Demyristoylation of the major substrate of protein kinase C (MARCKS) by the cytoplasmic fraction of brain synaptosomes

A peptide against the N-terminus of myristoylated alanine-rich C kinase substrate promotes neuronal differentiation in SH-SY5Y human neuroblastoma cells

Myristoylated alanine-rich protein kinase C substrate (MARCKS) plays crucial roles in neuronal functions and differentiation. However, specific effects of the myristoylated N-terminal sequence (MANS) peptide, a widely used MARCKS modulator comprising the initial 24 amino acids of MARCKS, on neuronal cells remain unclear. Therefore, in this study, we aimed to examine the effects and action mechanisms of the MANS peptide on SH-SY5Y human neuroblastoma cells, which served as the in vitro neuronal cell models. MANS treatment of SH-SY5Y cells resulted in significant neurite outgrowth within 24 hr, which was as prominent as that induced by seven days of treatment with all-trans retinoic acid, the most common agent used to induce SH-SY5Y cell differentiation. Levels of synaptophysin, a neuronal marker protein, were significantly increased in the MANS peptide-treated cells. Additionally, increased MARCKS levels and decreased MARCKS phosphorylation were observed in MANS peptide-treated cells. Notably, neurite outgrowth induced by the MANS peptide was significantly reduced in MARCKS-knocked-down cells. Overall, these results suggest the MANS peptide as a novel agent for SH-SY5Y cell differentiation, particularly for the analysis of MARCKS functions.

Insights into the post-translational modification and its emerging role in shaping the tumor microenvironment

More and more in-depth studies have revealed that the occurrence and development of tumors depend on gene mutation and tumor heterogeneity. The most important manifestation of tumor heterogeneity is the dynamic change of tumor microenvironment (TME) heterogeneity. This depends not only on the tumor cells themselves in the microenvironment where the infiltrating immune cells and matrix together forming an antitumor and/or pro-tumor network. TME has resulted in novel therapeutic interventions as a place beyond tumor beds. The malignant cancer cells, tumor infiltrate immune cells, angiogenic vascular cells, lymphatic endothelial cells, cancer-associated fibroblastic cells, and the released factors including intracellular metabolites, hormonal signals and inflammatory mediators all contribute actively to cancer progression. Protein post-translational modification (PTM) is often regarded as a degradative mechanism in protein destruction or turnover to maintain physiological homeostasis. Advances in quantitative transcriptomics, proteomics, and nuclease-based gene editing are now paving the global ways for exploring PTMs. In this review, we focus on recent developments in the PTM area and speculate on their importance as a critical functional readout for the regulation of TME. A wealth of information has been emerging to prove useful in the search for conventional therapies and the development of global therapeutic strategies.

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.

N-Myristoyltransferase as a Glycine and Lysine Myristoyltransferase in Cancer, Immunity, and Infections

Protein myristoylation, the addition of a 14-carbon saturated acyl group, is an abundant modification implicated in biological events as diverse as development, immunity, oncogenesis, and infections. N-Myristoyltransferase (NMT) is the enzyme that catalyzes this modification. Many elegant studies have established the rules guiding the catalysis including substrate amino acid sequence requirements with the indispensable N-terminal glycine, and a co-translational mode of action. Recent advances in technology such as the development of fatty acid analogs, small molecule inhibitors, and new proteomic strategies, allowed a deeper insight into the NMT activity and function. Here we focus on discussing recent work demonstrating that NMT is also a lysine myristoyltransferase, the enzyme's regulation by a previously unnoticed solvent channel, and the mechanism of NMT regulation by protein-protein interactions. We also summarize recent findings on NMT's role in cancer, immunity, and infections and the advances in pharmacological targeting of myristoylation. Our analyses highlight opportunities for further understanding and discoveries.

Interaction between Polysialic Acid and the MARCKS-ED Peptide at the Molecular Level

Polysialic acid (polySia) is a highly negatively charged linear homopolymer comprising α-2,8-linked sialic acids. It is abundant in the embryonic brain and modulates various functions such as differentiation and synaptic plasticity in the adult central nervous system by direct binding to its protein partners. One such example is the binding of polySia to myristoylated-alanine rich C-kinase substrate (MARCKS) to modulate neuritogenesis. To understand their interaction mechanism at the molecular level, we performed a binding assay which showed a direct binding of the MARCKS-ED peptide (KKKKKRFSFKKSFKLSGFSFKKNKK) with polySia in a concentration-dependent manner. Molecular dynamics simulations revealed that this binding is not exclusively dominated by electrostatics but can in part be attributed to the presence of near-regularly spaced Phe residues, that confer a compact 3D conformation based on pseudoglycine loop structures supported by Phe-Phe interactions. Our simulations, which are confirmed by circular dichroism measurements, also indicate that the peptide-polySia binding induces large-scale conformational rearrangement of polySia into coils at the binding site, whereas the peptide conformation is relatively unperturbed. As a consequence, we predict that each peptide can bind to a domain extending ∼14 polySia repeat units. Using the fluorescently tagged MARCKS-ED peptide on rat brainstem tissue sections, we demonstrate the ability of the peptide to detect polySia, similarly to polySia-specific antibody mAb735, especially in the spinal trigeminal nucleus and the dorsal vagal complex. This study provides information about the interaction between polySia and its CNS protein binding partner, MARCKS, and provides a fundamental platform for further studies to explore the prospect of the MARCKS-ED as an effective polySia-binding peptide for bioimaging and drug delivery applications.

