibited cells did not assemble zyxin-containing adhesions at the periphery, but maintained focal adhesions in the center. They generated membrane protrusions all around the cell, turned more frequently, and migrated less effectively. In contrast, Rho-associated kinase (ROCK) inhibition blocked MLC phosphorylation in the center, but not at the periphery. ROCK-inhibited cells assembled zyxin-containing adhesions at the periphery, but not focal adhesions in the center. They moved faster and more straight. On the other hand, inhibition of myosin phosphatase increased MLC phosphorylation and blocked peripheral membrane ruffling, as well as turnover of focal adhesions and cell migration. Our results suggest that myosin II activated by MLCK at the cell periphery controls membrane ruffling, and that the spatial regulation of MLC phosphorylation plays critical roles in controlling cell migration of fibroblasts.

Comparative Analysis of Cytokinesis in Budding Yeast, Fission Yeast and Animal Cells

Cytokinesis is a temporally and spatially regulated process through which the cellular constituents of the mother cell are partitioned into two daughter cells, permitting an increase in cell number. When cytokinesis occurs in a polarized cell it can create daughters with distinct fates. In eukaryotes, cytokinesis is carried out by the coordinated action of a cortical actomyosin contractile ring and targeted membrane deposition. Recent use of model organisms with facile genetics and improved light-microscopy methods has led to the identification and functional characterization of many proteins involved in cytokinesis. To date, this analysis indicates that some of the basic components involved in cytokinesis are conserved from yeast to humans, although their organization into functional machinery that drives cytokinesis and the associated regulatory mechanisms bear species-specific features. Here, we briefly review the current status of knowledge of cytokinesis in the budding yeast Saccharomyces cerevisiae, the fission yeast Schizosaccharomyces pombe and animal cells, in an attempt to highlight both the common and the unique features. Although these organisms diverged from a common ancestor about a billion years ago, there are eukaryotes that are far more divergent. To evaluate the overall evolutionary conservation of cytokinesis, it will be necessary to include representatives of these divergent branches. Nevertheless, the three species discussed here provide substantial mechanistic diversity.

Functional diversity of protein phosphatase-1, a cellular economizer and reset button

The protein serine/threonine phosphatase protein phosphatase-1 (PP1) is a ubiquitous eukaryotic enzyme that regulates a variety of cellular processes through the dephosphorylation of dozens of substrates. This multifunctionality of PP1 relies on its association with a host of function-specific targetting and substrate-specifying proteins. In this review we discuss how PP1 affects the biochemistry and physiology of eukaryotic cells. The picture of PP1 that emerges from this analysis is that of a "green" enzyme that promotes the rational use of energy, the recycling of protein factors, and a reversal of the cell to a basal and/or energy-conserving state. Thus PP1 promotes a shift to the more energy-efficient fuels when nutrients are abundant and stimulates the storage of energy in the form of glycogen. PP1 also enables the relaxation of actomyosin fibers, the return to basal patterns of protein synthesis, and the recycling of transcription and splicing factors. In addition, PP1 plays a key role in the recovery from stress but promotes apoptosis when cells are damaged beyond repair. Furthermore, PP1 downregulates ion pumps and transporters in various tissues and ion channels that are involved in the excitation of neurons. Finally, PP1 promotes the exit from mitosis and maintains cells in the G1 or G2 phases of the cell cycle.

Regulation of myosin phosphorylation and myofilament Ca2+ sensitivity in vascular smooth muscle

The Ca2+-dependent, reversible phosphorylation of the 20 kDa regulatory myosin light chain (MLC) plays a primary role in regulating the contraction of smooth muscle. However, it is well known that the Ca2+ signal is not the only factor which regulates such contraction, however, the alteration of the Ca2+ sensitivity in the contractile apparatus is also known to play an important role. The degree of MLC phosphorylation is determined by the balance of the activity between phosphorylation and dephosphorylation. Either the Ca2+-independent activation of MLC phosphorylation or the inhibition of MLC dephosphorylation causes a greater MLC phosphorylation for a given level of Ca2+ signal and thereby potentiates the myofilament Ca2+ sensitivity. The smooth muscle myosin light chain phosphatase (MLCP) consisting of three subunits was first isolated and cloned in the early '90s. The intensive investigation thereafter has uncovered the biochemical basis for regulating the activity of MLCP. The regulation of the MLCP activity is now considered to play a critical role in regulating the myofilament Ca2+ sensitivity. There are three major mechanisms in the regulation of MLCP; (1) the phosphorylation of a 110 kDa regulatory subunit of MLCP (2) the conformational change of the trimeric structure, and (3) the inhibition by a smooth muscle specific inhibitor protein, CPI-17. Furthermore, some kinases have been found to phosphorylate the MLC and activate the contraction of smooth muscle in a Ca2+-independent manner. Numerous protein kinases have been found to be involved in the regulation of MLC phosphorylation, and rho-kinase is one of the most frequently investigated kinases. The smooth muscle physiology is now asked to integrate the current understanding of the biochemical mechanisms and to clarify which kinases and/or proteins in the contractile apparatus play a physiological role in regulating the myofilament Ca2+ sensitivity and how such extracellular contractile stimulation modulates these mechanisms.

