Dissociation of phosphorylation and translocation of a myristoylated protein kinase C substrate (MARCKS protein) in C6 glioma and N1E-115 neuroblastoma cells

Myristoylated alanine-rich C kinase substrate (MARCKS): a multirole signaling protein in cancers

Emerging evidence implicates myristoylated alanine-rich C-kinase substrate (MARCKS), a major substrate of protein kinase C (PKC), in a critical role for cancer development and progression. MARCKS is tethered to the plasma membrane but can shuttle between the cytosol and plasma membrane via the myristoyl-electrostatic switch. Phosphorylation of MARCKS by PKC leads to its translocation from the plasma membrane to the cytosol where it functions in actin cytoskeletal remodeling, Ca²⁺ signaling through binding to calmodulin, and regulation of exocytic vesicle release in secretory cells such as neurons and airway goblet cells. Although the contribution of MARCKS to various cellular processes has been extensively studied, its roles in neoplastic disease have been conflicting. This review highlights the molecular and functional differences of MARCKS that exist between normal and tumor cells. We also discuss the recent advances in the potential roles of MARCKS in tumorigenesis, metastasis, and resistance to anti-cancer therapies, with a focus on addressing the inconsistent results regarding the function of MARCKS as a promoter or inhibitor of oncogenesis.

Regulation of PI3K by PKC and MARCKS: Single-Molecule Analysis of a Reconstituted Signaling Pathway

In chemotaxing ameboid cells, a complex leading-edge signaling circuit forms on the cytoplasmic leaflet of the plasma membrane and directs both actin and membrane remodeling to propel the leading edge up an attractant gradient. This leading-edge circuit includes a putative amplification module in which Ca(2+)-protein kinase C (Ca(2+)-PKC) is hypothesized to phosphorylate myristoylated alanine-rich C kinase substrate (MARCKS) and release phosphatidylinositol-4,5-bisphosphate (PIP2), thereby stimulating production of the signaling lipid phosphatidylinositol-3,4,5-trisphosphate (PIP3) by the lipid kinase phosphoinositide-3-kinase (PI3K). We investigated this hypothesized Ca(2+)-PKC-MARCKS-PIP2-PI3K-PIP3 amplification module and tested its key predictions using single-molecule fluorescence to measure the surface densities and activities of its protein components. Our findings demonstrate that together Ca(2+)-PKC and the PIP2-binding peptide of MARCKS modulate the level of free PIP2, which serves as both a docking target and substrate lipid for PI3K. In the off state of the amplification module, the MARCKS peptide sequesters PIP2 and thereby inhibits PI3K binding to the membrane. In the on state, Ca(2+)-PKC phosphorylation of the MARCKS peptide reverses the PIP2 sequestration, thereby releasing multiple PIP2 molecules that recruit multiple active PI3K molecules to the membrane surface. These findings 1) show that the Ca(2+)-PKC-MARCKS-PIP2-PI3K-PIP3 system functions as an activation module in vitro, 2) reveal the molecular mechanism of activation, 3) are consistent with available in vivo data, and 4) yield additional predictions that are testable in live cells. More broadly, the Ca(2+)-PKC-stimulated release of free PIP2 may well regulate the membrane association of other PIP2-binding proteins, and the findings illustrate the power of single-molecule analysis to elucidate key dynamic and mechanistic features of multiprotein signaling pathways on membrane surfaces.

MARCKS as a negative regulator of lipopolysaccharide signaling

Myristoylated alanine-rich C kinase substrate (MARCKS) is an intrinsically unfolded protein with a conserved cationic effector domain, which mediates the cross-talk between several signal transduction pathways. Transcription of MARCKS is increased by stimulation with bacterial LPS. We determined that MARCKS and MARCKS-related protein specifically bind to LPS and that the addition of the MARCKS effector peptide inhibited LPS-induced production of TNF-α in mononuclear cells. The LPS binding site within the effector domain of MARCKS was narrowed down to a heptapeptide that binds to LPS in an extended conformation as determined by nuclear magnetic resonance spectroscopy. After LPS stimulation, MARCKS moved from the plasma membrane to FYVE-positive endosomes, where it colocalized with LPS. MARCKS-deficient mouse embryonic fibroblasts (MEFs) responded to LPS with increased IL-6 production compared with the matched wild-type MEFs. Similarly, small interfering RNA knockdown of MARCKS also increased LPS signaling, whereas overexpression of MARCKS inhibited LPS signaling. TLR4 signaling was enhanced by the ablation of MARCKS, which had no effect on stimulation by TLR2, TLR3, and TLR5 agonists. These findings demonstrate that MARCKS contributes to the negative regulation of the cellular response to LPS.

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.

Phosphorylation of myristoylated alanine-rich C kinase substrate is involved in the cAMP-dependent amylase release in parotid acinar cells