Grb7-derived calmodulin-binding peptides inhibit proliferation, migration and invasiveness of tumor cells while they enhance attachment to the substrate

The growth factor receptor bound protein 7 (Grb7) is a Ca²⁺-dependent calmodulin (CaM)-binding adaptor protein implicated, among other functions, in cell proliferation, migration and tumor-associated angiogenesis. The goal of this study was to determine whether a peptide based on the CaM binding site of Grb7 disrupts cellular processes, relevant for the malignancy of tumor cells, in which this adaptor protein is implicated. We designed synthetic myristoylated and non-myristoylated peptides corresponding to the CaM-binding domain of human Grb7 with the sequence ²⁴³RKLWKRFFCFLRRS²⁵⁶ and a variant peptide with the mutated sequence RKLERFFCFLRRE (W246E-ΔK247-S256E). The two non-myristoylated peptides bind dansyl-CaM with higher efficiency in the presence than in the absence of Ca²⁺ and they enter into the cell, as tested with 5(6)-carboxytetramethylrhodamine (TAMRA)-labeled peptides. The myristoylated and non-myristoylated peptides inhibit the proliferation, migration and invasiveness of A431 tumor cells while they enhance their adhesion to the substrate. The myristoylated peptides have stronger inhibitory effect than the non-myristoylated counterparts, in agreement with their expected higher cell-permeant capacity. The myristoylated and non-myristoylated W246E-ΔK247-S256E mutant peptide has a lesser inhibitory effect on cell proliferation as compared to the wild-type peptide. We also demonstrated that the myristoylated peptides were more efficient than the CaM antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) inhibiting cell migration and equally efficient inhibiting cell proliferation.

Tip60- and sirtuin 2-regulated MARCKS acetylation and phosphorylation are required for diabetic embryopathy

Failure of neural tube closure results in severe birth defects and can be induced by high glucose levels resulting from maternal diabetes. MARCKS is required for neural tube closure, but the regulation and of its biological activity and function have remained elusive. Here, we show that high maternal glucose induced MARCKS acetylation at lysine 165 by the acetyltransferase Tip60, which is a prerequisite for its phosphorylation, whereas Sirtuin 2 (SIRT2) deacetylated MARCKS. Phosphorylated MARCKS dissociates from organelles, leading to mitochondrial abnormalities and endoplasmic reticulum stress. Phosphorylation dead MARCKS (PD-MARCKS) reversed maternal diabetes-induced cellular organelle stress, apoptosis and delayed neurogenesis in the neuroepithelium and ameliorated neural tube defects. Restoring SIRT2 expression in the developing neuroepithelium exerted identical effects as those of PD-MARCKS. Our studies reveal a new regulatory mechanism for MARCKS acetylation and phosphorylation that disrupts neurulation under diabetic conditions by diminishing the cellular organelle protective effect of MARCKS.

MARCKS and MARCKS-like proteins in development and regeneration

BACKGROUND

The Myristoylated Alanine-Rich C-kinase Substrate (MARCKS) and MARCKS-like protein 1 (MARCKSL1) have a wide range of functions, ranging from roles in embryonic development to adult brain plasticity and the inflammatory response. Recently, both proteins have also been identified as important players in regeneration. Upon phosphorylation by protein kinase C (PKC) or calcium-dependent calmodulin-binding, MARCKS and MARCKSL1 translocate from the membrane into the cytosol, modulating cytoskeletal actin dynamics and vesicular trafficking and activating various signal transduction pathways. As a consequence, the two proteins are involved in the regulation of cell migration, secretion, proliferation and differentiation in many different tissues.

MAIN BODY

Throughout vertebrate development, MARCKS and MARCKSL1 are widely expressed in tissues derived from all germ layers, with particularly strong expression in the nervous system. They have been implicated in the regulation of gastrulation, myogenesis, brain development, and other developmental processes. Mice carrying loss of function mutations in either Marcks or Marcksl1 genes die shortly after birth due to multiple deficiencies including detrimental neural tube closure defects. In adult vertebrates, MARCKS and MARCKL1 continue to be important for multiple regenerative processes including peripheral nerve, appendage, and tail regeneration, making them promising targets for regenerative medicine.

CONCLUSION

This review briefly summarizes the molecular interactions and cellular functions of MARCKS and MARCKSL1 proteins and outlines their vital roles in development and regeneration.

Myristoylation: An Important Protein Modification in the Immune Response

Protein N-myristoylation is a cotranslational lipidic modification specific to the alpha-amino group of an N-terminal glycine residue of many eukaryotic and viral proteins. The ubiquitous eukaryotic enzyme, N-myristoyltransferase, catalyzes the myristoylation process. Precisely, attachment of a myristoyl group increases specific protein–protein interactions leading to subcellular localization of myristoylated proteins with its signaling partners. The birth of the field of myristoylation, a little over three decades ago, has led to the understanding of the significance of protein myristoylation in regulating cellular signaling pathways in several biological processes especially in carcinogenesis and more recently immune function. This review discusses myristoylation as a prerequisite step in initiating many immune cell signaling cascades. In particular, we discuss the hitherto unappreciated implication of myristoylation during myelopoiesis, innate immune response, lymphopoiesis for T cells, and the formation of the immunological synapse. Furthermore, we discuss the role of myristoylation in inducing the virological synapse during human immunodeficiency virus infection as well as its clinical implication. This review aims to summarize existing knowledge in the field and to highlight gaps in our understanding of the role of myristoylation in immune function so as to further investigate into the dynamics of myristoylation-dependent immune regulation.

