Identification of the calmodulin-binding domain of neuron-specific protein kinase C substrate protein CAP-22/NAP-22. Direct involvement of protein myristoylation in calmodulin-target protein interaction

BASP1 down-regulates RANKL-induced osteoclastogenesis

The cytokine RANKL (Receptor Activator of NFκB Ligand) is the key driver of differentiation of monocytes/macrophages to form multi-nucleated, bone-resorbing osteoclasts, a process that is accompanied by significant changes in gene expression. We show that exposure to RANKL rapidly down-regulates expression of Brain Acid Soluble Protein 1 (BASP1) in cultured primary mouse bone marrow macrophages (BMMs), and that this reduced expression is causally linked to the osteoclastogenic process in vitro. Knocking down BASP1 expression in BMMs or eliminating its expression in these cells or in RAW 264.7 cells enhanced RANKL-induced osteoclastogenesis, promoted cell-cell fusion, and generated cultures containing larger osteoclasts with increased mineral degrading abilities relative to controls. Expression of exogenous BASP1 in BMMs undergoing osteoclastogenic differentiation produced the opposite effects. Upon exposure to RANKL, primary mouse BMMs in which BASP1 had been knocked down exhibited increased expression of the key osteoclastogenic transcription factor Nfatc1and of its downstream target genes Dc-stamp, Ctsk, Itgb3, and Mmp9 relative to controls. The knock-down cells also exhibited increased sensitivity to the pro-osteoclastogenic effects of RANKL. We conclude that BASP1 is a negative regulator of RANKL-induced osteoclastogenesis, which down-regulates the pro-osteoclastogenic gene expression pattern induced by this cytokine. Decreased expression of BASP1 upon exposure of BMMs to RANKL removes a negative regulator of osteoclastogenesis and promotes this process.

Role of myristoylation in modulating PCaP1 interaction with calmodulin

Plasma membrane-associated Cation-binding Protein 1 (PCaP1) belongs to the plant-unique DREPP protein family with largely unknown biological functions but ascertained roles in plant development and calcium (Ca2+) signaling. PCaP1 is anchored to the plasma membrane via N-myristoylation and a polybasic cluster, and its N-terminal region can bind Ca2+/calmodulin (CaM). However, the molecular determinants of PCaP1-Ca2+-CaM interaction and the functional impact of myristoylation in the complex formation and Ca2+ sensitivity of CaM remained to be elucidated. Herein, we investigated the direct interaction between Arabidopsis PCaP1 (AtPCaP1) and CaM1 (AtCaM1) using both myristoylated and non-myristoylated peptides corresponding to the N-terminal region of AtPCaP1. ITC analysis showed that AtCaM1 forms a high affinity 1:1 complex with AtPCaP1 peptides and the interaction is strictly Ca2+-dependent. Spectroscopic and kinetic Ca2+ binding studies showed that the myristoylated peptide dramatically increased the Ca2+-binding affinity of AtCaM1 and slowed the Ca2+ dissociation rates from both the C- and N-lobes, thus suggesting that the myristoylation modulates the mechanism of AtPCaP1 recognition by AtCaM1. Furthermore, NMR and CD spectroscopy revealed that the structure of both the N- and C-lobes of Ca2+-AtCaM1 changes markedly in the presence of the myristoylated AtPCaP1 peptide, which assumes a helical structure in the final complex. Overall, our results indicate that AtPCaP1 biological function is strictly related to the presence of multiple ligands, i.e., the myristoyl moiety, Ca2+ ions and AtCaM1 and only a full characterization of their equilibria will allow for a complete molecular understanding of the putative role of PCaP1 as signal protein.