Myristoylated alanine-rich C kinase substrate (MARCKS) is known as a major cellular substrate for protein kinase C (PKC). MARCKS has been implicated in the regulation of brain development and postnatal survival, cellular migration and adhesion, as well as phagocytosis, endocytosis, and exocytosis. The involvement of MARCKS phosphorylation in secretory function has been reported in Ca(2+)-mediated exocytosis. In rat parotid acinar cells, the activation of beta-adrenergic receptors provokes exocytotic amylase release via accumulation of intracellular cAMP levels. Here, we studied the involvement of MARCKS phosphorylation in the cAMP-dependent amylase release in rat parotid acinar cells. MARCKS protein was detected in rat parotid acinar cells by Western blotting. The beta-adrenergic agonist isoproterenol (IPR) induced MARCKS phosphorylation in a time-dependent manner. Translocation of a part of phosphorylated MARCKS from the membrane to the cytosol and enhancement of MARCKS phosphorylation at the apical membrane site induced by IPR were observed by immunohistochemistry. H89, a cAMP-dependent protein kinase (PKA) inhibitor, inhibited the IPR-induced MARCKS phosphorylation. The PKCdelta inhibitor rottlerin inhibited the IPR-induced MARCKS phosphorylation and amylase release. IPR activated PKCdelta, and the effects of IPR were inhibited by the PKA inhibitors. A MARCKS-related peptide partially inhibited the IPR-induced amylase release. These findings suggest that MARCKS phosphorylation via the activation of PKCdelta, which is downstream of PKA activation, is involved in the cAMP-dependent amylase release in parotid acinar cells.

Myelin-mediated inhibition of oligodendrocyte precursor differentiation can be overcome by pharmacological modulation of Fyn-RhoA and protein kinase C signalling

Failure of oligodendrocyte precursor cell (OPC) differentiation contributes significantly to failed myelin sheath regeneration (remyelination) in chronic demyelinating diseases. Although the reasons for this failure are not completely understood, several lines of evidence point to factors present following demyelination that specifically inhibit differentiation of cells capable of generating remyelinating oligodendrocytes. We have previously demonstrated that myelin debris generated by demyelination inhibits remyelination by inhibiting OPC differentiation and that the inhibitory effects are associated with myelin proteins. In the present study, we narrow down the spectrum of potential protein candidates by proteomic analysis of inhibitory protein fractions prepared by CM and HighQ column chromatography followed by BN/SDS/SDS–PAGE gel separation using Nano-HPLC-ESI-Q-TOF mass spectrometry. We show that the inhibitory effects on OPC differentiation mediated by myelin are regulated by Fyn-RhoA-ROCK signalling as well as by modulation of protein kinase C (PKC) signalling. We demonstrate that pharmacological or siRNA-mediated inhibition of RhoA-ROCK-II and/or PKC signalling can induce OPC differentiation in the presence of myelin. Our results, which provide a mechanistic link between myelin, a mediator of OPC differentiation inhibition associated with demyelinating pathologies and specific signalling pathways amenable to pharmacological manipulation, are therefore of significant potential value for future strategies aimed at enhancing CNS remyelination.

Sprouty4 negatively regulates protein kinase C activation by inhibiting phosphatidylinositol 4,5-biphosphate hydrolysis

Sproutys have been shown to negatively regulate growth factor-induced extracellular signal-regulated kinase (ERK) activation, and suggested to be an anti-oncogene. However, molecular mechanism of the suppression has not yet been clarified completely. Sprouty4 inhibits vascular endothelial growth factor (VEGF)-A-induced ERK activation, but not VEGF-C-induced ERK activation. It has been shown that VEGF-A-mediated ERK activation is strongly dependent on protein kinase C (PKC), whereas that by VEGF-C is dependent on Ras. This suggests that Sprouty4 inhibits the PKC pathway more specifically than the Ras pathway. In this study, we confirmed that Sprouty4 suppressed various signals downstream of PKC, such as phosphorylation of MARCKS and protein kinase D (PKD), as well as PKC-dependent nuclear factor (NF)-kappaB activation. Furthermore, Sprouty4 suppressed upstream signals of PKC, such as Ca(2+) mobilization, phosphatidylinositol 4,5-biphosphate (PIP(2)) breakdown and inositol 1,4,5-triphosphate (IP(3)) production in response to VEGF-A. Those effects were dependent on the C-terminal cysteine-rich region, but not on the N-terminal region of Sprouty4, which is critical for the suppression of fibroblast growth factor (FGF)-mediated ERK activation. Sprouty4 overexpression or deletion of the Sprouty4 gene did not affect phospholipase C (PLC) gamma-1 activation, which is an enzyme that catalyzes PIP(2) hydrolysis. Moreover, Sprouty4 inhibited not only VEGF-A-mediated PIP(2) hydrolysis but also inhibited the lysophosphatidic acid (LPA)-induced PIP(2) breakdown that is catalyzed by PLC beta/epsilon activated by G-protein coupled receptor (GPCR). Taken together, Sprouty4 has broader suppression activity for various stimuli than previously thought; it may function as an inhibitor for various types of PLC-dependent signaling as well as for ERK activation.

The MARCKS family of phospholipid binding proteins: regulation of phospholipase D and other cellular components

Myristoylated alanine-rich C kinase substrate (MARCKS) and MARCKS-related protein (MRP) are essential proteins that are implicated in coordination of membrane-cytoskeletal signalling events, such as cell adhesion, migration, secretion, and phagocytosis in a variety of cell types. The most prominent structural feature of MARCKS and MRP is a central basic effector domain (ED) that binds F-actin, Ca2+-calmodulin, and acidic phospholipids; phosphorylation of key serine residues within the ED by protein kinase C (PKC) prevents the above interactions. While the precise roles of MARCKS and MRP have not been established, recent attention has focussed on the high affinity of the MARCKS ED for phosphatidylinositol 4,5-bisphosphate (PIP2), and a model has emerged in which calmodulin- or PKC-mediated regulation of these proteins at specific membrane sites could in turn control spatial availability of PIP2. The present review summarizes recent progress in this area and discusses how the above model might explain a role for MARCKS and MRP in activation of phospholipase D and other PIP2-dependent cellular processes.

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.

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.