Protein myristoylation in health and disease

N-myristoylation is the attachment of a 14-carbon fatty acid, myristate, onto the N-terminal glycine residue of target proteins, catalysed by N-myristoyltransferase (NMT), a ubiquitous and essential enzyme in eukaryotes. Many of the target proteins of NMT are crucial components of signalling pathways, and myristoylation typically promotes membrane binding that is essential for proper protein localisation or biological function. NMT is a validated therapeutic target in opportunistic infections of humans by fungi or parasitic protozoa. Additionally, NMT is implicated in carcinogenesis, particularly colon cancer, where there is evidence for its upregulation in the early stages of tumour formation. However, the study of myristoylation in all organisms has until recently been hindered by a lack of techniques for detection and identification of myristoylated proteins. Here we introduce the chemistry and biology of N-myristoylation and NMT, and discuss new developments in chemical proteomic technologies that are meeting the challenge of studying this important co-translational modification in living systems.

The fibroblast growth factor receptor substrate 3 adapter is a developmentally regulated microtubule-associated protein expressed in migrating and differentiated neurons

Fibroblast growth factor (FGF) mediated signaling is essential to many aspects of neural development. Activated FGF receptors signal primarily through the FGF receptor substrate (Frs) adapters, which include Frs2/Frs2alpha and Frs3/Frs2beta. While some studies suggest that Frs3 can compensate for the loss of Frs2 in transfected cells, the lack of an effective Frs3 specific antibody has prevented efforts to determine the role(s) of the endogenous protein. To this end, we have generated a Frs3 specific antibody and have characterized the pattern of Frs3 expression in the developing nervous system, its subcellular localization as well as its biochemical properties. We demonstrate that Frs3 is expressed at low levels in the ventricular zone of developing cortex, between E12 and E15, and it co-localizes with nestin and acetylated alpha-tubulin in radial processes in the ventricular/subventricular zones as well as with betaIII tubulin in differentiated cortical neurons. Subcellular fractionation studies demonstrate that endogenous Frs3 is both soluble and plasma membrane associated while Frs3 expressed in 293T cells associates exclusively with lipid rafts. Lastly, we demonstrate that neuronal Frs3 binds microtubules comparable to the microtubule-associated protein, MAP2, while Frs2 does not. Collectively, these data suggest that neuronal Frs3 functions as a novel microtubule binding protein and they provide the first biochemical evidence that neuronal Frs3 is functionally distinct from Frs2/Frs2alpha.

Membrane binding of the neuronal calcium sensor recoverin - modulatory role of the charged carboxy-terminus

BACKGROUND

The Ca2+-binding protein recoverin operates as a Ca2+-sensor in vertebrate photoreceptor cells. It undergoes a so-called Ca2+-myristoyl switch when cytoplasmic Ca2+-concentrations fluctuate in the cell. Its covalently attached myristoyl-group is exposed at high Ca2+-concentrations and enables recoverin to associate with lipid bilayers and to inhibit its target rhodopsin kinase. At low Ca2+-concentrations the myristoyl group is inserted into a hydrophobic pocket of recoverin thereby relieving inhibitory constraint on rhodopsin kinase. Hydrophobic and electrostatic interactions of recoverin with membranes have not been clearly determined, in particular the function of the positively charged carboxy-terminus in recoverin 191QKVKEKLKEKKL202 in this context is poorly understood.

RESULTS

Binding of myristoylated recoverin to lipid bilayer depends on the charge distribution in phospholipids. Binding was tested by equilibrium centrifugation and surface plasmon resonance (SPR) assays. It is enhanced to a certain degree by the inclusion of phosphatidylserine (up to 60%) in the lipid mixture. However, a recoverin mutant that lacked the charged carboxy-terminus displayed the same relative binding amplitudes as wildtype (WT) recoverin when bound to neutral or acidic lipids. Instead, the charged carboxy-terminus of recoverin has a significant impact on the biphasic dissociation of recoverin from membranes. On the other hand, the nonmyristoylated WT and truncated mutant form of recoverin did not bind to lipid bilayers to a substantial amount as binding amplitudes observed in SPR measurements are similar to bulk refractive index changes.

CONCLUSION

Our data indicate a small, but evident electrostatic contribution to the overall binding energy of recoverin association with lipid bilayer. Properties of the charged carboxy-terminus are consistent with a role of this region as an internal effector region that prolongs the time recoverin stays on the membrane by influencing its Ca2+-sensitivity.

Thematic review series: Lipid Posttranslational Modifications. Lysosomal metabolism of lipid-modified proteins

Much is now understood concerning the synthesis of prenylated and palmitoylated proteins, but what is known of their metabolic fate? This review details metabolic pathways for the lysosomal degradation of S-fatty acylated and prenylated proteins. Central to these pathways are two lysosomal enzymes, palmitoyl-protein thioesterase (PPT1) and prenylcysteine lyase (PCL). PPT1 is a soluble lipase that cleaves fatty acids from cysteine residues in proteins during lysosomal protein degradation. Notably, deficiency in the enzyme causes a neurodegenerative lysosomal storage disorder, infantile neuronal ceroid lipofuscinosis. PCL is a membrane-associated flavin-containing lysosomal monooxygenase that metabolizes prenylcysteine to prenyl aldehyde through a completely novel mechanism. The eventual metabolic fates of other lipidated proteins (such as glycosylphosphatidylinositol-anchored and N-myristoylated proteins) are poorly understood, suggesting directions for future research.