Ca2+ sensitivity of smooth muscle and nonmuscle myosin II: modulated by G proteins, kinases, and myosin phosphatase

Ca2+ sensitivity of smooth muscle and nonmuscle myosin II reflects the ratio of activities of myosin light-chain kinase (MLCK) to myosin light-chain phosphatase (MLCP) and is a major, regulated determinant of numerous cellular processes. We conclude that the majority of phenotypes attributed to the monomeric G protein RhoA and mediated by its effector, Rho-kinase (ROK), reflect Ca2+ sensitization: inhibition of myosin II dephosphorylation in the presence of basal (Ca2+ dependent or independent) or increased MLCK activity. We outline the pathway from receptors through trimeric G proteins (Galphaq, Galpha12, Galpha13) to activation, by guanine nucleotide exchange factors (GEFs), from GDP. RhoA. GDI to GTP. RhoA and hence to ROK through a mechanism involving association of GEF, RhoA, and ROK in multimolecular complexes at the lipid cell membrane. Specific domains of GEFs interact with trimeric G proteins, and some GEFs are activated by Tyr kinases whose inhibition can inhibit Rho signaling. Inhibition of MLCP, directly by ROK or by phosphorylation of the phosphatase inhibitor CPI-17, increases phosphorylation of the myosin II regulatory light chain and thus the activity of smooth muscle and nonmuscle actomyosin ATPase and motility. We summarize relevant effects of p21-activated kinase, LIM-kinase, and focal adhesion kinase. Mechanisms of Ca2+ desensitization are outlined with emphasis on the antagonism between cGMP-activated kinase and the RhoA/ROK pathway. We suggest that the RhoA/ROK pathway is constitutively active in a number of organs under physiological conditions; its aberrations play major roles in several disease states, particularly impacting on Ca2+ sensitization of smooth muscle in hypertension and possibly asthma and on cancer neoangiogenesis and cancer progression. It is a potentially important therapeutic target and a subject for translational research.

Dimerization of cGMP-dependent protein kinase 1alpha and the myosin-binding subunit of myosin phosphatase: role of leucine zipper domains

Nitric oxide (NO) and nitrovasodilators induce vascular smooth muscle cell relaxation in part by cGMP-dependent protein kinase (cGK)-mediated activation of myosin phosphatase, which dephosphorylates myosin light chains. We recently found that cGMP-dependent protein kinase 1alpha binds directly to the myosin-binding subunit (MBS) of myosin phosphatase via the leucine/isoleucine zipper of cGK. We have now studied the role of the leucine zipper domain of MBS in dimerization with cGK and the leucine/isoleucine zipper and leucine zipper domains of both proteins in homodimerization. Mutagenesis of the MBS leucine zipper domain disrupts cGKIalpha-MBS dimerization. Mutagenesis of the MBS leucine zipper eliminates MBS homodimerization, while similar disruption of the cGKIalpha leucine/isoleucine zipper does not prevent formation of cGK dimers. The MBS leucine zipper domain is phosphorylated by cGK, but this does not have any apparent effect on heterodimer formation between the two proteins. MBS LZ mutants that are unable to bind cGK were poor substrates for cGK. These data support the theory that the MBS leucine zipper domain is necessary and sufficient to mediate both MBS homodimerization and binding of the protein to cGK. In contrast, the leucine/isoleucine zipper of cGK is required for binding to MBS, but not for cGK homodimerization. These data support that the MBS and cGK leucine zipper domains mediate the interaction between these two proteins. The contribution of these domains to both homodimerization and their specific interaction with each other suggest that additional regulatory mechanisms involving these domains may exist.