Protein kinase C prevents oligodendrocyte differentiation: modulation of actin cytoskeleton and cognate polarized membrane traffic

In a previous study, we showed that activation of protein kinase C (PKC) prevents oligodendrocyte differentiation at the pro-oligodendrocyte stage. The present study was undertaken to identify downstream targets of PKC action in oligodendrocyte progenitor cells. Activation of PKC induced the predominant phosphorylation of an 80-kD protein, identified as myristoylated alanine-rich C-kinase substrate (MARCKS). Upon phosphorylation, MARCKS is translocated from the plasma membrane to the cytosol. Furthermore, PKC activation perturbed the organization of the actin cytoskeleton, causing a redistribution of actin filaments to the submembranous or cortical actin cytoskeleton. As a consequence, transport of a protein traffic marker, the vesicular stomatitis virus glycoprotein, from the trans-Golgi network to the plasma membrane becomes perturbed. The effect of disruption of the actin filament network by cytochalasin D perfectly matched the effect of PKC. These data thus favor the existence of a causal relationship between actin rearrangement and docking and/or fusion of proteins to the plasma membrane. Interestingly, neither in control cells nor in PKC-activated cells did another protein traffic marker, influenza hemagglutinin (HA), reach the cell surface. However, an eminent and specific accumulation of HA just underneath the plasma membrane became apparent upon PKC activation. Yet, this effect could not be simulated by cytochalasin D treatment. Therefore, these observations imply that although MARCKS represents a prominent PKC target site in regulating differentiation, another target involves the differential control of cognate polarized trafficking pathways, which are apparently operating in oligodendrocyte progenitor cells.

Intense myristoylation of a single protein in the ocular lens

A single protein of the ocular lens was intensely myristoylated following short term incubation of cultured bovine lens epithelial cells and intact rat lenses with 3H-myristic acid. It was acidic (pI <5), about 19 kDa and present exclusively in the cytosol of both cultured epithelial cells and the epithelium of the young rat lens. Fiber cell proteins were not labeled. The myristoylated protein was not seen in the epithelium of the adult rat. Essentially no protein mass was evident in the 19-20 kDa range when samples of the labeled-soluble protein were fractionated by either HPLC coupled with SDS-PAGE or 2D-electrophoresis. These findings suggest that the myristoylated-soluble protein of 19 kDa in lens (p19L) is a rapidly-turning over minor protein likely associated with lens growth. The absence of any apparent membrane association for a myristoylated protein appears unusual. The trace nature of p19L has frustrated attempts at its identification by MALDI-MS.

Myristoylated alanine-rich C-kinase substrate is phosphorylated and translocated by a phorbol ester-insensitive and calcium-independent protein kinase C isoform in C6 glioma cell membranes

Myristoylated alanine-rich C-kinase substrate (MARCKS), a prominent substrate for conventional and novel protein kinase C (PKC) isoforms, is involved in the regulation of membrane-cytoskeletal interactions. Addition of [gamma-32P]ATP to the membrane fraction of digitonin-permeabilized C6 glioma cells resulted in phosphorylation and release of MARCKS, indicating involvement of an active membrane-bound kinase. Pretreatment of cells with 2 microM 4 beta-12-O-tetradecanoyl-phorbol-13-acetate (beta-TPA) for 18 h downregulated conventional (PKC alpha) and novel (PKC delta) isoforms of PKC by > 90% in both membrane and soluble fractions, but did not inhibit the rate of ATP-dependent phosphorylation or release of MARCKS, or decrease levels of membrane-bound PKC zeta or PKC mu. MARCKS phosphorylation was inhibited by staurosporine, bis-indolylmaleimide (a PKC-specific inhibitor), Gö6983 (inhibits all isoforms except PKC mu), and a peptide from the calmodulin-binding domain of MARCKS, but was unaffected by EGTA or Gö6976 (inhibits cPKCs and PKC mu). Peptide mapping indicated similar in vivo and in vitro phosphorylation at serine residue(s) known to be phosphorylated by PKC. These findings support a novel mechanism by which MARCKS may be regulated by an atypical PKC isoform in phorbol ester-downregulated cells.

MARCKS, membranes, and calmodulin: kinetics of their interaction

It is well documented that membrane binding of MARCKS (Myristoylated Alanine-Rich C-Kinase Substrate) requires both hydrophobic insertion of the N-terminal myristate into the bilayer and electrostatic interaction of the basic effector region with acidic lipids. The structure of a membrane-bound peptide corresponding to the effector region, residues 151-175 of bovine MARCKS, was recently determined using spin-labeled peptides and EPR. The kinetics of the peptide-membrane interaction were determined from stopped-flow fluorescence measurements; the adsorption of the peptide onto phospholipid vesicles is a diffusion-limited process. Five microM Ca2+-calmodulin decreases the lifetime of the peptide on a 100 nm diameter 10:1 PC/PS vesicle from 0.1 s to 0.01 s by rapidly pulling the peptide off the membrane. We propose a molecular mechanism, based on previous work by M. Eigen and colleagues, by which calmodulin may remove MARCKS(151-175) from the membrane at a diffusion-limited rate. Calmodulin may also use this mechanism to remove the pseudosubstrate region from the substrate binding site of enzymes such as calmodulin kinase II and myosin light chain kinase.