The myristoylation of TRIF-related adaptor molecule is essential for Toll-like receptor 4 signal transduction

TRIF-related adaptor molecule (TRAM) is the fourth Toll/IL-1 resistance domain-containing adaptor to be described that participates in Toll-like receptor (TLR) signaling. TRAM functions exclusively in the TLR4 pathway. Here we show by confocal microscopy that TRAM is localized in the plasma membrane and the Golgi apparatus, where it colocalizes with TLR4. Membrane localization of TRAM is the result of myristoylation because mutation of a predicted myristoylation site in TRAM (TRAM-G2A) brought about dissociation of TRAM from the membrane and its relocation to the cytosol. Further, TRAM, but not TRAM-G2A, was radiolabeled with [3H]myristate in vivo. Unlike wild-type TRAM, overexpression of TRAM-G2A failed to elicit either IFN regulatory factor 3 or NF-kappaB signaling. Moreover, TRAM-G2A was unable to reconstitute LPS responses in bone marrow-derived macrophages from TRAM-deficient mice. These observations provide clear evidence that the myristoylation of TRAM targets it to the plasma membrane, where it is essential for LPS responses through the TLR4 signal transduction pathway, and suggest a hitherto unappreciated manner in which LPS responses can be regulated.

Translocation of an endoproteolytically cleaved maxi-K channel isoform: mechanisms to induce human myometrial cell repolarization

Large conductance Ca(2+)- and voltage-activated K+ (maxi-K) channels modulate human myometrial smooth muscle cell (hMSMC) excitability; however, the role of individual alternatively spliced isoforms remains unclear. We have previously shown that the transcript of a human maxi-K channel isoform (mK44) is expressed predominantly in myometrial and aortic smooth muscle and forms a functional channel in heterologous expression systems. The mK44 isoform contains unique consensus motifs for both endoproteolytic cleavage and N-myristoylation, although the function of these post-translational modifications is unknown. The goal of these studies was to determine the role of post-translational modifications in regulating mK44 channel function in hMSMCs. An mK44-specific antibody indicated that this channel is localized intracellularly in hMSMCs and translocates to the cell membrane in response to increases in intracellular Ca(2+). Immunological analyses using an N-terminally myc-tagged mK44 construct demonstrated endoproteolytical cleavage of mK44 in hMSMCs resulting in membrane localization of the mK44 N-termini and intracellular retention of the pore-forming C-termini. Caffeine-induced Ca(2+) release from intracellular stores resulted in translocation of the C-termini of mK44 to the cell membrane and co-localization with its N-termini. Translocation of mK44 channels to the cell membrane was concomitant with repolarization of the hMSMCs. Endoproteolytic digest of mK44 did not occur in HEK293 cells or mouse fibroblasts. MK44 truncated at a putative N-myristoylation site did not produce current when expressed alone, but formed a functional channel when co-expressed with the N-terminus. These findings provide novel insight into cell-specific regulation of maxi-K channel function.

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.

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.

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.

N-terminal N-myristoylation of proteins: refinement of the sequence motif and its taxon-specific differences

N-terminal N-myristoylation is a lipid anchor modification of eukaryotic and viral proteins targeting them to membrane locations, thus changing the cellular function of modified proteins. Protein myristoylation is critical in many pathways; e.g. in signal transduction, apoptosis, or alternative extracellular protein export. The myristoyl-CoA:protein N-myristoyltransferase (NMT) recognizes the sequence motif of appropriate substrate proteins at the N terminus and attaches the lipid moiety to the absolutely required N-terminal glycine residue. Reliable recognition of capacity for N-terminal myristoylation from the substrate protein sequence alone is desirable for proteome-wide function annotation projects but the existing PROSITE motif is not practical, since it produces huge numbers of false positive and even some false negative predictions. As a first step towards a new prediction method, it is necessary to refine the sequence motif coding for N-terminal N-myristoylation. Relying on the in-depth study of the amino acid sequence variability of substrate proteins, on binding site analyses in X-ray structures or 3D homology models for NMTs from various taxa, and on consideration of biochemical data extracted from the scientific literature, we found indications that, at least within a complete substrate protein, the N-terminal 17 protein residues experience different types of variability restrictions. We identified three motif regions: region 1 (positions 1-6) fitting the binding pocket; region 2 (positions 7-10) interacting with the NMT's surface at the mouth of the catalytic cavity; and region 3 (positions 11-17) comprising a hydrophilic linker. Each region was characterized by physical requirements to single sequence positions or groups of positions regarding volume, polarity, backbone flexibility and other typical properties of amino acids (http://mendel.imp.univie.ac.at/myristate/). These specificity differences are confined partly to taxonomic ranges and are proposed for the design of NMT inhibitors in pathogenic fungal and protozoan systems including Aspergillus fumigatus, Leishmania major, Trypanosoma cruzi, Trypanosoma brucei, Giardia intestinalis, Entamoeba histolytica, Pneumocystis carinii, Strongyloides stercoralis and Schistosoma mansoni. An exhaustive search for NMT-homologues led to the discovery of two putative entomopoxviral NMTs.

Myristoylation-dependent N-terminal cleavage of the myristoylated alanine-rich C kinase substrate (MARCKS) by cellular extracts

The myristoylated alanine-rich C kinase substrate (MARCKS) has been proposed to regulate the plasticity of the actin cytoskeleton at its site of attachment to membranes. In macrophages, MARCKS is implicated in various cellular events including motility, adhesion and phagocytosis. In this report we show that macrophage extracts contain a protease which specifically cleaves human MARCKS, expressed in a cell-free system or in E. coli, between Lys-6 and Thr-7. Cleavage of MARCKS decreases its affinity for macrophage membranes by ca. one order of magnitude, highlighting the contribution of the myristoyl moiety of MARCKS to membrane binding. Importantly, cleavage requires myristoylation of MARCKS. Furthermore, MARCKS-related protein (MRP), the second member of the MARCKS family, is not digested. Since Thr-7 is lacking in MRP this suggests that Thr-7 at the P1 position is important for the recognition of lipid-modified substrates. A different product is observed when MARCKS is incubated with a calf brain cytosolic extract. This product can be remyristoylated in the presence of myristoyl-CoA and N-myristoyl transferase, demonstrating that cycles of myristoylation/demyristoylation of MARCKS can be achieved in vitro. Although the physiological relevance of these enzymes still needs to be demonstrated, our results reveal the presence of a new class of cleaving enzymes recognizing lipid-modified protein substrates.