Cortical recruitment of nonmuscle myosin II in early syncytial Drosophila embryos: its role in nuclear axial expansion and its regulation by Cdc2 activity

The nuclei of early syncytial Drosophila embryos migrate dramatically toward the poles. The cellular mechanisms driving this process, called axial expansion, are unclear, but myosin II activity is required. By following regulatory myosin light chain (RLC)-green fluorescent protein dynamics in living embryos, we observed cycles of myosin recruitment to the cortex synchronized with mitotic cycles. Cortical myosin is first seen in a patch at the anterocentral part of the embryo at cycle 4. With each succeeding cycle, the patch expands poleward, dispersing at the beginning of each mitosis and reassembling at the end of telophase. Each cycle of actin and myosin recruitment is accompanied by a cortical contraction. The cortical myosin cycle does not require microtubules but correlates inversely with Cdc2/cyclinB (mitosis-promoting factor) activity. A mutant RLC lacking inhibitory phosphorylation sites was fully functional with no effect on the cortical myosin cycle, indicating that Cdc2 must be modulating myosin activity by some other mechanism. An inhibitor of Rho kinase blocks the cortical myosin recruitment cycles and provokes a concomitant failure of axial expansion. These studies suggest a model in which cycles of myosin-mediated contraction and relaxation, tightly linked to Cdc2 and Rho kinase activity, are directly responsible for the axial expansion of the syncytial nuclei.

Integrin-linked kinase phosphorylates the myosin phosphatase target subunit at the inhibitory site in platelet cytoskeleton

The myosin phosphatase (MP) composed of the catalytic subunit of type 1 protein phosphatase and myosin phosphatase target subunit isoform 1 (MYPT1) was identified as the major serine/threonine phosphatase component in the platelet-cytoskeleton fraction. MYPT1 was phosphorylated by cytoskeletal kinase(s), but the identity of the kinase(s) and the effect of phosphorylation were not established. Incubation of platelet-cytoskeletal fraction with MgATP or MgATP[S] (magnesium adenosine 5'-[gamma-thio]triphosphate) caused a decrease in the 20 kDa light-chain of smooth-muscle myosin (MLC20) phosphatase and phosphorylase phosphatase activities. MYPT1 contains a phosphorylation site, Thr-695, involved in the inhibition of MP in a RhoA/Rho kinase-dependent manner. The cytoskeletal kinase(s) phosphorylated Thr-695 of glutathione S-transferase (GST)-MYPT1, as determined with an antibody specific for phosphorylated Thr-695. The level of Rho kinase was low in the cytoskeletal fraction and was detected primarily in the membrane and cytosolic fractions. The phosphorylation of Thr-695 by the cytoskeletal kinase(s) was not affected by Rho kinase inhibitor, Y-27632, suggesting that kinase(s) other than Rho kinase were involved. In-gel kinase assay identified a kinase at 54-59 kDa that phosphorylated the C-terminal fragment of MYPT1 (GST-MYPT1(667-1004)). Western blots detected both zipper-interacting protein kinase (ZIPK) and integrin-linked kinase (ILK) at 54-59 kDa in the cytoskeleton and membrane fractions. Cytoskeletal ZIPK and ILK were separated and partially purified by chromatography on SP-Sepharose and on MonoQ. ZIPK preferentially phosphorylated MLC20 and had low activity on MYPT1. ILK phosphorylated both MLC20 and MYPT1 and phosphorylation of MYPT1 occured on Thr-695. The above results raise the potential for regulation of MP activity in platelet cytoskeleton by ILK and suggest an alternative to the Rho-linked pathway.

Rho-binding kinase (LET-502) and myosin phosphatase (MEL-11) regulate cytokinesis in the earlyCaenorhabditis elegansembryo