Unmyristoylated MARCKS-related protein (MRP) binds to supported planar phosphatidylcholine membranes

We have recently shown that unmyristoylated MARCKS-related protein (MRP) does not bind to neutral phospholipid vesicles, unless negatively charged phospholipids are present. Similar behaviour has also been reported for MARCKS itself. Here we have compared the binding of MRP to neutral and negatively charged supported planar lipid bilayer membranes (SPLM) using two-mode waveguide spectroscopy. We find appreciable binding of unmyristoylated MRP to neutral SPLM. We propose that hydrophobic residues in the effector domain constitute an additional factor capable of mediating MRP-membrane interaction.

Phospholipase D activity is altered in X-linked adrenoleukodystrophy heterozygous carriers, but not in hemizygous patients

Abnormalities in levels of choline and its metabolites have been reported in the lesions of brains of X-linked adrenoleukodystrophy (X-ALD) patients. We have examined the turnover of the major choline-containing phospholipid, phosphatidylcholine (PtdCho), in fibroblasts from hemizygous X-ALD, heterozygous X-ALD, Zellweger syndrome (ZW), and male and female control individuals to assess possible alterations in PtdCho metabolism mediated by activation of protein kinase C (PKC). Hydrolysis of PtdCho by phospholipase D (PLD) and resynthesis of PtdCho from labeled choline were stimulated 2- to 4-fold by PKC activation with the phorbol ester, 4beta-12-O-tetradecanoylphorbol-13-acetate (beta-TPA), in all cells except those from heterozygous X-ALD individuals. No differences in quantity or intracellular distribution of PKC activity, PKC isoforms by Western blot analysis, or of the PKC substrate, myristoylated alanine-rich C kinase substrate (MARCKS), were apparent in any of the cells. Thus, altered PtdCho metabolism was not directly linked to either of these inherited defects that result in abnormal peroxisomal functions. Further, altered responsiveness of PLD in X-ALD heterozygotes was independent of changes in PKC and MARCKS.

Overexpression of myristoylated alanine-rich C-kinase substrate enhances activation of phospholipase D by protein kinase C in SK-N-MC human neuroblastoma cells

Signal transduction can involve the activation of protein kinase C (PKC) and the subsequent phosphorylation of protein substrates, including myristoylated alanine-rich C kinase substrate (MARCKS). Previously we showed that stimulation of phosphatidylcholine (PtdCho) synthesis by PMA in SK-N-MC human neuroblastoma cells required overexpression of MARCKS, whereas PKCalpha alone was insufficient. We have now investigated the role of MARCKS in PMA-stimulated PtdCho hydrolysis by phospholipase D (PLD). Overexpression of MARCKS enhanced PLD activity 1.3-2.5-fold compared with vector controls in unstimulated cells, and 3-4-fold in cells stimulated with 100 nM PMA. PMA-stimulated PLD activity was blocked by the PKC inhibitor bisindolylmaleimide. Activation of PLD by PMA was linear with time to 60 min, whereas stimulation of PtdCho synthesis by PMA in clones overexpressing MARCKS was observed after a 15 min time lag, suggesting that the hydrolysis of PtdCho by PLD preceded synthesis. The formation of phosphatidylbutanol by PLD was greatest when PtdCho was the predominantly labelled phospholipid, indicating that PtdCho was the preferred, but not the only, phospholipid substrate for PLD. Cells overexpressing MARCKS had 2-fold higher levels of PKCalpha than in vector control cells analysed by Western blot analysis; levels of PKCbeta and PLD were similar in all clones. The loss of both MARCKS and PKCalpha expression at higher subcultures of the clones was paralleled by the loss of stimulation of PLD activity and PtdCho synthesis by PMA. Our results show that MARCKS is an essential link in the PKC-mediated activation of PtdCho-specific PLD in these cells and that the stimulation of PtdCho synthesis by PMA is a secondary response.

Binding of MARCKS (myristoylated alanine-rich C kinase substrate)-related protein (MRP) to vesicular phospholipid membranes

The myristoylated alanine-rich C kinase substrate (MARCKS) protein family has two known members, MARCKS itself and MARCKS-related protein (MRP, also called MacMARCKS or F52). They are essential for brain development and are believed to regulate the structure of the actin cytoskeleton at the plasma membrane. Hence membrane binding is central to their function. MARCKS has been quite extensively characterized; MRP much less so. Despite the fact that MRP is only two thirds the size of MARCKS, it has hitherto been assumed that the two proteins have similar properties. Here we make a detailed study, including the effects of myristoylation, lipid composition, calmodulin and phosphorylation of the binding of MRP to phospholipid vesicles. We show that both the N-terminal myristoyl moiety and the central effector domain mediate binding. MRP behaves like MARCKS in the presence of neutral phospholipids. In contrast to MARCKS, however, the incorporation of 20% of negatively-charged phospholipids only marginally increases the affinity of myristoylated MRP. Co-operativity between the myristoyl moiety and the effector domain of MRP is weak and the protein has a significantly lower affinity for these vesicles compared with MARCKS. Furthermore, calmodulin or phosphorylation of the effector domain by the catalytic subunit of protein kinase C do not significantly decrease the binding of myristoylated MRP to negatively-charged phospholipid vesicles. Our results show that the mechanisms regulating the interactions of MARCKS and MRP with phospholipid vesicles are, at least quantitatively, different. In agreement with cellular studies, we therefore propose that MARCKS and MRP have different subcellular localization and, consequently, different functions.