Protein myristoylation in protein-lipid and protein-protein interactions

Various proteins in signal transduction pathways are myristoylated. Although this modification is often essential for the proper functioning of the modified protein, the mechanism by which the modification exerts its effects is still largely unknown. Here we discuss the roles played by protein myristoylation, in both protein-lipid and protein-protein interactions. Myristoylation is involved in the membrane interactions of various proteins, such as MARCKS and endothelial NO synthase. The intermediate hydrophobic nature of the modification plays an important role in the reversible membrane anchoring of these proteins. The anchoring is strengthened by a basic amphiphilic domain that works as a switch for the reversible binding. Protein myristoylation is also involved in protein-protein interactions, which are regulated by the interplay between protein phosphorylation, calmodulin binding, and membrane phospholipids.

Regulation of angiotensin II-induced neuromodulation by MARCKS in brain neurons

Angiotensin II (Ang II) exerts chronic stimulatory actions on tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DbetaH), and the norepinephrine transporter (NET), in part, by influencing the transcription of their genes. These neuromodulatory actions of Ang II involve Ras-Raf-MAP kinase signal transduction pathways (Lu, D., H. Yang, and M.K. Raizada. 1997. J. Cell Biol. 135:1609-1617). In this study, we present evidence to demonstrate participation of another signaling pathway in these neuronal actions of Ang II. It involves activation of protein kinase C (PKC)beta subtype and phosphorylation and redistribution of myristoylated alanine-rich C kinase substrate (MARCKS) in neurites. Ang II caused a dramatic redistribution of MARCKS from neuronal varicosities to neurites. This was accompanied by a time-dependent stimulation of its phosphorylation, that was mediated by the angiotensin type 1 receptor subtype (AT1). Incubation of neurons with PKCbeta subtype specific antisense oligonucleotide (AON) significantly attenuated both redistribution and phosphorylation of MARCKS. Furthermore, depletion of MARCKS by MARCKS-AON treatment of neurons resulted in a significant decrease in Ang II-stimulated accumulation of TH and DbetaH immunoreactivities and [3H]NE uptake activity in synaptosomes. In contrast, mRNA levels of TH, DbetaH, and NET were not influenced by MARKS-AON treatment. MARCKS pep148-165, which contains PKC phosphorylation sites, inhibited Ang II stimulation of MARCKS phosphorylation and reduced the amount of TH, DbetaH, and [3H]NE uptake in neuronal synaptosomes. These observations demonstrate that phosphorylation of MARCKS by PKCbeta and its redistribution from varicosities to neurites is important in Ang II-induced synaptic accumulation of TH, DbetaH, and NE. They suggest that a coordinated stimulation of transcription of TH, DbetaH, and NET, mediated by Ras-Raf-MAP kinase followed by their transport mediated by PKCbeta-MARCKS pathway are key in persistent stimulation of Ang II's neuromodulatory actions.

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.

Acylation of Escherichia coli hemolysin: a unique protein lipidation mechanism underlying toxin function

The pore-forming hemolysin (HlyA) of Escherichia coli represents a unique class of bacterial toxins that require a posttranslational modification for activity. The inactive protoxin pro-HlyA is activated intracellularly by amide linkage of fatty acids to two internal lysine residues 126 amino acids apart, directed by the cosynthesized HlyC protein with acyl carrier protein as the fatty acid donor. This action distinguishes HlyC from all bacterial acyltransferases such as the lipid A, lux-specific, and nodulation acyltransferases, and from eukaryotic transferases such as N-myristoyl transferases, prenyltransferases, and thioester palmitoyltransferases. Most lipids directly attached to proteins may be classed as N-terminal amide-linked and internal ester-linked acyl groups and C-terminal ether-linked isoprenoid groups. The acylation of HlyA and related toxins does not equate to these but does appear related to a small number of eukaryotic proteins that include inflammatory cytokines and mitogenic and cholinergic receptors. While the location and structure of lipid moieties on proteins vary, there are common effects on membrane affinity and/or protein-protein interactions. Despite being acylated at two residues, HlyA does not possess a "double-anchor" motif and does not have an electrostatic switch, although its dependence on calcium binding for activity suggests that the calcium-myristoyl switch may have relevance. The acyl chains on HlyA may provide anchorage points onto the surface of the host cell lipid bilayer. These could then enhance protein-protein interactions either between HlyA and components of a host signal transduction pathway to influence cytokine production or between HlyA monomers to bring about oligomerization during pore formation.

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.

