Phosphorylation-dependent binding of a synthetic MARCKS peptide to calmodulin

The myristoylated alanine-rich C-kinase substrates (MARCKS): A membrane-anchored mediator of the cell function

The myristoylated alanine-rich C-kinase substrate (MARCKS) and the MARCKS-related protein (MARCKSL1) are ubiquitous, highly conserved membrane-associated proteins involved in the structural modulation of the actin cytoskeleton, chemotaxis, motility, cell adhesion, phagocytosis, and exocytosis. MARCKS includes an N-terminal myristoylated domain for membrane binding, a highly conserved MARCKS Homology 2 (MH2) domain, and an effector domain (which is the phosphorylation site). MARCKS can sequester phosphatidylinositol-4, 5-diphosphate (PIP2) at lipid rafts in the plasma membrane of quiescent cells, an action reversed by protein kinase C (PKC), ultimately modulating the immune function. Being expressed mostly in innate immune cells, MARCKS promotes the inflammation-driven migration and adhesion of cells and the secretion of cytokines such as tumor necrosis factor (TNF). From a clinical point of view, MARCKS is overexpressed in patients with schizophrenia and bipolar disorders, while the brain level of MARCKS phosphorylation is associated with Alzheimer's disease. Furthermore, MARCKS is associated with the development and progression of numerous types of cancers. Data in autoimmune diseases are limited to rheumatoid arthritis models in which a connection between MARCKS and the JAK-STAT pathway is mediated by miRNAs. We provide a comprehensive overview of the structure of MARCKS, its molecular characteristics and functions from a biological and pathogenetic standpoint, and will discuss the clinical implications of this pathway.

Pathophysiological roles of myristoylated alanine-rich C-kinase substrate (MARCKS) in hematological malignancies

The myristoylated alanine-rich C-kinase substrate (MARCKS) protein has been at the crossroads of multiple signaling pathways that govern several critical operations in normal and malignant cellular physiology. Functioning as a target of protein kinase C, MARCKS shuttles between the phosphorylated cytosolic form and the unphosphorylated plasma membrane-bound states whilst regulating several molecular partners including, but not limited to calmodulin, actin, phosphatidylinositol-4,5-bisphosphate, and phosphoinositide-3-kinase. As a result of these interactions, MARCKS directly or indirectly modulates a host of cellular functions, primarily including cytoskeletal reorganization, membrane trafficking, cell secretion, inflammatory response, cell migration, and mitosis. Recent evidence indicates that dysregulated expression of MARCKS is associated with the development and progression of hematological cancers. While it is understood that MARCKS impacts the overall carcinogenesis as well as plays a part in determining the disease outcome in blood cancers, we are still at an early stage of interpreting the pathophysiological roles of MARCKS in neoplastic disease. The situation is further complicated by contradictory reports regarding the role of phosphorylated versus an unphosphorylated form of MARCKS as an oncogene versus tumor suppressor in blood cancers. In this review, we will investigate the current body of knowledge and evolving concepts of the physical properties, molecular network, functional attributes, and the likely pathogenic roles of MARCKS in hematological malignancies. Key emphasis will also be laid upon understanding the novel mechanisms by which MARCKS determines the overall disease prognosis by playing a vital role in the induction of therapeutic resistance. Additionally, we will highlight the importance of MARCKS as a valuable therapeutic target in blood cancers and will discuss the potential of existing strategies available to tackle MARCKS-driven blood cancers.

MARCKS and Lung Disease

MARCKS (myristoylated alanine-rich C kinase substrate) is a prominent PKC substrate expressed in all eukaryotic cells. It is known to bind to and cross-link actin filaments, to serve as a bridge between Ca²⁺/calmodulin and PKC signaling, and to sequester the signaling molecule phosphatidylinositol 4,5-bisphosphate in the plasma membrane. Since the mid-1980s, this evolutionarily conserved and ubiquitously expressed protein has been associated with regulating cellular events that require dynamic actin reorganization, including cellular adhesion, migration, and exocytosis. More recently, translational studies have implicated MARCKS in the pathophysiology of a number of airway diseases, including chronic obstructive pulmonary disease, asthma, lung cancer, and acute lung injury/acute respiratory distress syndrome. This article summarizes the structure and cellular function of MARCKS (also including MARCKS family proteins and MARCKSL1 [MARCKS-like protein 1]). Evidence for MARCKS's role in several lung diseases is discussed, as are the technological innovations that took MARCKS-targeting strategies from theoretical to therapeutic. Descriptions and updates derived from ongoing clinical trials that are investigating inhalation of a MARCKS-targeting peptide as therapy for patients with chronic bronchitis, lung cancer, and ARDS are provided.

Marcksb plays a key role in the secretory pathway of zebrafish Bmp2b

During vertebrate early embryogenesis, the ventral development is directed by the ventral-to-dorsal activity gradient of the bone morphogenetic protein (BMP) signaling. As secreted ligands, the extracellular traffic of BMP has been extensively studied. However, it remains poorly understood that how BMP ligands are secreted from BMP-producing cells. In this work, we show the dominant role of Marcksb controlling the secretory process of Bmp2b via interaction with Hsp70 in vivo. We firstly carefully characterized the role of Marcksb in promoting BMP signaling during dorsoventral axis formation through knockdown approach. We then showed that Marcksb cell autonomously regulates the trafficking of Bmp2b from producing cell to the extracellular space and both the total and the extracellular Bmp2b was decreased in Marcksb-deficient embryos. However, neither the zygotic mutant of marcksb (Zmarcksb) nor the maternal zygotic mutant of marcksb (MZmarcksb) showed any defects of dorsalization. In contrast, the MZmarcksb embryos even showed increased BMP signaling activity as measured by expression of BMP targets, phosphorylated Smad1/5/9 levels and imaging of Bmp2b, suggesting that a phenomenon of “genetic over-compensation” arose. Finally, we revealed that the over-compensation effects of BMP signaling in MZmarcksb was achieved through a sequential up-regulation of MARCKS-family members Marcksa, Marcksl1a and Marcksl1b, and MARCKS-interacting protein Hsp70.3. We concluded that the Marcksb modulates BMP signaling through regulating the secretory pathway of Bmp2b.

Bone morphogenetic proteins (BMPs) are extracellular proteins which belong to the transforming growth factor-β (TGF-β) superfamily. BMP signaling is essential for embryonic development, organogenesis, and tissue regeneration and homeostasis, and tightly linked to various diseases and tumorigenesis. However, as secreted proteins, how BMPs are transported and secreted from BMP-producing cells remains poorly understood. In this study, we showed that Marcksb interacts with a molecular chaperon–Hsp70.3 to mediate the secretory pathway of BMP ligands during early development of zebrafish. Moreover, we discovered a novel phenomenon of “genetic over-compensation” in the genetic knock-out mutants of marcksb. To our knowledge, this is the first report that reveals the molecules and their related trafficking system mediating the secretion of BMPs. Considering the wide distribution of BMP and MARCKS within the human body, our work may shed light on the studies of BMPs secretion in organogenesis and adult tissue homeostasis. The finding of MARCKS in controlling BMP secretion may provide potential therapeutic targets for modulating the activity of BMP signaling and thus will be of interest to clinical research.

Effects of Spin-Labels on Membrane Burial Depth of MARCKS-ED Residues

Site-directed spin-labeling electron paramagnetic resonance spectroscopy is a useful tool to obtain information about the environment of specific residues. One of its applications is to investigate membrane protein topology based on the accessibility of the spin label, with the assumption that the position of the spin label in the membrane is close to that of the native residue. This assumption is valid in proteins with well-ordered structures, but could be problematic in small peptides because the labeling may cause a perturbation that is large enough to change local interactions between the peptide and the membrane. To quantitatively characterize such effects, we have simulated the association of a 25-amino-acid peptide, MARCKS-ED, to membranes with and without spin labels. Our simulations show that the depths of spin labels are ∼6-17 Å deeper than the unlabeled charged and polar residues in the wild-type. When the hydrophobic residue Phe is labeled, however, the spin-label depth is close to that of the native residue as well as the experimental value. Our study suggests that one should be cautious in interpretation of spin label data when charged and polar residues in small peptides are labeled.