Identification and characterization of cathepsin B as the cellular MARCKS cleaving enzyme

The importance of regulating the cellular concentrations of the myristoylated alanine-rich C kinase substrate (MARCKS), a major cellular substrate of protein kinase C, is indicated by the fact that mice lacking MARCKS exhibit gross abnormalities of central nervous system development and die shortly after birth. We previously identified a novel means of regulating cellular MARCKS concentrations that involved a specific proteolytic cleavage of the protein and implicated a cysteine protease in this process (Spizz, G., and Blackshear, P. J. (1996) J. Biol. Chem. 271, 553-562). Here we show that p40, the carboxyl-terminal fragment resulting from this cleavage of MARCKS, was associated with the mitochondrial/lysosomal pellet fraction of human diploid fibroblasts and that its generation in cells was sensitive to treatment with NH4Cl. These data suggest the involvement of lysosomes in the generation and/or stability of p40. The MARCKS-cleaving enzyme (MCE) activity was peripherally associated with a 10,000 x g pellet fraction from bovine liver, and it co-purified with the activity and immunoreactivity of a lysosomal protease, cathepsin B. Cathepsin B catalyzed the generation of p40 from MARCKS in a cell-free system and behaved similarly to the MCE with respect to mutants of MARCKS previously shown to be poor substrates for the MCE. Treatment of fibroblasts with a cell-permeable, specific inhibitor of cathepsin B, CA074-Me, resulted in parallel time- and concentration-dependent inhibition of cathepsin B and MCE activity. Incubation of a synthetic MARCKS phosphorylation site domain peptide with purified cathepsin B resulted in cleavage of the peptide at sites consistent with preferred cathepsin B substrate sites. These data provide evidence for the identity of the MCE as cathepsin B and suggest that this cleavage most likely takes place within lysosomes, perhaps as a result of specific lysosomal targeting sequences within the MARCKS primary sequence. The data also suggest a direct interaction between MARCKS and cathepsin B in cells and leave open the possibility that MARCKS may in some way regulate the protease for which it is a substrate.

Inhibitors of actin polymerization and calmodulin binding enhance protein kinase C-induced translocation of MARCKS in C6 glioma cells

MARCKS (myristoylated alanine-rich C-kinase substrate) is known to interact with calmodulin, actin filaments, and anionic phospholipids at a central basic domain which is also the site of phosphorylation by protein kinase C (PKC). In the present study, cytochalasin D (CD) and calmodulin antagonists were used to examine the influence of F-actin and calmodulin on membrane interaction of MARCKS in C6 glioma cells. CD treatment for 1 h disrupted F-actin filaments, increased membrane bound immunoreactive MARCKS (from 51% to 62% of total), yet markedly enhanced the amount of MARCKS translocated to the cytosolic fraction in response to the phorbol ester 4beta-12-O-tetradecanoylphorbol 13-acetate. In contrast, CD treatment had no effect on phorbol ester-stimulated phosphorylation of MARCKS or on translocation of PKC alpha to the membrane fraction. Staurosporine also increased membrane association of MARCKS in a PKC-independent manner, as no change in MARCKS phosphorylation was noted and bis-indolylmaleimide (a more specific PKC inhibitor) did not alter MARCKS distribution. Staurosporine inhibited the phorbol ester-induced translocation of MARCKS but not of PKC alpha in both CD pretreated and untreated cells. Calmodulin antagonists (trifluoperazine, calmidazolium) had little effect on the cellular distribution or phosphorylation of MARCKS, but were synergistic with phorbol ester in translocating MARCKS from the membrane without a further increase in its phosphorylation. We conclude that cytoskeletal integrity is not required for phosphorylation and translocation of MARCKS in response to activated PKC, but that interaction with both F-actin and calmodulin might serve to independently modulate PKC-regulated localization and function of MARCKS at cellular membranes.

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-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.

Protein kinase C isoforms and growth, differentiation and phosphatidylcholine turnover in human neuroblastoma cells

Neuroblastoma and glioma cells differentially express isoforms of protein kinase C (PKC) and myristoylated PKC substrates (e.g. MARCKS). Correlation with metabolism of membrane phospholipids suggests that PKC-alpha and MARCKS may be required to mediate phosphatidylcholine turnover stimulated by phorbol ester (beta-TPA). To evaluate relationships to neural cell differentiation, SK-N-SH human neuroblastoma cells were treated with 20 nM beta-TPA. In beta-TPA-treated cells, growth arrest and differentiation occurred (neurite extension; 40-60% decrease in cell number, total protein and RNA). By day 4, mRNA for PKC-alpha and MARCKS increased and, after an initial decrease, PKC-alpha protein also increased. At day 4, phosphatidylcholine synthesis was 3-5 fold greater than in control cells. In contrast, C6 glioma cells treated with beta-TPA showed no growth arrest, decreased PKC-alpha protein (< 20%) and lower phosphatidylcholine synthesis. Thus, induced differentiation of human neuroblastoma cells involved increased expression of PKC-alpha and MARCKS and synthesis of phosphatidylcholine, consistent with involvement of PKC-alpha and MARCKS in regulation of phosphatidylcholine turnover during neurite growth.