Myristoylated and nonmyristoylated pools of sea urchin sperm flagellar creatine kinase exist side-by-side: myristoylation is necessary for efficient lipid association

In sperm of the sea urchin Strongylocentrotus purpuratus, a functional phosphocreatine shuttle, that requires the existence of mitochondrial and cytosolic creatine kinase (CK) isoforms in distinct locations, is essential for sperm motility. S. purpuratus sperm have an unusually large, 145 kDa CK isoform, present exclusively in the sperm tail (TCK), that is enriched in flagellum membrane preparations. Purified TCK contains two very similar proteins, designated TCKI and TCKII, of which only TCKII associates readily with liposomes and detergent micelles in vitro. Here we demonstrate by gas chromatography/mass spectrometry combined with selective ion monitoring that ions diagnostic for the presence of myristoylglycine in proteins are found in TCKII, but not TCKI. By contrast, TCKI, but not TCKII, served in vitro as a substrate for recombinant, polyhistidine-tagged N-myristoyltransferase and was myristoylated to high stoichiometries (0.58 +/- 0.14 pmol of myristate/pmol of TCK), in the presence of myristoyl-CoA, on glycine in amide linkage. In vitro myristoylated TCKI associated with phosphatidylcholine (PC)/phosphatidylserine (PS) (75:25) liposomes and Triton X-100 detergent micelles in gel filtration assays and with PC/PS liposomes in a centrifugation assay in the same manner as did TCKII. In gel filtration experiments, TCKI required at least 25-fold higher PC/PS liposome concentrations than TCKII to obtain 50% association. A partition coefficient of 0.8 x 10(5) M(-1) was determined for TCKII with PC/PS (75:25) liposomes in the centrifugation assay. Thus, myristoylated and nonmyristoylated forms of TCK exist side-by-side in the sea urchin flagellum, and myristoylation is essential for efficient liposome association of TCK.

Specific proteolytic cleavage of the myristoylated alanine-rich C kinase substrate between Asn 147 and Glu 148 also occurs in brain

The myristoylated alanine-rich C kinase substrate (MARCKS) is a major ubiquitous substrate of protein kinase C. The expression of the protein is regulated during cell cycle progression and cell proliferation. Specific proteolytic cleavage of the protein between Asn 147 and Glu 148 was described recently in cultured cells, and the corresponding proteolytic activity was observed in various tissue extracts except for brain. We purified a 40 kDa fragment of MARCKS from bovine brain that we characterized as the C-terminal specific fragment found in other tissues. The identification of the fragment was achieved by in vitro phosphorylation by protein kinase C, calcium-dependent interaction with calmodulin, mass spectrometric analysis, and N-terminal sequencing. These data suggest that specific proteolytic cleavage of MARCKS also occurs in brain and may be a general mechanism of down-regulation of the protein.

Myristoylation

N-myristoylation is an acylation process absolutely specific to the N-terminal amino acid glycine in proteins. This maturation process concerns about a hundred proteins in lower and higher eukaryotes involved in oncogenesis, in secondary cellular signalling, in infectivity of retroviruses and, marginally, of other virus types. Thy cytosolic enzyme responsible for this activity, N-myristoyltransferase (NMT), studied since 1987, has been purified from different sources. However, the studies of the specificities of the various NMTs have not progressed in detail except for those relating to the yeast cytosolic enzyme. Still to be explained are differences in species specificity and between various putative isoenzymes, also whether the data obtained from the yeast enzyme can be transposed to other NMTs. The present review discusses data on the various addressing processes subsequent to myristoylation, a patchwork of pathways that suggests myristoylation is only the first step of the mechanisms by which a protein associates with the membrane. Concerning the enzyme itself, there are evidences that NMT is also present in the endoplasmic reticulum and that its substrate specificity is different from that of the cytosolic enzyme(s). These differences have major implications for their differential inhibition and for their respective roles in several pathologies. For instance, the NMTs from mammalians are clearly different from those found in several microorganisms, which raises the question whether the NMT may be a new targets for fungicides. Finally, since myristoylation has a central role in virus maturation and oncogenesis, specific NMT inhibitors might lead to potent antivirus and anticancer agents.

Understanding covalent modifications of proteins by lipids: where cell biology and biophysics mingle

Much effort has been expended on the in vitro characterization of enzymes that covalently attach lipids to proteins. Less information is available about properties conferred on modified proteins by their attached lipid groups, but biophysical studies of simple model systems have begun to shed light on this issue. Recent evidence suggests that the specificity of lipid modifications may be dependent upon the intracellular compartmentalization of the lipid and protein substrates of lipidating enzymes. The function and targeting of their lipidated products appear to be regulated dynamically through addition or subtraction of lipid moieties, other covalent or noncovalent modifications, as well as several devices that at this point can only be inferred. This field of research illustrates the necessity of integrating cell-biological and biophysical perspectives.

Partial constitutive activation of pheromone responses by a palmitoylation-site mutant of a G protein alpha subunit in yeast

G protein alpha subunits are often myristoylated and/or palmitoylated near their amino terminus. The G protein alpha subunit in the yeast Saccharomyces cerevisiae (GPA1 gene product, Gpa1p) is known to be myristoylated, and this modification is essential for G protein activity in vivo. Here we examined whether Gpa1p is palmitoylated and determined the functional consequences of this modification. [3H]-Palmitic acid was incorporated into Gpa1p in cells expressing myc-tagged Gpa1p or Gpa1p-Gst. The label was released upon hydroxylamine treatment. Substitution of the conserved Cys 3 for Ser blocked incorporation of the label (Gpa1pC3S). Palmitoylation was also blocked by a mutation that prevents myristoylation (Gly2Ala), whereas the palmitoylation-site mutation had no effect on myristoylation of Gpa1p. Gpa1pC3S complemented the gpa1 delta mutation in vivo and formed a complex with G beta gamma that was able to undergo nucleotide exchange in vitro. However, basal and pheromone-induced FUSl-lacZ transcription were 2-5-fold higher in the C3S mutant. Pheromone-induced growth arrest was also enhanced by the mutation, but recovery from arrest was not affected. Like wild-type Gpa1p, the C3S mutant was predominantly membrane-associated. Upon Triton X-114 partitioning or high pH treatment, no difference in the membrane-binding properties of the wild-type Gpa1p and the C3S mutant was detected. By sucrose density gradient centrifugation of membranes, however, most of the mutant protein was mislocalized to a non-plasma membrane compartment, whereas G beta gamma localization was unaltered. Taken together, our data suggest that Gpa1p is palmitoylated via a thioester bond at Cys 3 and that palmitoylation plays a role in modulating Gpa1p signaling and membrane localization.