CHARMM-GUI HMMM Builder for Membrane Simulations with the Highly Mobile Membrane-Mimetic Model

Slow diffusion of the lipids in conventional all-atom simulations of membrane systems makes it difficult to sample large rearrangements of lipids and protein-lipid interactions. Recently, Tajkhorshid and co-workers developed the highly mobile membrane-mimetic (HMMM) model with accelerated lipid motion by replacing the lipid tails with small organic molecules. The HMMM model provides accelerated lipid diffusion by one to two orders of magnitude, and is particularly useful in studying membrane-protein associations. However, building an HMMM simulation system is not easy, as it requires sophisticated treatment of the lipid tails. In this study, we have developed CHARMM-GUI HMMM Builder (http://www.charmm-gui.org/input/hmmm) to provide users with ready-to-go input files for simulating HMMM membrane systems with/without proteins. Various lipid-only and protein-lipid systems are simulated to validate the qualities of the systems generated by HMMM Builder with focus on the basic properties and advantages of the HMMM model. HMMM Builder supports all lipid types available in CHARMM-GUI and also provides a module to convert back and forth between an HMMM membrane and a full-length membrane. We expect HMMM Builder to be a useful tool in studying membrane systems with enhanced lipid diffusion.

Structural basis for the association of MAP6 protein with microtubules and its regulation by calmodulin

Microtubules are highly dynamic αβ-tubulin polymers. In vitro and in living cells, microtubules are most often cold- and nocodazole-sensitive. When present, the MAP6/STOP family of proteins protects microtubules from cold- and nocodazole-induced depolymerization but the molecular and structure determinants by which these proteins stabilize microtubules remain under debate. We show here that a short protein fragment from MAP6-N, which encompasses its Mn1 and Mn2 modules (MAP6(90-177)), recapitulates the function of the full-length MAP6-N protein toward microtubules, i.e. its ability to stabilize microtubules in vitro and in cultured cells in ice-cold conditions or in the presence of nocodazole. We further show for the first time, using biochemical assays and NMR spectroscopy, that these effects result from the binding of MAP6(90-177) to microtubules with a 1:1 MAP6(90-177):tubulin heterodimer stoichiometry. NMR data demonstrate that the binding of MAP6(90-177) to microtubules involve its two Mn modules but that a single one is also able to interact with microtubules in a closely similar manner. This suggests that the Mn modules represent each a full microtubule binding domain and that MAP6 proteins may stabilize microtubules by bridging tubulin heterodimers from adjacent protofilaments or within a protofilament. Finally, we demonstrate that Ca(2+)-calmodulin competes with microtubules for MAP6(90-177) binding and that the binding mode of MAP6(90-177) to microtubules and Ca(2+)-calmodulin involves a common stretch of amino acid residues on the MAP6(90-177) side. This result accounts for the regulation of microtubule stability in cold condition by Ca(2+)-calmodulin.

Characterization of the structure and intermolecular interactions between the connexin 32 carboxyl-terminal domain and the protein partners synapse-associated protein 97 and calmodulin

In Schwann cells, connexin 32 (Cx32) can oligomerize to form intracellular gap junction channels facilitating a shorter pathway for metabolite diffusion across the layers of the myelin sheath. The mechanisms of Cx32 intracellular channel regulation have not been clearly defined. However, Ca(2+), pH, and the phosphorylation state can regulate Cx32 gap junction channels, in addition to the direct interaction of protein partners with the carboxyl-terminal (CT) domain. In this study, we used different biophysical methods to determine the structure and characterize the interaction of the Cx32CT domain with the protein partners synapse-associated protein 97 (SAP97) and calmodulin (CaM). Our results revealed that the Cx32CT is an intrinsically disordered protein that becomes α-helical upon binding CaM. We identified the GUK domain as the minimal SAP97 region necessary for the Cx32CT interaction. The Cx32CT residues affected by the binding of CaM and the SAP97 GUK domain were determined as well as the dissociation constants for these interactions. We characterized three Cx32CT Charcot-Marie-Tooth disease mutants (R219H, R230C, and F235C) and identified that whereas they all formed functional channels, they all showed reduced binding affinity for SAP97 and CaM. Additionally, we report that in RT4-D6P2T rat schwannoma cells, Cx32 is differentially phosphorylated and exists in a complex with SAP97 and CaM. Our studies support the importance of protein-protein interactions in the regulation of Cx32 gap junction channels and myelin homeostasis.

Regulation of mucin secretion and inflammation in asthma: a role for MARCKS protein?

BACKGROUND

A major characteristic of asthmatic airways is an increase in mucin (the glycoprotein component of mucus) producing and secreting cells, which leads to increased mucin release that further clogs constricted airways and contributes markedly to airway obstruction and, in the most severe cases, to status asthmaticus. Asthmatic airways show both a hyperplasia and metaplasia of goblet cells, mucin-producing cells in the epithelium; hyperplasia refers to enhanced numbers of goblet cells in larger airways, while metaplasia refers to the appearance of these cells in smaller airways where they normally are not seen. With the number of mucin-producing and secreting cells increased, there is a coincident hypersecretion of mucin which characterizes asthma. On a cellular level, a major regulator of airway mucin secretion in both in vitro and in vivo studies has been shown to be MARCKS (myristoylated alanine-rich C kinase substrate) protein, a ubiquitous substrate of protein kinase C (PKC).

GENERAL SIGNIFICANCE

In this review, properties of MARCKS and how the protein may regulate mucin secretion at a cellular level will be discussed. In addition, the roles of MARCKS in airway inflammation related to both influx of inflammatory cells into the lung and release of granules containing inflammatory mediators by these cells will be explored. This article is part of a Special Issue entitled: Biochemistry of Asthma.

Identification of single and double sites of phosphorylation by ECD FT-ICR/MS in peptides related to the phosphorylation site domain of the myristoylated alanine-rich C kinase protein

A series of phosphorylated test peptides was studied by electron capture dissociation Fourier transform ion cyclotron resonance mass spectrometry (ECD FT-ICR MS). The extensive ECD-induced fragmentation made identification of phosphorylation sites for these peptides straightforward. The site(s) of initial phosphorylation of a synthetic peptide with a sequence identical to that of the phosphorylation site domain (PSD) of the myristoylated alanine-rich C kinase (MARCKS) protein was then determined. Despite success in analyzing fragmentation of the smaller test peptides, a unique site on the PSD for the first step of phosphorylation could not be identified because the phosphorylation reaction produced a heterogeneous mixture of products. Some molecules were phosphorylated on the serine closest to the N-terminus, and others on one of the two serines closest to the C-terminus of the peptide. Although no definitive evidence for phosphorylation on either of the remaining two serines in the PSD was found, modification there could not be ruled out by the ECD fragmentation data.

Ca2+-dependent calmodulin binding to FcRn affects immunoglobulin G transport in the transcytotic pathway

The Fcgamma receptor FcRn transports immunoglobulin G (IgG) so as to avoid lysosomal degradation and to carry it bidirectionally across epithelial barriers to affect mucosal immunity. Here, we identify a calmodulin-binding site within the FcRn cytoplasmic tail that affects FcRn trafficking. Calmodulin binding to the FcRn tail is direct, calcium-dependent, reversible, and specific to residues comprising a putative short amphipathic alpha-helix immediately adjacent to the membrane. FcRn mutants with single residue substitutions in this motif, or FcRn mutants lacking the cytoplasmic tail completely, exhibit a shorter half-life and attenuated transcytosis. Chemical inhibitors of calmodulin phenocopy the mutant FcRn defect in transcytosis. These results suggest a novel mechanism for regulation of IgG transport by calmodulin-dependent sorting of FcRn and its cargo away from a degradative pathway and into a bidirectional transcytotic route.