A high-affinity cocaine binding site associated with the brain acid soluble protein 1

Cocaine exerts its stimulant effect by inhibiting dopamine (DA) reuptake, leading to increased dopamine signaling. This action is thought to reflect the binding of cocaine to the dopamine transporter (DAT) to inhibit its function. However, cocaine is a relatively weak inhibitor of DAT, and many DAT inhibitors do not share cocaine’s behavioral actions. Further, recent reports show more potent actions of the drug, implying the existence of a high-affinity receptor for cocaine. We now report high-affinity binding of cocaine associated with the brain acid soluble protein 1 (BASP1) with a dissociation constant (Kd) of 7 nM. Knocking down BASP1 in the striatum inhibits [3H]cocaine binding to striatal synaptosomes. Depleting BASP1 in the nucleus accumbens but not the dorsal striatum diminishes locomotor stimulation in mice. Our findings imply that BASP1 is a pharmacologically relevant receptor for cocaine.

The actin-cytoskeleton associating protein BASP1 regulates neural progenitor localization in the neural tube

Brain acid soluble protein 1 (BASP1; previously NAP22 or CAP23) is an actin-associating protein that is highly expressed in the nervous system throughout development. While its roles at the neuromuscular junction and in certain non-neuronal tissues have been previously characterized, its function in the early neural tube is unclear. Using in ovo electroporation in the chicken (Gallus gallus) embryonic neural tube, we show that BASP1 overexpression resulted in the appearance of ectopic neural progenitor cells within the marginal zone of the neural tube. BASP1 knockdown did not affect the position of neural progenitors but did alter the complexity of axons developing from differentiated neurons. This suggests a role for BASP1 in regulating the apical polarity of progenitor cells and axon trajectories from developing neurons.

BASP1 labels neural stem cells in the neurogenic niches of mammalian brain

The mechanisms responsible for determining neural stem cell fate are numerous and complex. To begin to identify the specific components involved in these processes, we generated several mouse neural stem cell (NSC) antibodies against cultured mouse embryonic neurospheres. Our immunohistochemical data showed that the NSC-6 antibody recognized NSCs in the developing and postnatal murine brains as well as in human brain organoids. Mass spectrometry revealed the identity of the NSC-6 epitope as brain abundant, membrane-attached signal protein 1 (BASP1), a signaling protein that plays a key role in neurite outgrowth and plasticity. Western blot analysis using the NSC-6 antibody demonstrated multiple BASP1 isoforms with varying degrees of expression and correlating with distinct developmental stages. Herein, we describe the expression of BASP1 in NSCs in the developing and postnatal mammalian brains and human brain organoids, and demonstrate that the NSC-6 antibody may be a useful marker of these cells.

GAP-43 and BASP1 in Axon Regeneration: Implications for the Treatment of Neurodegenerative Diseases

Growth-associated protein-43 (GAP-43) and brain acid-soluble protein 1 (BASP1) regulate actin dynamics and presynaptic vesicle cycling at axon terminals, thereby facilitating axonal growth, regeneration, and plasticity. These functions highly depend on changes in GAP-43 and BASP1 expression levels and post-translational modifications such as phosphorylation. Interestingly, examinations of GAP-43 and BASP1 in neurodegenerative diseases reveal alterations in their expression and phosphorylation profiles. This review provides an overview of the structural properties, regulations, and functions of GAP-43 and BASP1, highlighting their involvement in neural injury response and regeneration. By discussing GAP-43 and BASP1 in the context of neurodegenerative diseases, we also explore the therapeutic potential of modulating their activities to compensate for neuron loss in neurodegenerative diseases.

A Non-Canonical Calmodulin Target Motif Comprising a Polybasic Region and Lipidated Terminal Residue Regulates Localization