Rho-binding kinase and myosin phosphatase regulate the contraction of actomyosin filaments in non-muscle and smooth muscle cells. Previously, we described the role of C. elegans genes encoding Rho-binding kinase(let-502) and myosin phosphatase targeting subunit (mel-11)in epidermal cell-shape changes that drive morphogenesis and in spermathecal contraction. Here we analyze their roles in a third contractile event,cytokinesis within early embryos. We demonstrate that these genes function together to regulate the rate of cleavage furrow contraction, with Rho-binding kinase/LET-502 mediating contraction, whereas myosin phosphatase/MEL-11 acts as a brake to contraction: early embryonic cleavage often fails or is slowed when let-502 is mutated, whereas mel-11 mutations result in ectopic furrowing and faster furrow ingression. These phenotypes correspond to changes in the levels of phosphorylated regulatory non-muscle myosin light chain (rMLC). The gene products of let-502 and mel-11colocalize at cleavage furrows, and their mutations alleviate one another's defects. rMLC is phosphorylated in let-502; mel-11 double mutants,indicating that a kinase is able to phosphorylate rMLC in the absence of both LET-502 and MEL-11. Genetic and molecular epistasis experiments place LET-502 and MEL-11 in a cytokinetic pathway. LET-502 and MEL-11 regulate the activity of non-muscle myosin after actin, non-muscle myosin heavy chain/NMY-2,regulatory non-muscle myosin light chain/MLC-4 and early formin/CYK-1 have formed a contractile ring. Proteins including Rho GTPase activating protein/CYK-4 and late CYK-1, which are required for late stages of cytokinesis, function downstream of LET-502 and MEL-11.

Phosphorylation of the myosin-binding subunit of myosin phosphatase by Raf-1 and inhibition of phosphatase activity

Raf-1 serine/threonine protein kinase plays an important role in cell survival, proliferation, and migration; however, the specific targets of Raf-1 in diverse cellular processes are not clearly defined. Myosin phosphatase activity is critical to the regulation of cytoskeletal reorganization, cytokinesis, and cell motility. Here, we describe the association of Raf-1 with myosin phosphatase and phosphorylation of the regulatory myosin-binding subunit (MBS) of myosin phosphatase by Raf-1. Treatment of cells with phorbol 12-myristate 13-acetate has been shown to stimulate Raf-1 protein kinase. To determine the effect of enzymatic activation of Raf-1 on MBS phosphorylation, COS-1 cells were transiently transfected with FLAG-tagged full-length Raf-1. A significantly higher phosphorylation of purified glutathione S-transferase-tagged truncated MBS protein (amino acids 654-880) occurred in the presence of FLAG-Raf-1 immunoprecipitated from phorbol 12-myristate 13-acetate-treated cells compared with untreated cells ( approximately 3.0-fold). Using a sequential kinase-phosphatase assay and phosphorylated myosin light chain as substrate in the phosphatase reaction, we showed that Raf-1-associated protein phosphatase-specific activity was inhibited (relative phosphatase activity without and with adenosine 5'-O-(3-thiotriphosphate): 100 and approximately 30%, respectively). Previously, ionizing radiation has been shown to activate Raf-1 (Kasid, U., Suy, S., Dent, P., Ray, S., Whiteside, T. L., and Sturgill, T. W. (1996) Nature 382, 813-816). Exposure of cells to ionizing radiation resulted in the increased association of Raf-1 with MBS (3-6-fold versus unirradiated control) and inhibition of Raf-1-associated protein phosphatase-specific activity (relative phosphatase activity without and with ionizing radiation: 100 and approximately 54%, respectively). Our studies identify MBS as a new substrate of Raf-1 and implicate a role for Raf-1 in the regulation of pathways involving myosin phosphatase activity.

Protein phosphatase 1 – targeted in many directions

Protein phosphatase 1 (PP1) is a major eukaryotic protein serine/threonine phosphatase that regulates an enormous variety of cellular functions through the interaction of its catalytic subunit (PP1c) with over fifty different established or putative regulatory subunits. Most of these target PP1c to specific subcellular locations and interact with a small hydrophobic groove on the surface of PP1c through a short conserved binding motif – the RVxF motif – which is often preceded by further basic residues. Weaker interactions may subsequently enhance binding and modulate PP1 activity/specificity in a variety of ways. Several putative targeting subunits do not possess an RVxF motif but nevertheless interact with the same region of PP1c. In addition, several ‘modulator’ proteins bind to PP1c but do not possess a domain targeting them to a specific location. Most are potent inhibitors of PP1c and possess at least two sites for interaction with PP1c, one of which is identical or similar to the RVxF motif.Regulation of PP1c in response to extracellular and intracellular signals occurs mostly through changes in the levels, conformation or phosphorylation status of targeting subunits. Understanding of the mode of action of PP1c complexes may facilitate development of drugs that target particular PP1c complexes and thereby modulate the phosphorylation state of a very limited subset of proteins.