PKC regulates a farnesyl-electrostatic switch on K-Ras that promotes its association with Bcl-XL on mitochondria and induces apoptosis

K-Ras associates with the plasma membrane (PM) through farnesylation that functions in conjunction with an adjacent polybasic sequence. We show that phosphorylation by protein kinase C (PKC) of S181 within the polybasic region promotes rapid dissociation of K-Ras from the PM and association with intracellular membranes, including the outer membrane of mitochondria where phospho-K-Ras interacts with Bcl-XL. PKC agonists promote apoptosis of cells transformed with oncogenic K-Ras in a S181-dependent manner. K-Ras with a phosphomimetic residue at position 181 induces apoptosis via a pathway that requires Bcl-XL. The PKC agonist bryostatin-1 inhibited the growth in vitro and in vivo of cells transformed with oncogenic K-Ras in a S181-dependent fashion. These data demonstrate that the location and function of K-Ras are regulated directly by PKC and suggest an approach to therapy of K-Ras-dependent tumors with agents that stimulate phosphorylation of S181.

Nerve Ending “Signal” Proteins GAP‐43, MARCKS, and BASP1

Mechanisms of growth cone pathfinding in the course of neuronal net formation as well as mechanisms of learning and memory have been under intense investigation for the past 20 years, but many aspects of these phenomena remain unresolved and even mysterious. "Signal" proteins accumulated mainly in the axon endings (growth cones and the presynaptic area of synapses) participate in the main brain processes. These proteins are similar in several essential structural and functional properties. The most prominent similarities are N-terminal fatty acylation and the presence of an "effector domain" (ED) that dynamically binds to the plasma membrane, to calmodulin, and to actin fibrils. Reversible phosphorylation of ED by protein kinase C modulates these interactions. However, together with similarities, there are significant differences among the proteins, such as different conditions (Ca2+ contents) for calmodulin binding and different modes of interaction with the actin cytoskeleton. In light of these facts, we consider GAP-43, MARCKS, and BASP1 both separately and in conjunction. Special attention is devoted to a discussion of apparent inconsistencies in results and opinions of different authors concerning specific questions about the structure of proteins and their interactions.

Direct Involvement of Protein Myristoylation in Myristoylated Alanine-rich C Kinase Substrate (MARCKS)-Calmodulin Interaction

MARCKS, a major in vivo substrate of protein kinase C, interacts with plasma membranes in a phosphorylation-, myristoylation-, and calmodulin-dependent manner. Although we have previously observed that myristoylated and non-myristoylated MARCKS proteins behave differently during calmodulin-agarose chromatography, the role of protein myristoylation in the MARCKS-calmodulin interaction remained to be elucidated. Here we demonstrate that the myristoyl moiety together with the N-terminal protein domain is directly involved in the MARCKS-calmodulin interaction. Both myristoylated and non-myristoylated recombinant MARCKS bound to calmodulin-agarose at low ionic strengths, but only the former retained the affinity at high ionic strengths. A quantitative analysis obtained with dansyl (5-dimethylaminonaphthalene-1-sulfonyl)-calmodulin showed that myristoylated MARCKS has an affinity higher than the non-myristoylated protein. Furthermore, a synthetic peptide based on the N-terminal sequence was found to bind calmodulin only when it was myristoylated. Only the N-terminal peptide but not the canonical calmodulin-binding domain showed the ionic strength-independent calmodulin binding. A mutation study suggested that the importance of the positive charge in the N-terminal protein domain in the binding.

Calmodulin is a phospholipase C-beta interacting protein

Phospholipase C-beta 3 (PLC beta 3) is an important effector enzyme in G protein-coupled signaling pathways. Activation of PLC beta 3 by G alpha and G beta gamma subunits has been fairly well characterized, but little is known about other protein interactions that may also regulate PLC beta 3 function. A yeast two-hybrid screen of a mouse brain cDNA library with the amino terminus of PLC beta 3 has yielded potential PLC beta 3 interacting proteins including calmodulin (CaM). Physical interaction between CaM and PLC beta 3 is supported by a positive secondary screen in yeast and the identification of a CaM binding site in the amino terminus of PLC beta 3. Co-precipitation of in vitro translated and transcribed amino- and carboxyl-terminal PLC beta 3 revealed CaM binding at a putative amino-terminal binding site. Direct physical interaction of PLC beta 3 and PLC beta 1 isoforms with CaM is supported by pull-down of both isoenzymes with CaM-Sepharose beads from 1321N1 cell lysates. CaM inhibitors reduced M1-muscarinic receptor stimulation of inositol phospholipid hydrolysis in 1321N1 astrocytoma cells consistent with a physiologic role for CaM in modulation of PLC beta activity. There was no effect of CaM kinase II inhibitors, KN-93 and KN-62, on M1-muscarinic receptor stimulation of inositol phosphate hydrolysis, consistent with a direct interaction between PLC beta isoforms and CaM.

Crystal structure of a MARCKS peptide containing the calmodulin-binding domain in complex with Ca2+-calmodulin

The calmodulin-binding domain of myristoylated alanine-rich C kinase substrate (MARCKS), which interacts with various targets including calmodulin, actin and membrane lipids, has been suggested to function as a crosstalk point among several signal transduction pathways. We present here the crystal structure at 2 A resolution of a peptide consisting of the MARCKS calmodulin (CaM)-binding domain in complex with Ca2+-CaM. The domain assumes a flexible conformation, and the hydrophobic pocket of the calmodulin N-lobe, which is a common CaM-binding site observed in previously resolved Ca2+-CaM-target peptide complexes, is not involved in the interaction. The present structure presents a novel target-recognition mode of calmodulin and provides insight into the structural basis of the flexible interaction module of MARCKS.

Myristoylated alanine-rich C kinase substrate (MARCKS) sequesters spin-labeled phosphatidylinositol 4,5-bisphosphate in lipid bilayers

The myristoylated alanine-rich protein kinase C substrate (MARCKS) may function to sequester phosphoinositides within the plane of the bilayer. To characterize this interaction with phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)), a novel spin-labeled derivative, proxyl-PIP(2), was synthesized and characterized. In the presence of molecules known to bind PI(4,5)P(2) the EPR spectrum of this label exhibits an increase in line width because of a decrease in label dynamics, and titration of this probe with neomycin yields the expected 1:1 stoichiometry. Thus, this probe can be used to quantitate the interactions made by the PI(4,5)P(2) head group within the bilayer. In the presence of a peptide comprising the effector domain of MARCKS the EPR spectrum broadens, but the changes in line shape are modulated by both changes in label correlation time and spin-spin interactions. This result indicates that at least some proxyl-PIP(2) are in close proximity when bound to MARCKS and that MARCKS associates with multiple PI(4,5)P(2) molecules. Titration of the proxyl-PIP(2) EPR signal by the MARCKS-derived peptide also suggests that multiple PI(4,5)P(2) molecules interact with MARCKS. Site-directed spin labeling of this peptide shows that the position and conformation of this protein segment at the membrane interface are not altered significantly by binding to PI(4,5)P(2). These data are consistent with the hypothesis that MARCKS functions to sequester multiple PI(4,5)P(2) molecules within the plane of the membrane as a result of interactions that are driven by electrostatic forces.

Calmodulin and cyclin D anchoring sites on the Src-suppressed C kinase substrate, SSeCKS

SSeCKS and its human orthologue, Gravin, are large scaffolding proteins that are thought to facilitate mitogenic control by anchoring key signal mediators such as protein kinase (PK) C, PKA, the plasma membrane associated isoform of alpha-1,4-galactosyltransferase (GalTase), beta2-adrenergic receptor, and cyclins. SSeCKS is also a major PKC substrate and phosphatidylserine-dependent PKC binding protein whose phosphorylation sites shares homology with a site in the MARCKS protein that encodes phosphorylation-sensitive calmodulin (CaM) binding activity. In the present study, we mapped the in vitro binding sites for CaM and cyclins on SSeCKS. Four CaM binding sites were identified by binding assays that conform to the so-called 1-5-10 motif. Notably, CaM binding was antagonized by prephosphorylation of SSeCKS by PKC. We also identified two major cyclin binding (CY) sites that overlap a major PKC phosphorylation site in SSeCKS (Ser(507/515)), and showed that cyclin D binding is attenuated if SSeCKS is prephosphorylated by PKC. These data suggest that the scaffolding activities of SSeCKS are modulated by mitogenically stimulated kinases such as PKC.