Interaction of calmodulin-binding domain peptides of nitric oxide synthase with membrane phospholipids: regulation by protein phosphorylation and Ca(2+)-calmodulin

Endothelial nitric oxide synthase (eNOS) is unique among the NO synthase isozymes in being modified with myristoyl group, which appears to be necessary for its membrane association. However, the presence of myristoylated eNOS in cytosolic fraction after the stimulation-dependent translocation of the enzyme from membrane to cytosol suggests that other regions may be involved in the eNOS-membrane interaction and its regulation. In this study, we have synthesized a 20-amino acid peptide corresponding to the putative calmodulin-binding domain of human eNOS and studied the interaction of the peptide with calmodulin and with various membrane phospholipids. The peptide formed a stoichiometric complex with calmodulin. Upon addition of various acidic phospholipids, the peptide showed a drastic conformational change from random coil to alpha-helix, as was evidenced by circular dichroism spectroscopy. These results suggest that the same domain of eNOS binds both calmodulin and membrane phospholipids. Furthermore, we found that the synthetic peptide was phosphorylated in vitro by protein kinase C. Phosphorylation of the peptide decreased its interaction with membrane phospholipids. Thus, our results raise the possibility that the calmodulin-binding domain is directly involved in the membrane association of eNOS and that phosphorylation of the domain and Ca(2+)-calmodulin may regulate the interaction. Synthetic peptides corresponding to the calmodulin-binding domains of macrophage and neuronal isozymes showed similar abilities to bind phospholipids, suggesting that the calmodulin-binding domains of NO synthase serve as the phospholipid-binding domains as well.

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.

Regulation of cellular signalling by fatty acid acylation and prenylation of signal transduction proteins

Covalent modification by fatty acylation and prenylation occurs on a wide variety of cellular signalling proteins. The enzymes that catalyze attachment of these lipophilic moieties to proteins have recently been identified and characterized. Each lipophilic group confers unique properties to the modified protein, resulting in alterations in protein/protein interactions, membrane binding and targeting, and intracellular signalling. The biochemistry and cell biology of protein myristoylation, farnesylation and geranylgeranylation is reviewed here, with emphasis on the Src family of tyrosine kinases, Ras proteins and G protein coupled signalling systems.

Site-specific phosphorylation of synapsin I by mitogen-activated protein kinase and Cdk5 and its effects on physiological functions

Posttranslational modifications of synapsin I, a major phosphoprotein in synaptic terminals, were studied by mass spectrometry. In addition to a well known phosphorylation site by calmodulin-dependent protein kinase II (CaM kinase II), a hitherto unrecognized site (Ser553) was found phosphorylated in vivo. The phosphorylation site is immediately followed by a proline, suggesting that the protein is an in vivo substrate of so-called proline-directed protein kinase(s). To identify the kinase involved, three proline-directed protein kinases expressed highly in the brain, i.e. mitogen-activated protein (MAP) kinase, Cdk5-p23, and glycogen synthase kinase 3beta, were tested for the in vitro phosphorylation of synapsin I. Only MAP kinase and Cdk5-p23 phosphorylated synapsin I stoichiometrically. The phosphorylation sites were determined to be Ser551 and Ser553 with Cdk5-p23, and Ser62, Ser67, and Ser551 with MAP kinase. Upon phosphorylation with MAP kinase, synapsin I showed reduced F-actin bundling activity, while no significant effect on the interaction was observed with the protein phosphorylated with Cdk5-p23. These results raise the possibility that the so-called proline-directed protein kinases together with CaM kinase II and cAMP-dependent protein kinase play an important role in the regulation of synapsin I function.

Regulation of membrane and subunit interactions by N-myristoylation of a G protein alpha subunit in yeast

Initiation of the mating process in yeast Saccharomyces cerevisiae requires the action of secreted pheromones and G protein-coupled receptors. As in other eukaryotes, the yeast G protein alpha subunit undergoes N-myristoylation (GPA1 gene product, Gpa1p). This modification appears to be essential for function, since a myristoylation site mutation exhibits the null phenotype in vivo (gpa1(G2A)). Here we examine how myristoylation affects Gpa1p activity in vitro. We show that the G2A mutant of Gpa1p, when fused with glutathione S-transferase, can still form a complex with the G protein betagamma subunits. The complex is stabilized by GDP and is dissociated upon treatment with guanosine 5'-O-(thiotriphosphate). In addition, there is no apparent difference in the relative binding affinity of Gbetagamma for mutant and wild-type Gpa1p. Using sucrose density gradient fractionation of cell membranes, Gpa1p associates normally with the plasma membrane whereas Gpa1pG2A is mislocalized to a microsomal membrane fraction. A portion of Gbetagamma is also mislocalized in these cells, as it is in a gpa1Delta strain. In contrast, wild-type Gpa1p reaches the plasma membrane in cells that do not express Gbetagamma or cell surface receptors. These findings indicate that mislocalization of Gpa1pG2A is not caused by a redistribution of Gbetagamma, nor is it the result of any difference in Gbetagamma binding affinity. These data suggest that myristoylation is required for specific targeting of Gpa1p to the plasma membrane, where it is needed to interact with the receptor and to regulate the release of Gbetagamma.