Differential alterations of ethanolamine and choline phosphoglyceride metabolism by clofibrate and retinoic acid in human fibroblasts are not mediated by phorbol ester-sensitive protein kinase C

Peroxisomal proliferators and retinoids have been reported to interact to regulate lipid metabolism, particularly beta-oxidation of fatty acids. Based on postulated interactions of these agents at the levels of receptors and response elements, we examined whether interactions exist between the peroxisomal proliferator, clofibrate (CLF), and retinoic acid (RA) in modulation of phospholipid turnover in cultured human skin fibroblasts. Treatment of cultured cells with either 25 microM CLF or 1 microM RA alone decreased [14C]ethanolamine incorporation into ethanolamine phosphoglycerides (EPG) by 20-30%, and simultaneous exposure to both agents resulted in additive inhibition. By contrast, [3H]choline incorporation into phospholipid was stimulated 5-30% by incubation with either agent; when CLF and RA were administered together, the stimulatory effects were additive. Different types of pulse-chase studies examining effects on uptake, biosynthesis, and degradation of labelled phospholipids indicated stimulation of EPG degradation and inhibition of phosphatidylcholine degradation by CLF; no effect on catabolism of either phospholipid was observed with RA. Combinations of modifiers of protein kinase activity [4 beta-12-O-tetradecanoylphorbol-13-acetate (beta-TPA), 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride, N-(2'-guanidinoethyl)-5-isoquinolinesulfonamide hydrochloride, bis-indolylmaleimide, staurosporine indicated that beta-TPA-responsive protein kinases were not involved. Accordingly, CLF and RA regulate biosynthesis and degradation of ethanolamine and choline phosphoglycerides in cultured skin fibroblasts by different mechanisms that do not involve classical protein kinase C (PKC) isoforms, even though turnover of phospholipids generating lipid activators of PKC occurs.

Lipopolysaccharide stimulates differential expression of myristoylated protein kinase C substrates in murine microglia

Microglia rapidly respond to lipoplysaccharide (LPS) by transformation from resting to active states and secretion of several neuro- and immuno-regulators including tumour necrosis factor alpha (TNF-alpha), interleukin 1 beta (IL-1 beta), and interleukin 6 (IL-6). With longer LPS treatment, microglia are converted to reactive or phagocytic states with characteristics similar to macrophages in inflammation and injury processes. We have investigated LPS-mediated changes in two myristoylated substrates of protein kinase C (PKC): MARCKS (myristoylated alaninerich C kinase substrate) and MRP (MARCKS-related protein). Within 6 hours of addition, LPS induced a twofold increase in [3H]myristoylated and immunoreactive MARCKS protein and a sevenfold increase in MRP. The differential effect of LPS on expression of MRP vs. MARCKS was even more dramatic at the level of transcription: S1 nuclease protection assays revealed a 40-fold increase in MRP mRNA levels (maximum at 4-6 hours), whereas a threefold increase was observed for MARCKS. TNF alpha and colony-stimulating factor 1 (CSF-1), two cytokines which are induced by LPS, did not reproduce the observed effect of LPS on MARCKS and MRP gene transcription. CSF-1 also induced differential transcription of MRP, but of lower magnitude (threefold) and more sustained than by LPS. Accordingly, these two substrates for PKC are differentially up-regulated by LPS, apparently independent of TNF alpha or CSF-1.

Protein kinase C-mediated phosphorylation of the myristoylated alanine-rich C-kinase substrate protects it from specific proteolytic cleavage

The myristoylated alanine-rich C kinase substrate (MARCKS) is a major cellular substrate of protein kinase C. Its concentration in cells is important for the normal development of the central nervous system, and perhaps other physiological processes. We found that MARCKS concentrations in cells were regulated in part by a specific proteolytic cleavage; this resulted in two fragments, each representing about half of the intact protein, that co-existed with MARCKS in cells and tissues. These fragments were present in significant concentrations in quiescent fibroblasts; they disappeared, and the amount of intact MARCKS increased, within 15 s of activation of protein kinase C by serum. In vitro experiments demonstrated that phosphorylated MARCKS was a poor substrate for a protease activity present in cell extracts, whereas dephosphorylated MARCKS was a good substrate. Both the protease activity and the specific MARCKS cleavage products were essentially absent in brain, but present in many other cells and tissues. The protease activity, which had the characteristics of a cysteine protease, cleaved MARCKS between Asn147 and Glu148 of the bovine sequence, three amino acids to the amino-terminal side of the MARCKS phosphorylation site domain. These studies demonstrate that MARCKS is subjected to specific cleavage by a cellular protease, in a manner dependent on the phosphorylation state of the substrate. This represents a novel means of regulating cellular MARCKS concentrations; these data also raise the interesting possibility that MARCKS is involved in regulating the activity of this novel cellular protease.

Phorbol ester stimulation of phosphatidylcholine synthesis in four cultured neural cell lines: correlations with expression of protein kinase C isoforms

Phosphatidylcholine (PtdCho) can provide lipid second messengers involved in signal transduction pathways. As a measure of phospholipid turnover in response to extracellular stimulation, we investigated differential enhancement of [3H]choline incorporation into PtdCho by phorbol esters. In C6 rat glioma and SK-N-SH human neuroblastoma cells, [3H]PtdCho synthesis was 2-4 fold stimulated by beta-12-O-tetradecanoylphorbol-13-acetate (beta-TPA) when [3H]choline was incubated simultaneously with, or 15 min prior to, beta-TPA treatment. By contrast, in N1E-115 mouse and SK-N-MC human neuroblastoma cells, phorbol esters had no appreciable effect on [3H]choline incorporation; however, in all cells, 200 microM oleic acid enhanced PtdCho synthesis, indicating a stimulable process. Alterations by thymeleatoxin (TMT), an activator of conventional PKC isoforms (alpha, beta and gamma), were similar to beta-TPA. We investigated whether expression of specific PKC isoforms might correlate with these effects of phorbol esters on PtdCho synthesis. All cell lines bound phorbol esters, had PKC activity that was translocated by phorbol esters and differentially expressed isoforms of PKC. Northern and western blot analyses, using specific cDNA and antibodies for PKC-alpha, -beta, -gamma, -delta, -epsilon, and -zeta, revealed that expression of alpha-isoform predominated in C6 and SK-N-SH cells. In contrast, TPA-responsive beta-isoform predominated in SK-N-MC cells. gamma-PKC was not detected in any cells and only in C6 cells was PKC-delta present and translocated by beta-TPA treatment. PKC-epsilon was not detected in SK-N-MC cell lines but translocated with TPA treatment in the other three cell lines. PKC-zeta was present in all cells but was unaltered by TPA treatment. Accordingly, stimulation of PtdCho turnover by phorbol esters correlated only with expression of PKC-alpha; presence of PKC-beta alone was insufficient for a TPA response.