Chromaffin cell F-actin disassembly and potentiation of catecholamine release in response to protein kinase C activation by phorbol esters is mediated through myristoylated alanine-rich C kinase substrate phosphorylation

The large majority of chromaffin vesicles are excluded from the plasma membrane by a cortical F-actin network. Treatment of chromaffin cells with phorbol 12-myristate 13-acetate produces disassembly of cortical F-actin, increasing the number of vesicles at release sites (Vitale, M. L., Seward, E. P., and Trifaró, J. M. (1995) Neuron 14, 353-363). Here, we provide evidence for involvement of myristoylated alanine-rich protein kinase C substrate (MARCKS), a protein kinase C substrate, in chromaffin cell secretion. MARCKS binds and cross-links F-actin, the latter is inhibited by protein kinase C-induced MARCKS phosphorylation. MARCKS was found in chromaffin cells by immunoblotting. MARCKS was also detected by immunoprecipitation. In intact or permeabilized cells MARCKS phosphorylation increased upon stimulation with 10(-7) m phorbol 12-myristate 13-acetate. This was accompanied by cortical F-actin disassembly and potentiation of secretion. MARCKS phosphorylation, cortical F-actin disassembly, and potentiation of Ca(2+)-evoked secretion were inhibited by a peptide (MARCKS phosphorylation site domain sequence (MPSD)) with amino acid sequence corresponding to MARCKS phosphorylation site. MPSD was phosphorylated in the process. A similar peptide (alanine-substituted phosphorylated site domain) with four serine residues of MPSD substituted by alanines was ineffective. These results provide the first evidence for MARCKS involvement in chromaffin cell secretion and suggest that regulation of cortical F-actin cross-linking might be involved in this process.

Overexpression of the myristoylated alanine-rich C-kinase substrate inhibits cell adhesion to extracellular matrix components

Mice lacking the myristoylated alanine-rich C-kinase substrate, or MARCKS protein, exhibit abnormalities consistent with a defect in the ability of neurons to migrate appropriately during forebrain development. To investigate the possibility that this phenotype could be due to disruption of normal cellular adhesion to extracellular matrix, an assay was developed in which 293 cells co-expressing MARCKS and green fluorescent protein were tested for their adhesion competence on various substrates. Fluorescence-activated cell sorting of adherent and non-adherent green fluorescent protein-expressing cells demonstrated that wild-type MARCKS inhibited adhesion of cells to fibronectin, whereas a non-myristoylated mutant did not inhibit adhesion of cells to a variety of substrates. The fibronectin competitive inhibitor RGD peptide inhibited adhesion of cells expressing all MARCKS variants equally. Cytochalasin D inhibited the adhesion of cells expressing non-myristoylated MARCKS, but did not further decrease the adhesion of cells expressing adhesion-inhibitory proteins. Confocal microscopy demonstrated the presence of inhibitory, myristoylated MARCKS at the plasma membrane, suggesting that localization at this region might be important for MARCKS to inhibit cellular adhesion. These data suggest a possible myristoylation-dependent function of MARCKS to inhibit cellular adhesion to extracellular matrix proteins, indicating a potential mechanism for the cell migration defects seen in the MARCKS-deficient mice.

Overexpression of the myristoylated alanine-rich C kinase substrate decreases uptake and K(+)-evoked release of noradrenaline in the human neuroblastoma SH-SY5Y

The aim of this study was to investigate a possible role of the myristoylated alanine-rich C kinase substrate (MARCKS) in the mechanism of noradrenaline uptake and release in the human neuroblastoma cell line SH-SY5Y. A stable cell line showing a twofold overexpression of MARCKS was prepared by transfecting SH-SY5Y with pCEP4 containing MARCKS cDNA in the sense orientation. This cell line showed no changes in the expression of neurofilaments or markers of noradrenergic large dense-cored vesicles compared with both untransfected SH-SY5Y and SH-SY5Y transfected with pCEP4 only (mock transfected). Similarly, no differences in the rate of cell growth could be detected between these three cell lines. In contrast, specific uptake and depolarization-evoked (100 mM K(+)) release of noradrenaline from the cell line overexpressing MARCKS was inhibited by approximately 50% compared with mock-transfected SH-SY5Y. K(+)-evoked noradrenaline release enhanced by pretreatment with 12-O-tetradecanoylphorbol 13-acetate (100 nM) was also inhibited by 50%. In contrast, carbachol-evoked noradrenaline release was unaffected. Thus, in SH-SY5Y cells, overexpression of MARCKS leads to a decrease in the K(+)-evoked noradrenaline release possibly by increased actin cross-linking preventing the movement of noradrenaline containing large dense-cored vesicles to the plasma membrane in response to depolarization.

A newly characterized human endometrial adenocarcinoma cell line (CAC-1) differentiates in response to retinoic acid treatment

A new cell line of poorly differentiated human endometrial adenocarcinoma cells termed "CAC-1" cells has been established. These cells are epithelial, as indicated by positive cytokeratin and negative vimentin staining. They are rounded and possess a high nuclear-to-cytoplasmic ratio, desmosomes, surface microvilli, intercelular lumens, and pleomorphic nuclei containing multiple nucleoli. These cells have been in long-term culture for 2 years. Our previous studies demonstrated that moderately differentiated (RL95-2) cells differentiated in response to retinoic acid treatment, illustrated by their reorganization of actin filaments and cell enlargement (Carter et al., 1996; Anticancer Res. 16, 17-24). CAC-1 cells exhibited a similar response because they also organized actin filaments and enlarged in response to retinoic acid treatment. Concurrently, retinoic acid treatment caused a 40% decrease in cell detachment in an in vitro detachment assay compared to controls. A slight lag in cell growth was observed when CAC-1 cells were treated with 1 microM 13-cis or all-trans retinoic acid during a 12-day growth curve. In addition, we examined the effects of retinoic acid on protein kinase C-alpha (PKC-alpha) and myristoylated alanine-rich C-kinase substrate (MARCKS). Treatment with retinoic acid caused cytoplasmic PKC-alpha to increase concomitant with a decrease in PKC-alpha in the membrane. In contrast, MARCKS increased in the membrane in response to retinoic acid treatment. These data indicate that retinoid treatment causes inactivation of PKC-alpha, allowing MARCKS to relocalize to the membrane, where it can cross-link actin filaments. CAC-1 cells represent an ideal model for investigating the effects of retinoids on differentiation induction concomitant with actin reorganization.

Role of MARCKS in regulating endothelial cell proliferation

Myristoylated alanine-rich C kinase substrate (MARCKS), as a specific protein kinase C (PKC) substrate, mediates PKC signaling through its phosphorylation and subsequent modification of its association with filamentous actin (F-actin) and calmodulin (CaM). PKC has long been implicated in cell proliferation, and recent studies have suggested that MARCKS may function as a cell growth suppressor. Therefore, in the present study, we investigated MARCKS protein expression, distribution, and phosphorylation in preconfluent and confluent bovine pulmonary microvascular endothelial cells (BPMEC) in the presence or absence of the vascular endothelial growth factor (VEGF). In addition, we examined functional alterations of MARCKS in these cells by studying the association of MARCKS with F-actin and CaM-dependent myosin light chain (MLC) phosphorylation. Our results indicate that MARCKS protein is downregulated during BPMEC proliferation. Decreased MARCKS association with F-actin, increased actin polymerization, and CaM-dependent MLC phosphorylation appear to mediate cell shape changes and motility during BPMEC growth. In contrast, VEGF stimulated MARCKS phosphorylation without alteration of protein expression during BPMEC proliferation, which may result in reduced interaction between MARCKS and actin or CaM, leading to actin reorganization and MLC phosphorylation. Our data suggest a regulatory role of MARCKS during endothelial cell proliferation.