Role of myosin light chain phosphorylation in the regulation of cytokinesis

Phosphorylation of regulatory light chain (RMLC) of myosin II at Ser19/Thr18 is likely to play important roles in controlling the morphological changes seen during cell division of cultured mammalian cells. Phosphorylation of RMLC regulates the activity of myosin II, an essntial motor for cytokinesis, and phosphorylation of RMLC shows dramatic changes during mitosis. Two exzymes, myosin phosphatase and kinase, control phosphorvlation of RMLC. Myosin phosphatase is activated during mitosis, apparently as a result of mitosis-specific phosphorylation of the myosin phosphatase targeting subunit (MYPT). This activation of myosin phosphatase is likely to result in RMLC dephosphorylation, causing the disassemly of stress fibers and focal adhesions during prophase. The phosphorylation of MYPT is lost in cyotokinesis, which would decrease myosin phosphatase activity. At the same time, ROCK (Rho-kinase) probably phosphorylates MYPT at its inhibitory sites, further decreasing the activity of myosin phosphatase. These changes in MYPT phosphorylation would raise RMLC phosphorylation, leading to the activation of myosin II for cyotokinesis. RMLC phosphorylation is also regulated by several RMLC kinases including ROCK (Rho-kinase), MLCK and citron kinase, all of which are localized at cleavage furrows. Future studies should examine whether these multiple kinases are redundant or whether they control distinct aspects of cell division.

Combinatorial control of protein phosphatase-1

The catalytic subunit of the type 1 Ser/Thr protein phosphatases (PP1) can interact with many different regulatory (R) subunits. These R subunits function as activity-modulators, targeting subunits and/or substrates. The specificity of the R subunits can be accounted for by their interaction with specific subsets of binding pockets on the catalytic subunit and by the presence of subcellular targeting sequences. Hormones, growth factors and metabolites control the function of PP1 holoenzymes mainly by modulating the interaction of the subunits.

Phosphorylation of a novel myosin binding subunit of protein phosphatase 1 reveals a conserved mechanism in the regulation of actin cytoskeleton

The myotonic dystrophy kinase-related kinases RhoA binding kinase and myotonic dystrophy kinase-related Cdc42 binding kinase (MRCK) are effectors of RhoA and Cdc42, respectively, for actin reorganization. Using substrate screening in various tissues, we uncovered two major substrates, p130 and p85, for MRCKalpha-kinase. p130 is identified as myosin binding subunit p130, whereas p85 is a novel related protein. p85 contains N-terminal ankyrin repeats, an alpha-helical C terminus with leucine repeats, and a centrally located conserved motif with the MRCKalpha-kinase phosphorylation site. Like MBS130, p85 is specifically associated with protein phosphatase 1delta (PP1delta), and this requires the N terminus, including the ankyrin repeats. This association is required for the regulation of both the catalytic activities and the assembly of actin cytoskeleton. The N terminus, in association with PP1delta, is essential for actin depolymerization, whereas the C terminus antagonizes this action. The C-terminal effects consist of two independent events that involved both the conserved phosphorylation inhibitory motif and the alpha-helical leucine repeats. The former was able to interact with PP1delta only in the phosphorylated state and result in inactivation of PP1delta activity. This provides further evidence that phosphorylation of a myosin binding subunit protein by specific kinases confers conformational changes in a highly conserved region that plays an essential role in the regulation of its catalytic subunit activities.

Serine phosphorylation of focal adhesion kinase in interphase and mitosis: a possible role in modulating binding to p130(Cas)

Focal adhesion kinase (FAK) is an important regulator of integrin signaling in adherent cells and accordingly its activity is significantly modulated during mitosis when cells detach from the extracellular matrix. During mitosis, FAK becomes heavily phosphorylated on serine residues concomitant with its inactivation and dephosphorylation on tyrosine. Little is known about the regulation of FAK activity by serine phosphorylation. In this report, we characterize two novel sites of serine phosphorylation within the C-terminal domain of FAK. Phosphorylation-specific antibodies directed to these sites and against two previously characterized sites of serine phosphorylation were used to study the regulated phosphorylation of FAK in unsynchronized and mitotic cells. Among the four major phosphorylation sites, designated pS1-pS4, phosphorylation of pS1 (Ser722) is unchanged in unsynchronized and mitotic cells. In contrast, pS3 and pS4 (Ser843 and Ser910) exhibit increased phosphorylation during mitosis. In vitro peptide binding experiments provide evidence that phosphorylation of pS1 (Ser722) may play a role in modulating FAK binding to the SH3 domain of the adapter protein p130(Cas).