Cholecystokinin induced signaling in rat glioma C6 cells

We investigated the effect of sulphated cholecystokinin octapeptide (CCK-8S) on free intracellular calcium concentration, phosphatidylinositol metabolism, and protein phosphorylation in C6 cells, a rat glioma cell line which was shown to express CCKB type receptors. Increase in [Ca2+]i by both influx across the cell membrane and release from internal stores was demonstrated by utilizing a laser confocal imaging system. Because CCK-8S produced a transient elevation of inositol triphosphate level participation of InsP3 in calcium signaling in C6 cells is very likely. Protein kinase C seems to be involved in CCK-8S induced signaling in rat glioma C6 cells as demonstrated by using in vivo phosphorylation experiments.

Ca2+ x calmodulin prevents myristoylated alanine-rich kinase C substrate protein phosphorylation by protein kinase Cs in C6 rat glioma cells

Ionomycin stimulated membrane-associated protein kinase Cs (PKCs) activity in C6 rat glioma cells as much as the potent PKCs stimulator 12-O-tetradecanoyl phorbol 13-acetate (TPA). However, while TPA, as expected, powerfully stimulated the phosphorylation of the PKCs' 85-kDa myristoylated alanine-rich protein kinase C substrate (MARCKS) protein, ionomycin unexpectedly did not. Instead, ionomycin reduced the basal MARCKS phosphorylation. Pretreating the glioma cells with ionomycin prevented TPA-stimulated PKCs from phosphorylating the MARCKS protein. The stimulation of membrane PKCs activity and the prevention of MARCKS phosphorylation by ionomycin required external Ca2+ because they were both abolished by adding 5 mM EGTA to the culture medium. Recently (Chakravarthy, B. R., Isaacs, R. J., Morley, P., Durkin, J. P., and Whitfield, J. F. (1995) J. Biol. Chem. 270, 1362-1368), we proposed that Ca2+ x calmodulin complexes block MARCKS phosphorylation by the activated PKCs in keratinocytes stimulated by raising the external Ca2+ concentration. In the present experiments calmodulin prevented MARCKS phosphorylation by TPA-stimulated PKCs in glioma cell lysates, and this blockade was lifted by a calmodulin antagonist, the calmodulin-binding domain peptide. But, physiologically more significant, pretreating intact glioma cells with a cell-permeable calmodulin antagonist, calmidazolium, prevented ionomycin from blocking MARCKS phosphorylation by PKCs in unstimulated and TPA-stimulated cells. The effect of ionomycin on MARCKS phosphorylation was not due to the stimulation of Ca2+ x calmodulin-dependent phosphoprotein phosphatase, calcineurin, because cyclosporin A, a potent inhibitor of this phosphatase, did not stop ionomycin from preventing MARCKS phosphorylation. The ability of ionomycin to prevent TPA-stimulated PKCs from phosphorylating MARCKS depended on whether ionomycin was added before, with, or after TPA. Maximum blockade occurred when ionomycin was added before TPA but was less effective when added with or after TPA. These results indicate that Ca2+ x calmodulin can profoundly affect PKCs' signaling at the substrate level.

Calbindin expression in renal tubular epithelial cells. Altered sodium phosphate co-transport in association with cytoskeletal rearrangement

Sodium-phosphate transport in the opossum kidney (OK) cell line was studied in an OK clonal cell line that was transfected with an episomal vector expressing high levels of rat calbindin (28 kDa). High level expression of calbindin buffered the influx of calcium induced by ionomycin by 53% and raised the basal intracellular calcium from 100 +/- 6 to 150 +/- 8 nM. The decrement in sodium phosphate uptake induced by parathyroid hormone or forskolin was identical in the two cell lines. However, phorbol esters (10(-10)-10(-7) M), which decreased sodium phosphate uptake in the parental OK line, increased it in the calbindin-expressing line. Similarly, the parental clone did not respond to phosphate deprivation, while the calbindin-expressing clone did increase phosphate uptake in response to phosphate deprivation. In the calbindin-expressing cells, phorbol 12-myristate 13-acetate or low phosphate medium, which increased phosphate transport, produced actin filament aggregation, dissociation of the myristoylated alanine-rich C kinase substrate protein from sub-apical actin, and membrane-associated tyrosine phosphate staining. Agonists that reduced sodium phosphate uptake (cAMP, parathyroid hormone) did not affect these cellular features. The cytoskeletal rearrangement, redistribution of the myristoylated alanine-rich C kinase substrate protein, and membrane tyrosine phosphorylation are suggested to be involved in the events by which phosphate transport is increased in this cell line.