Calmodulin (CaM) is a Ca²⁺-sensor that regulates a wide variety of target proteins, many of which interact through short basic helical motifs bearing two hydrophobic ‘anchor’ residues. CaM comprises two globular lobes, each containing a pair of EF-hand Ca²⁺-binding motifs that form a Ca²⁺-induced hydrophobic pocket that binds an anchor residue. A central flexible linker allows CaM to accommodate diverse targets. Several reported CaM interactors lack these anchors but contain Lys/Arg-rich polybasic sequences adjacent to a lipidated N- or C-terminus. Ca²⁺-CaM binds the myristoylated N-terminus of CAP23/NAP22 with intimate interactions between the lipid and a surface comprised of the hydrophobic pockets of both lobes, while the basic residues make electrostatic interactions with the negatively charged surface of CaM. Ca²⁺-CaM binds farnesylcysteine, derived from the farnesylated polybasic C-terminus of KRAS4b, with the lipid inserted into the C-terminal lobe hydrophobic pocket. CaM sequestration of the KRAS4b farnesyl moiety disrupts KRAS4b membrane association and downstream signaling. Phosphorylation of basic regions of N-/C-terminal lipidated CaM targets can reduce affinity for both CaM and the membrane. Since both N-terminal myristoylated and C-terminal prenylated proteins use a Singly Lipidated Polybasic Terminus (SLIPT) for CaM binding, we propose these polybasic lipopeptide elements comprise a non-canonical CaM-binding motif.

The brain acid-soluble protein 1 (BASP1) interferes with the oncogenic capacity of MYC and its binding to calmodulin

The MYC protein is a transcription factor with oncogenic potential controlling fundamental cellular processes such as cell proliferation, metabolism, differentiation, and apoptosis. The MYC gene is a major cancer driver, and elevated MYC protein levels are a hallmark of most human cancers. We have previously shown that the brain acid-soluble protein 1 gene (BASP1) is specifically downregulated by the v-myc oncogene and that ectopic BASP1 expression inhibits v-myc-induced cell transformation. The 11-amino acid effector domain of the BASP1 protein interacts with the calcium sensor calmodulin (CaM) and is mainly responsible for this inhibitory function. We also reported recently that CaM interacts with all MYC variant proteins and that ectopic CaM increases the transactivation and transformation potential of the v-Myc protein. Here, we show that the presence of excess BASP1 or of a synthetic BASP1 effector domain peptide leads to displacement of v-Myc from CaM. The protein stability of v-Myc is decreased in cells co-expressing v-Myc and BASP1, which may account for the inhibition of v-Myc. Furthermore, suppression of v-Myc-triggered transcriptional activation and cell transformation is compensated by ectopic CaM, suggesting that BASP1-mediated withdrawal of CaM from v-Myc is a crucial event in the inhibition. In view of the tumor-suppressive role of BASP1 which was recently also reported for human cancer, small compounds or peptides based on the BASP1 effector domain could be used in drug development strategies aimed at tumors with high MYC expression.

A Unique Family of Neuronal Signaling Proteins Implicated in Oncogenesis and Tumor Suppression

The neuronal proteins GAP43 (neuromodulin), MARCKS, and BASP1 are highly expressed in the growth cones of nerve cells where they are involved in signal transmission and cytoskeleton organization. Although their primary structures are unrelated, these signaling proteins share several structural properties like fatty acid modification, and the presence of cationic effector domains. GAP43, MARCKS, and BASP1 bind to cell membrane phospholipids, a process reversibly regulated by protein kinase C-phosphorylation or by binding to the calcium sensor calmodulin (CaM). GAP43, MARCKS, and BASP1 are also expressed in non-neuronal cells, where they may have important functions to manage cytoskeleton architecture, and in case of MARCKS and BASP1 to act as cofactors in transcriptional regulation. During neoplastic cell transformation, the proteins reveal differential expression in normal vs. tumor cells, and display intrinsic tumor promoting or tumor suppressive activities. Whereas GAP43 and MARCKS are oncogenic, tumor suppressive functions have been ascribed to BASP1 and in part to MARCKS depending on the cell type. Like MARCKS, the myristoylated BASP1 protein is localized both in the cytoplasm and in the cell nucleus. Nuclear BASP1 participates in gene regulation converting the Wilms tumor transcription factor WT1 from an oncoprotein into a tumor suppressor. The BASP1 gene is downregulated in many human tumor cell lines particularly in those derived from leukemias, which display elevated levels of WT1 and of the major cancer driver MYC. BASP1 specifically inhibits MYC-induced cell transformation in cultured cells. The tumor suppressive functions of BASP1 and MARCKS could be exploited to expand the spectrum of future innovative therapeutic approaches to inhibit growth and viability of susceptible human tumors.