Endotoxin causes phosphorylation of MARCKS in pulmonary vascular endothelial cells

Protein kinase C (PKC) has been implicated in lipopolysaccharide (LPS)-induced endothelial cell (EC) monolayer permeability. Myristoylated alanine-rich C kinase substrate (MARCKS), as a specific PKC substrate, appears to mediate PKC signaling by PKC-dependent phosphorylation of MARCKS and subsequent modification of the association of MARCKS with filamentous actin and calmodulin (CaM). Therefore, in the present study, we investigated LPS-induced MARCKS phosphorylation in bovine pulmonary artery EC (BPAEC). LPS potentiated MARCKS phosphorylation in BPAEC in a time- and dose-dependent manner. The PKC inhibitor, calphostin C, significantly decreased LPS-induced phosphorylation of MARCKS. In addition, downregulation of PKC with phorbol 12-myristate 13-acetate (PMA) did not affect the LPS-induced MARCKS phosphorylation, suggesting that LPS and PMA activate different isoforms of PKC. Pretreatment with SB203580, a specific inhibitor of p38 MAP kinase, or genistein, a tyrosine kinase inhibitor, prevented LPS-induced MARCKS phosphorylation. Phosphorylation at appropriate sites will induce translocation of MARCKS from the cell membrane to the cytosol. However, LPS, in contrast to PMA, did not generate MARCKS translocation in BPAEC, suggesting that MARCKS translocation may not play a role in LPS-induced actin rearrangement and EC permeability. LPS also enhanced both thrombin- and PMA-induced phosphorylation of MARCKS, suggesting that LPS was able to prime these signaling pathways in BPAEC. Because the CaM-dependent phosphorylation of myosin light chains (MLC) results in EC contraction, we studied the effect of LPS on MLC phosphorylation in BPAEC. LPS induced diphosphorylation of MLC in a time-dependent manner, which occurred at lower doses of LPS, than those required to induce MARCKS phosphorylation. In addition, there was no synergism between LPS and thrombin in the induction of MLC phosphorylation. These data indicate that MLC phosphorylation is independent of MARCKS phosphorylation. In conclusion, LPS stimulated MARCKS phosphorylation in BPAEC. This phosphorylation appears to involve activation of PKC, p38 MAP kinase, and tyrosine kinases. Further studies are needed to explore the role of MARCKS phosphorylation in LPS-induced actin rearrangement and EC permeability.

Interaction between actin and the effector peptide of MARCKS-related protein. Identification of functional amino acid segments

It is widely assumed that the members of the MARCKS protein family, MARCKS (an acronym for myristoylated alanine-rich C kinase substrate) and MARCKS-related protein (MRP), interact with actin via their effector domain, a highly basic segment composed of 24-25 amino acid residues. To clarify the mechanisms by which this interaction takes place, we have examined the effect of a peptide corresponding to the effector domain of MRP, the so-called effector peptide, on both the dynamic and the structural properties of actin. We show that in the absence of cations the effector peptide polymerizes monomeric actin and causes the alignment of the formed filaments into bundle-like structures. Moreover, we document that binding of calmodulin or phosphorylation by protein kinase C both inhibit the actin polymerizing activity of the MRP effector peptide. Finally, several effector peptides were synthesized in which positively charged or hydrophobic segments were deleted or replaced by alanines. Our data suggest that a group of six positively charged amino acid residues at the N-terminus of the peptide is crucial for its interaction with actin. While its actin polymerizing activity critically depends on the presence of all three positively charged segments of the peptide, hydrophobic amino acid residues rather modulate the polymerization velocity.

Thrombin-induced phosphorylation of MARCKS does not alter its interactions with calmodulin or actin

Myristoylated alanine-rich C kinase substrate (MARCKS) is a calmodulin (CaM)- and actin-binding protein and prominent protein kinase C (PKC) substrate. In vitro phosphorylation of MARCKS by PKC has been shown to induce the release of both CaM and actin, leading to the suggestion that MARCKS may regulate CaM availability during agonist-induced signalling. In support of this hypothesis we previously demonstrated that thrombin-induced MARCKS phosphorylation in endothelial cells (EC) parallels activation of myosin light chain kinase, a CaM-dependent enzyme. To test this theory further, we transfected CHO cells, which normally do not express significant levels of MARCKS, with a MARCKS cDNA. The thrombin-stimulated phosphorylation of myosin light chains and the sensitivity to CaM antagonists in the MARCKS overexpressing cells was the same as that in control CHO cells. MARCKS associated with the actin cytoskeleton in EC was markedly increased upon treatment with the PKC activator, PMA, but only modestly enhanced by thrombin treatment. Similarly, colocalisation of MARCKS with actin was enhanced when the EC were challenged with PMA but not thrombin. These data may be partially explained by PKC-independent phosphorylation of MARCKS in response to thrombin stimulation.

Platelet secretion induced by phorbol esters stimulation is mediated through phosphorylation of MARCKS: a MARCKS-derived peptide blocks MARCKS phosphorylation and serotonin release without affecting pleckstrin phosphorylation*

Previous experiments suggest that actin disassembly, perhaps at a specific site, is required for platelet secretion. Platelet stimulation by phorbol 12-myristate 13-acetate (PMA) induced pleckstrin phosphorylation, platelet aggregation, and secretion. Inhibition of protein kinase C (PKC) is accompanied by inhibition of pleckstrin phosphorylation and serotonin secretion. Here, we demonstrate the presence of myristoylated alanine-rich C kinase substrate (MARCKS), another PKC substrate, in platelets and its phosphorylation during PMA stimulation. MARCKS is known to bind actin and to cross-link actin filaments; the latter is inhibited by PKC-induced MARCKS phosphorylation. MARCKS phosphorylation and serotonin release from permeabilized platelets have the same time course and were blocked by a peptide (MPSD) with the amino acid sequence corresponding to the phosphorylation site domain of MARCKS. Pleckstrin and myosin light chain phosphorylation was not modified. A peptide (Ala-MPSD) in which the four serine residues of MPSD were substituted by alanines was ineffective. These results provide the first evidence that MARCKS may play a role in platelet secretion. Moreover, pleckstrin phosphorylation has a different time course than that of MARCKS or serotonin release and was not modified when MARCKS phosphorylation and serotonin release were inhibited, suggesting that pleckstrin is either not directly involved in secretion or that it might only be involved upstream in the cascade of events leading to exocytosis.

Ziskind-Somerfeld Research Award. Protein kinase C signaling in the brain: molecular transduction of mood stabilization in the treatment of manic-depressive illness

Understanding the biology of the pharmacological stabilization of mood will undoubtedly serve to provide significant insight into the pathophysiology of manic-depressive illness (MDI). Accumulating evidence from our laboratories and those of other researchers has identified the family of protein kinase C isozymes as a shared target in the brain for the long-term action of both lithium and valproate. In rats chronically treated with lithium, there is a reduction in the hippocampus of the expression of two protein kinase isozymes, alpha and epsilon, as well as a reduction in the expression of a major PKC substrate, MARCKS, which has been implicated in long-term neuroplastic events in the developing and adult brain. In addition, we have been investigating the down-stream impact of these mood stabilizers on another kinase system, GSK-3 beta and on the AP-1 family of transcription factors. Further studies have generated promising preliminary data in support of the antimanic action of tamoxifen, and antiestrogen that is also a PKC inhibitor. Future studies must address the therapeutic relevance of these protein targets in the brain using innovative strategies in both animal and clinical investigations to ultimately create opportunities for the discovery of the next generations of mood stabilizers for the treatment of MDI.

Phagocytic and macropinocytic activity in MARCKS-deficient macrophages and fibroblasts

Macrophages express high levels of the myristoylated, alanine-rich, C kinase substrate (MARCKS), an actin cross-linking protein. To investigate a possible role of MARCKS in macrophage function, fetal liver-derived macrophages were generated from wild-type and MARCKS knockout mouse embryos. No differences between the wild-type and MARCKS-deficient macrophages with respect to morphology (Wright's stain) or actin distribution (staining with rhodamine-phalloidin, under basal conditions or after treatment with phorbol esters, lipopolysaccharide, or both) were observed. We then evaluated phagocytosis mediated by different receptors: Fc receptors tested with IgG-coated sheep red blood cells, complement C3b receptors tested with C3b-coated yeast, mannose receptors tested with unopsonized zymosan, and nonspecific phagocytosis tested with latex beads. We also studied fluid phase endocytosis in macrophages and mouse embryo fibroblasts by using FITC-dextran to quantitate this process. In most cases, there were no differences between the cells derived from wild-type and MARCKS-deficient mice. However, a minor but significant and reproducible difference in rates of zymosan phagocytosis at 45-60 min was observed, with lower rates of phagocytosis in the MARCKS-deficient cells. Our data indicate that MARCKS deficiency may lead to slightly decreased rates of zymosan phagocytosis.