Membrane association of the myristoylated alanine-rich C kinase substrate (MARCKS) protein. Mutational analysis provides evidence for complex interactions

The myristoylated alanine-rich C kinase substrate (MARCKS) protein, a prominent cellular substrate for protein kinase C, is associated with membranes in various cell types. MARCKS is myristoylated at its amino terminus; this modification is thought to play the major role in anchoring MARCKS to cellular membranes. Recent studies have suggested that the protein's basic phosphorylation site/calmodulin binding domain may also be involved in the membrane association of MARCKS through electrostatic interactions. The present studies used mutations in the primary structure of the protein to investigate the nature of the association between MARCKS and cell membranes. In chick embryo fibroblasts, activation of protein kinase C led to a decrease in MARCKS membrane association as determined by cell fractionation techniques. Cell-free assays revealed that nonmyristoylated MARCKS exhibited almost no affinity for fibroblast membranes, despite readily demonstrable binding of the wild-type protein. Similar experiments in which the four serines in the phosphorylation site domain were mutated to aspartic acids, mimicking phosphorylation, decreased, but did not eliminate, membrane binding when compared to either the wild-type protein or a comparable tetra-asparagine mutant. Addition of calmodulin in the presence of Ca2+ also inhibited binding of the wild-type protein to membranes, presumably by neutralizing the phosphorylation site domain, or by physically interfering with its membrane association. Surprisingly, expression of a nonmyristoylatable mutant form of MARCKS in intact cells led to only a 46% decrease in its plasma membrane association, as determined by cell fractionation and immunoelectron microscopy. These results are consistent with a complex model of the interaction of MARCKS with cellular membranes, in which the myristoyl moiety, the positively charged phosphorylation site domain, and possibly other domains make independent contributions to membrane binding in intact cells.

Characterization of the phosphatidylserine-binding region of rat MARCKS (myristoylated, alanine-rich protein kinase C substrate). Its regulation through phosphorylation of serine 152

We reported previously that recombinant myristoylated, alanine-rich protein kinase C substrate (MARCKS) expressed in Escherichia coli as well as MARCKS purified from rat brain specifically bound to phosphatidylserine (PS) in a calcium-independent manner and that the binding was regulated through phosphorylation of MARCKS (Nakaoka, T., Kojima, N., Hamamoto, T., Kurosawa, N., Lee, Y. C., Kawasaki, H., Suzuki, K., and Tsuji, S. (1993) J. Biochem. (Tokyo) 114, 449-452). In this study, to identify the minimum PS-binding region of MARCKS and the regulatory phosphorylation site, the binding of MARCKS to PS was examined in deletion mutants producing glutathione S-transferase (GST) fusion proteins. The mutant proteins GST-6-180 and GST-127-160 had almost the same ability to bind to immobilized PS as MARCKS purified from rat brain, whereas GST-127-152 did not bind to it. In addition, the binding of GST-6-156 to immobilized PS was 62% of that of GST-6-180, but that of GST-6-152 was only 8% and that of GST-6-135 was not detected. The effect of phosphorylation by protein kinase C was examined in several mutants of GST-6-180 whose serine residues were substituted with alanine. After phosphorylation, the mutants GST-6-180[S156A and S163A], GST-6-180]S156A], and GST-6-180[S163A] did not bind to immobilized PS like native MARCKS and GST-6-180. However, even after phosphorylation, GST-6-180-[S152A] and GST-6-180[S152A and S156A] could bind to immobilized PS. These results strongly suggest that MARCKS binds to PS molecules in the inner leaflet of the plasma membrane through residues 127-156, with residues 153-156 (FKKS) being particularly important in the binding of MARCKS to PS, and that the binding is regulated through the protein kinase C-catalyzed phosphorylation of the serine at residue 152.

Bombesin, endothelin and platelet-derived growth factor induce rapid translocation of the myristoylated alanine-rich C-kinase substrate in Swiss 3T3 cells

We analyzed the effect of growth factors on the localization of the 80-kDa acidic myristoylated alanine-rich C-kinase substrate (80-kDa MARCKS), the major protein kinase C (PKC) substrate, in Swiss 3T3 fibroblasts. Virtually all 80-kDa MARCKS of quiescent cultures of these cells was membrane bound. However, within 40 min after addition of bombesin (10 nM) to these cells, the content of 80-kDa MARCKS in the cytoplasmic fraction increased 25-fold. Phosphorylated 80-kDa MARCKS was detectable in the cytoplasmic fraction as early as 30 s after addition of bombesin and the translocation was sustained for 6 h i.e. until 80-kDa MARCKS became down-regulated. The ability of bombesin to stimulate translocation of 80-kDa MARCKS was dose-dependent (concentration required to produce 50% of the effect was 0.6 nM bombesin) and was abolished by the specific antagonist [Leu14,13 psi 14CH2NH]bombesin. Furthermore, platelet-derived growth factor (PDGF) stimulated a dose-dependent (concentration required to produce 50% of the effect was 3 ng/ml) translocation which was comparable to that induced by bombesin in terms of kinetics and magnitude. Translocation was independent of continuous protein synthesis, but dependent on active PKC. Depletion or inhibition of PKC activity abolished the 80-kDa MARCKS translocation induced by either bombesin or PDGF. Furthermore, the neuropeptides beta-endothelin, bradykinin, and vasopressin, which are known to stimulate PKC activity, also promoted translocation. In contrast, epidermal growth factor, insulin and forskolin, which do not activate PKC, failed to cause such an effect. Translocation of 80-kDa MARCKS was also observed in Rat1 cells treated with phorbol ester, PDGF and beta-endothelin. We conclude that the translocation of 80-kDa MARCKS from the membrane to the cytosol is an early response to a variety of growth-promoting factors that stimulate PKC through different signal-transduction pathways.