Protein Lipidation: Occurrence, Mechanisms, Biological Functions, and Enabling Technologies

Protein lipidation, including cysteine prenylation, N-terminal glycine myristoylation, cysteine palmitoylation, and serine and lysine fatty acylation, occurs in many proteins in eukaryotic cells and regulates numerous biological pathways, such as membrane trafficking, protein secretion, signal transduction, and apoptosis. We provide a comprehensive review of protein lipidation, including descriptions of proteins known to be modified and the functions of the modifications, the enzymes that control them, and the tools and technologies developed to study them. We also highlight key questions about protein lipidation that remain to be answered, the challenges associated with answering such questions, and possible solutions to overcome these challenges.

High-order oligomers of intrinsically disordered brain proteins BASP1 and GAP-43 preserve the structural disorder

Brain acid-soluble protein-1 (BASP1) and growth-associated protein-43 (GAP-43) are presynaptic membrane proteins participating in axon guidance, neuroregeneration and synaptic plasticity. They are presumed to sequester phosphatidylinositol-4,5-bisphosphate (PIP2 ) in lipid rafts. Previously we have shown that the proteins form heterogeneously sized oligomers in the presence of anionic phospholipids or SDS at submicellar concentration. BASP1 and GAP-43 are intrinsically disordered proteins (IDPs). In light of this, we investigated the structure of their oligomers. Using partial cross-linking of the oligomers with glutaraldehyde, the aggregation numbers of BASP1 and GAP-43 were estimated as 10-14 and 6-7 monomer subunits, respectively. The cross-linking pattern indicated that the subunits are circularly arranged. The circular dichroism (CD) spectra of the monomers were characteristic of coil-like IDPs showing unordered structure with a high population of polyproline-II conformation. The oligomerization was accompanied by a minor CD spectral change attributable to formation of a small amount of α-helix. The number of residues in the α-helical conformation was estimated as 13 in BASP1 and 18 in GAP-43. However, the overall structure of the oligomers remained disordered, indicating a high degree of 'fuzziness'. This was confirmed by measuring the hydrodynamic dimensions of the oligomers using polyacrylamide gradient gel electrophoresis and size-exclusion chromatography, and by assaying their sensitivity to proteolytic digestion. There is evidence that the observed α-helical folding occurs within the basic effector domains, which are presumably tethered together via anionic molecules of SDS or PIP2 . We conclude that BASP1 and GAP-43 oligomers preserve a mostly disordered structure, which may be of great importance for their function in PIP2 signaling pathway.

Mechanisms of transcriptional regulation by WT1 (Wilms' tumour 1)

The WT1 (Wilms' tumour 1) gene encodes a zinc finger transcription factor and RNA-binding protein that direct the development of several organs and tissues. WT1 manifests both tumour suppressor and oncogenic activities, but the reasons behind these opposing functions are still not clear. As a transcriptional regulator, WT1 can either activate or repress numerous target genes resulting in disparate biological effects such as growth, differentiation and apoptosis. The complex nature of WT1 is exemplified by a plethora of isoforms, post-translational modifications and multiple binding partners. How WT1 achieves specificity to regulate a large number of target genes involved in diverse physiological processes is the focus of the present review. We discuss the wealth of the growing molecular information that defines our current understanding of the versatility and utility of WT1 as a master regulator of organ development, a tumour suppressor and an oncogene.

N-myristoylation regulates the axonal distribution of the Fragile X-related protein FXR2P