Phosphorylation of MYPT1 by protein kinase C attenuates interaction with PP1 catalytic subunit and the 20 kDa light chain of myosin

The effect of phosphorylation in the N-terminal region of myosin phosphatase target subunit 1 (MYPT1) on the interactions with protein phosphatase 1 catalytic subunit (PP1c) and with phosphorylated 20 kDa myosin light chain (P-MLC20) was studied. Protein kinase C (PKC) phosphorylated threonine-34 (1 mol/mol), the residue preceding the consensus PP1c-binding motif ((35)KVKF(38)) in MYPT1(1-38), but this did not affect binding of the peptide to PP1c. PKC incorporated 2 mol P(i) into MYPT1(1-296) suggesting a second site of phosphorylation within the ankyrin repeats (residues 40-296). This phosphorylation diminished the stimulatory effect of MYPT1(1-296) on the P-MLC20 phosphatase activity of PP1c. Binding of PP1c or P-MLC20 to phosphorylated MYPT1(1-296) was also attenuated. It is concluded that phosphorylation of MYPT1 by PKC may therefore result in altered dephosphorylation of myosin.

Localization and activity of myosin light chain kinase isoforms during the cell cycle

Phosphorylation on Ser 19 of the myosin II regulatory light chain by myosin light chain kinase (MLCK) regulates actomyosin contractility in smooth muscle and vertebrate nonmuscle cells. The smooth/nonmuscle MLCK gene locus produces two kinases, a high molecular weight isoform (long MLCK) and a low molecular weight isoform (short MLCK), that are differentially expressed in smooth and nonmuscle tissues. To study the relative localization of the MLCK isoforms in cultured nonmuscle cells and to determine the spatial and temporal dynamics of MLCK localization during mitosis, we constructed green fluorescent protein fusions of the long and short MLCKs. In interphase cells, localization of the long MLCK to stress fibers is mediated by five DXRXXL motifs, which span the junction of the NH(2)-terminal extension and the short MLCK. In contrast, localization of the long MLCK to the cleavage furrow in dividing cells requires the five DXRXXL motifs as well as additional amino acid sequences present in the NH(2)-terminal extension. Thus, it appears that nonmuscle cells utilize different mechanisms for targeting the long MLCK to actomyosin structures during interphase and mitosis. Further studies have shown that the long MLCK has twofold lower kinase activity in early mitosis than in interphase or in the early stages of postmitotic spreading. These findings suggest a model in which MLCK and the myosin II phosphatase (Totsukawa, G., Y. Yamakita, S. Yamashiro, H. Hosoya, D.J. Hartshorne, and F. Matsumura. 1999. J. Cell Biol. 144:735-744) act cooperatively to regulate the level of Ser 19-phosphorylated myosin II during mitosis and initiate cytokinesis through the activation of myosin II motor activity.

Distinct roles of ROCK (Rho-kinase) and MLCK in spatial regulation of MLC phosphorylation for assembly of stress fibers and focal adhesions in 3T3 fibroblasts

ROCK (Rho-kinase), an effector molecule of RhoA, phosphorylates the myosin binding subunit (MBS) of myosin phosphatase and inhibits the phosphatase activity. This inhibition increases phosphorylation of myosin light chain (MLC) of myosin II, which is suggested to induce RhoA-mediated assembly of stress fibers and focal adhesions. ROCK is also known to directly phosphorylate MLC in vitro; however, the physiological significance of this MLC kinase activity is unknown. It is also not clear whether MLC phosphorylation alone is sufficient for the assembly of stress fibers and focal adhesions. We have developed two reagents with opposing effects on myosin phosphatase. One is an antibody against MBS that is able to inhibit myosin phosphatase activity. The other is a truncation mutant of MBS that constitutively activates myosin phosphatase. Through microinjection of these two reagents followed by immunofluorescence with a specific antibody against phosphorylated MLC, we have found that MLC phosphorylation is both necessary and sufficient for the assembly of stress fibers and focal adhesions in 3T3 fibroblasts. The assembly of stress fibers in the center of cells requires ROCK activity in addition to the inhibition of myosin phosphatase, suggesting that ROCK not only inhibits myosin phosphatase but also phosphorylates MLC directly in the center of cells. At the cell periphery, on the other hand, MLCK but not ROCK appears to be the kinase responsible for phosphorylating MLC. These results suggest that ROCK and MLCK play distinct roles in spatial regulation of MLC phosphorylation.