Age-dependent differences in glutamate-induced phosphorylation systems in rat hippocampal slices

Glutamate receptor induced changes in the activity of different phosphorylation systems were measured in hippocampal slices from 12- and 56-day-old rats, by determining the endogenous phosphorylation of 2.5% perchloric acid (PCA) soluble proteins. We identified among these proteins an 85, 80 kDa and the tau protein as specific substrates for protein kinase A (PKA), MARCKS, and neurogranin as specific substrates for protein kinase C (PKC), and prostaglandin-D-synthase as substrate for casein kinase II (CKII). In addition, a 35 kDa protein was phosphorylated by calcium/calmodulin dependent kinase II and protein kinase C and a 21 kDa protein was a substrate for all investigated kinases. The basal endogenous phosphorylation of 2.5% PCA soluble proteins changed during development qualitatively and quantitatively. Thus, the phosphorylation degree of nearly all proteins declines during maturation. Activation of mGluR induced an increased phosphorylation of PKA, PKC, and CKII substrates in hippocampal slices from 12-day-old rats, but in slices of 56-day-old rats only PKA and to a lower extent PKC substrates were affected. In contrast, stimulation of NMDA receptors led to an enhancement of CKII and PKA dependent phosphorylation only in slices of young animals, whereas the endogenous phosphorylation of some proteins in adult slices was actually decreased. These data showing developmental changes in the coupling of metabotropic and ionotropic glutamate receptors to different phosphorylation systems are discussed in the light of altered physiological properties of the mature hippocampus.

Transgenic complementation of MARCKS deficiency with a nonmyristoylatable, pseudo-phosphorylated form of MARCKS: evidence for simultaneous positive and dominant-negative effects on central nervous system development

MARCKS is a widely expressed protein kinase C substrate that is essential for normal prenatal development of the central nervous system in mice. MARCKS-deficient mice exhibit universal perinatal mortality and numerous developmental abnormalities of the brain and retina. To determine which domains of the protein were important in complementing these neurodevelopmental anomalies, we have interbred MARCKS knockout mice with transgenic mice expressing an epitope-tagged human MARCKS transgene that can completely correct the MARCKS-deficient phenotype. Previous structure-function studies showed that a nonmyristoylatable form of MARCKS could correct all of the neuroanatomical abnormalities, and resulted in approximately 25% viable pups that grew to adulthood and were fertile. The present experiment attempted a similar complementation strategy in which a nonmyristoylatable, "pseudo-phosphorylated" form of the protein was used, which has been shown to be almost completely cytosolic in cell expression studies. Surprisingly, this transgene was able to complement almost all of the cerebral anatomical abnormalities characteristic of the knockout mice. However, these mice also exhibited a universal, novel phenotype: profound retinal ectopia, in which retinal tissue was often found in the vitreous humor as well as extraocularly. Retrospective evaluation of the original MARCKS knockout phenotype revealed that this anomaly was present in about 43% of the knockout mice, and was clearly detectable as early as embryonic day 12.5, before retinal cell differentiation begins. These data suggest that a nonmyristoylatable, pseudo-phosphorylated form of MARCKS can complement most if not all cerebral aspects of the MARCKS-deficient phenotype, but that it appears to worsen a retinal phenotype, perhaps by exerting a dominant-negative effect on a coexpressed MARCKS homologue.

Poly(ADP-ribose) modulates the properties of MARCKS proteins

In mammalian cells, the formation of DNA strand breaks is accompanied by synthesis of poly(ADP-ribose). This nucleic acid-like homopolymer may modulate protein functions by covalent and/or noncovalent interactions. Here we show that poly(ADP-ribose) binds strongly to the proteins of the myristoylated alanine-rich C kinase substrate (MARCKS) family, MARCKS and MARCKS-related protein (also MacMARCKS or F52). MARCKS proteins are myristoylated proteins associated with membranes and the actin cytoskeleton. As targets for both protein kinase C (PKC) and calmodulin (CaM), MARCKS proteins are thought to mediate cross-talk between these two signal transduction pathways. Dot blot assays show that poly(ADP-ribose) binds to MARCKS proteins at the highly basic effector domain. Complex formation between MARCKS-related protein and CaM as well as phosphorylation of MARCKS-related protein by the catalytic subunit of PKC are strongly inhibited by equimolar amounts of poly(ADP-ribose), suggesting a high affinity of poly(ADP-ribose) for MARCKS-related protein. Binding of MARCKS-related protein to membranes is also inhibited by poly(ADP-ribose). Finally, poly(ADP-ribose) efficiently reverses the actin-filament bundling activity of a peptide corresponding to the effector domain and inhibits the formation of actin filaments in vitro. Our results suggest that MARCKS proteins and actin could be targets of the poly(ADP-ribose) DNA damage signal pathway.

S100B(betabeta) inhibits the protein kinase C-dependent phosphorylation of a peptide derived from p53 in a Ca2+-dependent manner

S100B(betabeta) is a dimeric Ca2+-binding protein that is known to inhibit the protein kinase C (PKC)-dependent phosphorylation of several proteins. To further characterize this inhibition, we synthesized peptides based on the PKC phosphorylation domains of p53 (residues 367-388), neuromodulin (residues 37-53), and the regulatory domain of PKC (residues 19-31), and tested them as substrates for PKC. All three peptides were shown to be good substrates for the catalytic domain of PKC. As for full-length p53 (Baudier J, Delphin C, Grunwald D, Khochbin S, Lawrence JJ. 1992. Proc Natl Acad Sci USA 89:11627-11631), S100B(betabeta) binds the p53 peptide and inhibits its PKC-dependent phosphorylation (IC50 = 10 +/- 7 microM) in a Ca2+-dependent manner. Similarly, phosphorylation of the neuromodulin peptide and the PKC regulatory domain peptide were inhibited by S100B(betabeta) in the presence of Ca2+ (IC50 = 17 +/- 5 microM; IC50 = 1 +/- 0.5 microM, respectively). At a minimum, the C-terminal EF-hand Ca2+-binding domain (residues 61-72) of each S100beta subunit must be saturated to inhibit phosphorylation of the p53 peptide as determined by comparing the Ca2+ dependence of inhibition ([Ca]IC50 = 29.3 +/- 17.6 microM) to the dissociation of Ca2+ from the C-terminal EF-hand Ca2+-binding domain of S100B(betabeta).

The C-terminal conserved domain of MARCKS is phosphorylated in vivo by proline-directed protein kinase. Application of ion trap mass spectrometry to the determination of protein phosphorylation sites

MARCKS, the major protein kinase C substrate in various cells and tissues, binds to calmodulin, acidic membrane phospholipids, and actin filaments, and these interactions are regulated by protein phosphorylation. We have previously shown that MARCKS purified from bovine brain is phosphorylated not only by protein kinase C but also by so-called proline-directed protein kinases in the well conserved N-terminal half of the molecule (Taniguchi, H., Manenti, S., Suzuki, M., and Titani, K. (1994) J. Biol. Chem. 269, 18299-18302). Although the presence of other phosphorylation sites in the C-terminal peptide was also noticed, the ambiguity in the C-terminal domain of the bovine protein hampered a more detailed analysis. In the present study, we analyzed MARCKS purified from rat brain by electrospray ionization/ion trap mass spectrometry. The results obtained revealed two additional novel phosphorylation sites in the C-terminal region. Both phosphorylation sites (Ser291 and Ser299) are immediately followed by proline, suggesting that these sites are also phosphorylated by the proline-directed protein kinase(s). Since Ser299 is within the C-terminal domain, which is well conserved among various species, the function of the domain, whatever it is, seems to be controlled by phosphorylation.