The myristoyl moiety of myristoylated alanine-rich C kinase substrate (MARCKS) and MARCKS-related protein is embedded in the membrane

Members of the myristoylated alanine-rich protein kinase C substrate (MARCKS) family are involved in several cellular processes such as secretion, motility, mitosis, and transformation. In addition to their ability to bind calmodulin and to cross-link actin filaments, reversible binding to the plasma membrane is most certainly an important component of the so far unknown functions of these proteins. We have therefore investigated the binding of murine MARCKS-related protein (MRP) to lipid vesicles. The partition coefficient, Kp, describing the affinity of myristoylated MRP for acidic lipid vesicles (20% phosphatidylserine, 80% phosphatidylcholine) is 5-8 x 10(3) M-1, which is only 2-4 times larger than the partition coefficient for the unmyristoylated protein. Interestingly, the affinity of MRP for acidic lipid membranes is 20-30-fold smaller than reported for murine MARCKS (Kim, J., Shishido, T., Jiang, X., Aderem, A. A., and McLaughlin, S. (1994) J. Biol. Chem. 269, 28214-28219). Since only a marginal binding could be observed with neutral phosphatidylcholine vesicles, we propose that electrostatic interactions are the major determinant of the binding of MRP to pure lipid membranes. Although the myristoyl moiety does not contribute drastically to the binding of MRP to vesicles, photolabeling experiments with a photoreactive phospholipid probe show that the fatty acid is embedded in the bilayer. The same membrane topology was found for bovine brain MARCKS. Since the relatively low affinity of MRP for vesicles is insufficient to account for a stable anchoring of the protein to cellular membranes, insertion of the myristoyl moiety into the bilayer might favor the interaction of MRP with additional factors required for the binding of the protein to intracellular membranes.

Mammalian myristoyl CoA: protein N-myristoyltransferase

Myristoyl CoA:Protein N-myristoyltransferase (NMT) is the enzyme which catalyses the covalent transfer of myristate from myristoyl CoA to the amino-terminal glycine residue of protein substrates. Although NMT is ubiquitous in eukaryotic cells, the enzyme levels and cellular distribution vary among tissues. In this article, we describe the properties of mammalian NMT(s) with reference to subcellular distribution, molecular weights, substrate specificity and the possible involvement of NMT in pathological processes. The cytosolic fraction of bovine brain contains majority of NMT activity. In contrast, rabbit colon and rat liver NMT activity was predominantly particulate. Regional differences in NMT activity have been observed in both rabbit intestine and bovine brain. Results from our laboratory along with the existing knowledge, provide evidence for the existence of tissue specific isozymes of NMT.

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.

Fatty acylation of alpha z. Effects of palmitoylation and myristoylation on alpha z signaling

As the first step in an investigation of roles played by fatty acylation of G protein alpha chains in membrane targeting and signal transmission, we inserted monoclonal antibody epitopes, hemagglutinin (HA) or Glu-Glu (EE), at two internal sites in three alpha subunits. At site I, only HA-tagged alpha q and alpha z functioned normally. alpha s, alpha q, and alpha z subunits tagged at site II with the EE epitope showed normal expression, membrane localization, and signaling activity. Using epitope-tagged alpha z, we investigated effects of mutations in sites for fatty acylation. Mutational substitution of Ala for Gly2 (G2A) prevented incorporation of myristate and decreased but did not abolish incorporation of palmitate. Substitution of Ala for Cys3 (C3A) prevented incorporation of palmitate but had no effect on incorporation of myristate. Substitution of Ala for both Gly2 and Cys3 (G2AC3A) prevented incorporation of both myristate and palmitate. All three mutations substantially disrupted association of alpha z with the particulate fraction. Gz-mediated inhibition of adenylyl cyclase, triggered by activation of the D2-dopamine receptor, was, respectively, abolished (G2AC3A), impaired (G2A), and enhanced (C3A). Constitutive inhibition of adenylyl cyclase by alpha z was unchanged (G2AC3A), strongly diminished (G2A), or strongly enhanced (C3A). A nonacylated, mutationally activated alpha z mutant inhibited adenylyl cyclase, although less potently than normally acylated, mutationally activated alpha z. From these findings we conclude: (a) fatty acylations of alpha z increase its association with membranes; (b) myristoylation is not required for palmitoylation of alpha z or for its productive interactions with adenylyl cyclase; (c) palmitoylation is not required for, but may instead inhibit, signaling by alpha z.

Protein lipidation in cell signaling

The ability of cells to communicate with and respond to their external environment is critical for their continued existence. A universal feature of this communication is that the external signal must in some way penetrate the lipid bilayer surrounding the cell. In most cases of such signal acquisition, the signaling entity itself does not directly enter the cell but rather transmits its information to specific proteins present on the surface of the cell membrane. These proteins then communicate with additional proteins associated with the intracellular face of the membrane. Membrane localization and function of many of these proteins are dependent on their covalent modification by specific lipids, and it is the processes involved that form the focus of this article.

Lipid modifications of trimeric G proteins

G protein alpha subunits and beta gamma dimers are covalently modified by lipids. The emerging picture is one in which attached lipids provide more than just a nonspecific "glue" for sticking G proteins to membranes. We are only beginning to understand how different lipid modifications of different G protein subunits affect specific protein-protein interactions and localization to specific cellular sites. In addition, regulation of these modifications, particularly palmitoylation, can provide new ways to regulate signals transmitted by G proteins.