The Ca(2+)-dependent interaction of S100B(beta beta) with a peptide derived from p53

S100B(beta beta) was found to interact with the tumor suppressor protein, p53, and inhibit its PKC-dependent phosphorylation and tetramer formation [Baudier, J., Delphin, C., Grunwald, D., Khochbin, S., and Lawrence, J. J. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 11627-11631]. Since PKC-dependent phosphorylation at the C-terminus of p53 is known to effect transcription and p53 tetramer formation [Sakaguchi, K., Sakamoto, H., Lewis, M. S., Anderson, C. W., Erickson, J. W., Appella, E., and Xie, D. (1997) Biochemistry 36, 10117-10124], we examined the interaction of S100B(beta beta) with a peptide derived from the C-terminal regulatory domain of p53 (residues 367-388). In this paper, we report that S100B(beta beta) binds to the p53 peptide (CaK3 < or = 23.5 +/- 6.6 microM) in a Ca(2+)-dependent manner, and that the presence of the p53 peptide was found to increase the binding affinity of Ca2+ to S100B(beta beta) by 3-fold using EPR and PRR methods, whereas the peptide had no effect on Zn2+ binding to S100B(beta beta). Fluorescence and NMR spectroscopy experiments show that the p53 peptide binds to a region of S100B(beta beta) that probably includes residues in the "hinge" (S41, L44, E45, E46, I47), C-terminal loop (A83, C84, H85, E86, F87, F88), and helix 3 (V52, V53, V56, T59). Together these data support the notion that S100B(beta beta) inhibits PKC-dependent phosphorylation by binding directly to the C-terminus of p53.

MARCKS, a major protein kinase C substrate, assumes non-helical conformations both in solution and in complex with Ca2+-calmodulin

MARCKS, a major cellular substrate for protein kinase C, plays important roles in various cellular functions and its functions are regulated by calmodulin. We have studied the conformational properties of recombinant human MARCKS in solution and in complex with calmodulin. Circular dichroism (CD) spectra showed a high content of random coil in physiological solution. When MARCKS or MARCKS-derived calmodulin-binding peptide was complexed with Ca2+-calmodulin, little change was observed in the CD spectra, suggesting that MARCKS binds with calmodulin in a non-helical conformation, which is unique among the calmodulin-binding proteins.

Mechanisms of MARCKS gene activation during Xenopus development

The myristoylated alanine-rich protein kinase C substrate (MARCKS) is a high affinity cellular substrate for protein kinase C. The MARCKS gene is under multiple modes of transcriptional control, including cytokine- and transformation-dependent, cell-specific, and developmental regulation. This study evaluated the transcriptional control of MARCKS gene expression during early development of Xenopus laevis. Xenopus MARCKS was highly conserved with its mammalian and avian homologues; its mRNA and protein were abundant in the maternal pool and increased after the mid-blastula transition (MBT). The Xenopus MARCKS gene was similar to those of other species, except that a second intron interrupted the 5'- untranslated region. By transiently transfecting XTC-2 cells and microinjecting Xenopus embryos with reporter gene constructs containing serial deletions of 5'-flanking MARCKS sequences, we identified a 124-base pair minimal promoter that was critical for promoter activity. Developmental gel shift assays revealed that a CBF/NF-Y/CP-1-like factor and an Sp1-like factor bound to this region in a manner correlating with the onset of Xenopus MARCKS transcription at MBT. Mutations in the promoter that abolished binding of these two factors also completely inhibited transcriptional activation of the MARCKS gene at MBT. The binding sites for these two factors are highly conserved in the human and mouse MARCKS promoters, suggesting that these elements might also regulate MARCKS transcription in other species. These studies not only increase our knowledge of the transcriptional regulation of the MARCKS genes but also have implications for the mechanisms responsible for zygotic activation of the Xenopus genome at MBT.

Kinetics of interaction of the myristoylated alanine-rich C kinase substrate, membranes, and calmodulin

Membrane binding of the myristoylated alanine-rich C kinase substrate (MARCKS) requires both its myristate chain and basic "effector" region. Previous studies with a peptide corresponding to the effector region, MARCKS-(151-175), showed that the 13 basic residues interact electrostatically with acidic lipids and that the 5 hydrophobic phenylalanine residues penetrate the polar head group region of the bilayer. Here we describe the kinetics of the membrane binding of fluorescent (acrylodan-labeled) peptides measured with a stopped-flow technique. Even though the peptide penetrates the polar head group region, the association of MARCKS-(151-175) with membranes is extremely rapid; association occurs with a diffusion-limited association rate constant. For example, kon = 10(11) M-1 s-1 for the peptide binding to 100-nm diameter phospholipid vesicles. As expected theoretically, kon is independent of factors that affect the molar partition coefficient, such as the mole fraction of acidic lipid in the vesicle and the salt concentration. The dissociation rate constant (koff) is approximately 10 s-1 (lifetime = 0.1 s) for vesicles with 10% acidic lipid in 100 mM KCl. Ca2+-calmodulin (Ca2+.CaM) decreases markedly the lifetime of the peptide on vesicles, e.g. from 0.1 to 0.01 s in the presence of 5 micrM Ca2+.CaM. Our results suggest that Ca2+.CaM collides with the membrane-bound MARCKS-(151-175) peptide and pulls the peptide off rapidly. We discuss the biological implications of this switch mechanism, speculating that an increase in the level of Ca2+-calmodulin could rapidly release phosphatidylinositol 4, 5-bisphosphate that previous work has suggested is sequestered in lateral domains formed by MARCKS and MARCKS-(151-175).

Introduction of a phosphate at serine741 of the calmodulin-binding domain of the neuronal nitric oxide synthase (NOS-I) prevents binding of calmodulin

The calmodulin-binding domain of neuronal nitric oxide synthase (NOS-I) is represented by a segment of 26 amino acids. We tested whether the phosphorylation of a serine in the calmodulin-binding domain changes the affinity of calmodulin for this binding site. We monitored the binding of calmodulin to synthetic peptides by surface plasmon resonance spectroscopy, an electrophoretic mobility assay, circular dichroism spectroscopy and competitive inhibitory studies. All four experimental approaches showed that binding of calmodulin to the calmodulin-binding site is blocked by introduction of a phosphate. Phosphorylation of the calmodulin-binding domain of NOS-I could be a negative feedback loop to turn off NOS-I activity.

Thrombin-induced phosphorylation of the myristoylated alanine-rich C kinase substrate (MARCKS) protein in bovine pulmonary artery endothelial cells

Myristoylated alanine-rich C kinase substrates (MARCKS) is a prominent protein kinase C (PKC) substrate that is targeted to the plasma membrane by an aminoterminal myristoyl group. In its nonphosphorylated form, MARCKS cross-links Factin and binds calmodulin (CaM) reciprocally. However, upon phosphorylation by PKC, MARCKS release the actin or CaM MARCKS may therefore act as a CaM sink in resting cells and regulate CaM availability during cell activation. We have demonstrated previously that thrombin-induced myosin light chain (MLC) phosphorylation and increased monolayer permeability in bovine pulmonary artery endothelial cells (BPAEC) require both PKC-and CaM-dependent pathways. We therefore decided to investigate the phosphorylation of MARCKS in BPAEC to ascertain whether this occurs in a temporally relevant manner to participate in the thrombin-induced events. MARCKS is phosphorylated in response to thrombin with a time course similar to that seen with MLC. As expected, MARCKS is also phosphorylated by phorbol 12-myristate 13 acetate (PMA), a PKC activator, but with a slower onset and more prolonged duration. Bradykinin also enhances MARCKS phosphorylation in BPAEC, but histamine does not. MARCKS is distributed evently between the membrane and cytosol in BPAEC, and neither thrombin nor PMA caused significant translocation of the protein. Specific PKC inhibitors attenuated MARCKS phosphorylation by either thrombin or PMA. Since thrombin-induced MLC phosphorylation is also attenuated by these inhibitors, MARCKS may be involved in MLC kinase activation and subsequent BPAEC contraction. W7, a CaM antagonist, enhances the phosphorylation of MARCKS. This was expected since CaM binding to MARCKS has been shown to decrease MARCKS phosphorylation by PKC. On the other hand, tyrosine kinase inhibitors, genistein and tyrphostin, attenuate MARCKS phosphorylation but have no effect on MLC phosphorylation, suggesting that MARCKS may be phosphorylated by kinases other than PKC. Phosphorylation of MARCKS outside the PKC phosphorylation domain would not be expected to induce the release of CaM. These data provide support for the hypothesis that MARCKS may serve as a regulator of CaM availability in BPAEC.

Myristoylation does not modulate the properties of MARCKS-related protein (MRP) in solution

The members of the myristoylated alanine-rich C kinase substrate (MARCKS) family are proteins essential for brain development and phagocytosis. MARCKS proteins bind to actin filaments and calmodulin (CaM) and are phosphorylated by protein kinase C. In order to investigate how these interactions are regulated, we have characterized the properties of both the myristoylated (myr) and unmyristoylated (unmyr) forms of recombinant MARCKS-related protein (MRP), a 20-kDa member of the MARCKS family. Ultracentrifugation and circular dichroic spectroscopy reveal that MRP is an elongated protein, with an axis ratio estimated between 7 and 12 and with an apparent random coil conformation. MRP binds to CaM with high affinity (Kd,myr = 4 nM; Kd,unmyr = 7 nM) and with a second order rate constant, k+1,unmyr, of 1.6 x 10(8) M-1 s-1. In contrast to classical ligands such as the myosin light chain kinase, binding of MRP to CaM does not induce the formation of an alpha-helix in MRP. The catalytic subunit of protein kinase C (PKM) phosphorylates myr MRP with high affinity ([S]0.5 = 3.5 microM), positive cooperativity (nH = 2.5) and a turnover number of 130 min-1. CaM inhibits the phosphorylation of myr MRP with a half-maximum rate of phosphorylation at a [CaM]/[MRP] ratio of 0.7, indicating that CaM might efficiently regulate the phosphorylation of MRP in vivo. Interestingly, Ca2+ inhibits the binding of MRP to CaM as well as its phosphorylation by PKM in the millimolar concentration range, suggesting that MRP has a weak affinity for Ca2+. Finally, unmyr MRP can be stoichiometrically myristoylated by N-myristoyl transferase in vitro. Since neither binding of CaM nor phosphorylation by PKM inhibits myristoylation, the N terminus of unmyr MRP is exposed on the surface of the protein and is well separated from the effector domain. In view of the observations that unmyr and myr MRP do not exhibit significant differences in their properties in solution, the function of myristoylation is most probably to modulate the interactions of MRP with membranes.

Defining protein-protein interactions using site-directed spin-labeling: the binding of protein kinase C substrates to calmodulin

EPR spectroscopy was used to examine protein-protein interactions between calmodulin and spin-labeled peptides based on the protein kinase C substrate domains of the myristoylated alanine rich C kinase substrate (MARCKS) and neuromodulin. When bound to calmodulin, the C- and N-terminal ends of a 25 residue MARCKS derived peptide exhibited large amplitude motion on the nanosecond time scale and were accessible to paramagnetic agents in aqueous solution. However, residues 5-23 were highly protected and in contact with side chains from calmodulin. These data are consistent with an alpha-helical configuration for this segment of MARCKS and with structures that have been obtained for other calmodulin-substrate complexes. For the 17 residue neuromodulin derived peptide, which is Ca2+ independent in its binding to calmodulin, oxygen collision rates demonstrate that one helical face of this peptide interacts strongly with calmodulin. The data are consistent with an interaction of this face specifically with the C-terminal lobe of calmodulin, where this lobe is either in an "open" or "semiopen" configuration. The EPR data also indicate that the N-terminal lobe of calmodulin is in contact with the peptide, but that this lobe is not as strongly associated with the peptide target. Overall, the binding pocket for neuromodulin appears to be less compact and more dynamic than that formed by MARCKS. This behavior has not previously been seen for calmodulin substrates, and it may play a role in the Ca2+ independent binding of this class of substrates. This work demonstrates the utility of EPR spectroscopy to define protein-protein interactions; in addition, oxygen collision frequencies obtained at buried sites appear to provide information on the conformational dynamics of proteins.

Myristoylation-dependent and electrostatic interactions exert independent effects on the membrane association of the myristoylated alanine-rich protein kinase C substrate protein in intact cells

The myristoylated alanine-rich protein kinase C substrate (MARCKS) is a widely expressed, prominent substrate for protein kinase C. MARCKS is largely associated with membranes in cells, and hydrophobic interactions involving the amino-terminal myristoyl moiety are thought to play a role in anchoring MARCKS to cellular membranes. In addition, experiments in cell-free systems have suggested that electrostatic interactions between the positively charged phosphorylation site/calmodulin binding domain (PSD) of MARCKS and negatively charged membrane lipids are also involved in this association. Although it has been inferred from phosphorylation experiments, the electrostatic nature of the interaction between the PSD and membranes has not been demonstrated directly in intact cells. We expressed human MARCKS mutated in the myristoylation site and the PSD in REF52 cells; the cells were then fractionated by ultracentrifugation. Both nonmyristoylatable MARCKS and MARCKS in which the four serines in the PSD were mutated to aspartic acids, mimicking phosphorylation, exhibited decreased membrane affinity when compared to the fully myristoylated, wild-type, tetra-Ser protein or a myristoylated, tetra-Asn mutant. A double mutant, nonmyristoylatable protein in which the four serines in the PSD were mutated to aspartic acids exhibited negligible membrane association. Similar results were obtained in 293 cells that stably expressed chicken MARCKS mutated in the same domains. The double mutant, nonmyristoylatable tetra-Asp chicken protein exhibited little membrane association as determined by both subcellular fractionation and immunoelectron microscopy. These results indicate that myristoylation and electrostatic interactions involving the PSD exert independent, essentially additive effects on the membrane association of MARCKS in intact cells.

Inhibition of the phosphorylation of a myristoylated alanine-rich C kinase substrate by methyl methanesulfonate in cultured NIH 3T3 cells

The effect of methyl methanesulfonate (MMS) on the phosphorylation of an acidic 80-kDa myristoylated alanine-rich C kinase substrate (MARCKS) protein was investigated in NIH 3T3 fibroblasts. An alkylating agent, MMS inhibited protein kinase C activity and the phosphorylation of MARCKS. MMS treatment also lowered the cellular amounts of second messengers of inositol-1,4,5-trisphosphate and diacylglycerol. Data suggest that MMS decreased the phosphorylation of phospholipase C, a protein whose activity is influenced by its phosphorylation state. We present here the first report that MMS intervenes in a signal cascade by inhibiting the phosphorylation of phospholipase C, which in turn leads to the inactivation of protein kinase C and the subsequent inhibition of MARCKS phosphorylation.

Membrane structure of protein kinase C and calmodulin binding domain of myristoylated alanine rich C kinase substrate determined by site-directed spin labeling

Cysteine-substituted peptides based on the membrane, calmodulin, and protein kinase C binding domain of the myristoylated alanine rich C kinase substrate (MARCKS) were synthesized and derivatized with a sulfhydryl reactive proxyl nitroxide. These spin-labeled peptides were used in combination with continuous wave power saturation electron paramagnetic resonance (EPR) spectroscopy to determine the position and structure of the peptide on membranes containing phosphatidylserine. These peptides bind at the membrane interface, with nitroxide side chains in the central and C-terminal regions lying several angstroms below the level of the head group. In contrast, the N-terminus of the peptide is extended out of the membrane interface so that the two N-terminal residues are positioned on the aqueous side of the head group. When bound to the membrane, the N-terminal segment of this peptide is sensitive to the membrane surface charge density. Higher charge densities decrease the amplitude of side chain motions at the N-terminus and bring this end of the peptide closer to the membrane interface. When the location of successive residues along the bilayer normal is compared, no helical trend is seen, and no evidence for aggregation of the peptide is found. The EPR spectra of double spin-labeled peptides also show no evidence for a helical structure. Thus, these basic peptides are in an extended configuration at the membrane interface with hydrophobic side chains oriented inward toward the membrane hydrocarbon.