Fragile X syndrome, the leading cause of inherited intellectual disability and autism, is caused by loss of function of Fragile X mental retardation protein (FMRP). FMRP is an RNA binding protein that regulates local protein synthesis in the somatodendritic compartment. However, emerging evidence also indicates important roles for FMRP in axonal and presynaptic functions. In particular, FMRP and its homologue FXR2P localize axonally and presynaptically to discrete endogenous structures in the brain termed Fragile X granules (FXGs). FXR2P is a component of all FXGs and is necessary for the axonal and presynaptic localization of FMRP to these structures. We therefore sought to identify and characterize structural features of FXR2P that regulate its axonal localization. Sequence analysis reveals that FXR2P harbors a consensus N-terminal myristoylation sequence (MGXXXS) that is absent in FMRP. Using click chemistry with wild type and an unmyristoylatable G2A mutant we demonstrate that FXR2P is N-myristoylated on glycine 2, establishing it as a lipid-modified RNA binding protein. To investigate the role of FXR2P N-myristoylation in neurons we generated fluorescently tagged wild type and unmyristoylatable FXR2P (WT and G2A, respectively) and expressed them in primary cortical cultures. Both FXR2P(WT) and FXR2P(G2A) are expressed at equivalent overall levels and are capable of forming FMRP-containing axonal granules. However, FXR2P(WT) granules are largely restricted to proximal axonal segments while granules formed with unmyristoylatable FXR2P(G2A) are localized throughout the axonal arbor, including in growth cones. These studies indicate that N-terminal myristoylation of the RNA binding protein FXR2P regulates its localization within the axonal arbor. Moreover, since FMRP localization within axonal domains requires its association with FXR2P, these findings suggest that FXR2P lipid modification is a control point for the axonal and presynaptic distribution of FMRP.

Repression of transcription by WT1-BASP1 requires the myristoylation of BASP1 and the PIP2-dependent recruitment of histone deacetylase

The Wilms' tumor 1 protein WT1 is a transcriptional regulator that is involved in cell growth and differentiation. The transcriptional corepressor BASP1 interacts with WT1 and converts WT1 from a transcriptional activator to a repressor. Here, we demonstrate that the N-terminal myristoylation of BASP1 is required in order to elicit transcriptional repression at WT1 target genes. We show that myristoylated BASP1 binds to nuclear PIP2, which leads to the recruitment of PIP2 to the promoter regions of WT1-dependent target genes. BASP1's myristoylation and association with PIP2 are required for the interaction of BASP1 with HDAC1, which mediates the recruitment of HDAC1 to the promoter and elicits transcriptional repression. Our findings uncover a role for myristoylation in transcription, as well as a critical function for PIP2 in gene-specific transcriptional repression through the recruitment of histone deacetylase.

Immunoelectron microscopic study of BASP1 and MARCKS location in the early and late rat spermatids

Immunoelectron microscopy was used to locate the proteins BASP1 and MARCKS in the post-meiotic spermatids of male rat testis. It was shown that in early spermatids, BASP1 and MARCKS accumulate in chromatoid bodies, which are characteristic organelles for these cells. During spermatogenesis, while the spermatid nucleus is still active, the chromatoid body periodically moves to the cell nucleus and absorbs the precursors of definite mRNAs and small RNAs. mRNAs are preserved in the chromatoid body until the corresponding proteins are needed, but their "fresh" mRNA cannot be formed due to the nucleus inactivation. The chromatoid body (0.5-1.5μm in diameter) has a cloud-like fibrous appearance with many fairly round cavities. In the chromatoid body, BASP1 and MARCKS are distributed mainly around the cavities and at periphery. Based on the known functions of BASP1 and MARCKS in neurons, it is conceivable that these proteins participate in non-random movements of the chromatoid body to the nucleus and in Ca(2+)-calmodulin enrichment. In late spermatids, BASP1 and MARCKS are located in the outer dense fiber layer belonging to a metabolically active spermatozoon region, the tail mid-piece. In spermatozoa, as in chromatoid body, BASP1 and MARCKS may bind Ca(2+)-calmodulin and therefore contribute to the activation of calcium-dependent biochemical processes.

The myristate moiety and amino terminus of vaccinia virus l1 constitute a bipartite functional region needed for entry

Vaccinia virus (VACV) L1 is a myristoylated envelope protein which is required for cell entry and the fusion of infected cells. L1 associates with members of the entry-fusion complex (EFC), but its specific role in entry has not been delineated. We recently demonstrated (Foo CH, et al., Virology 385:368-382, 2009) that soluble L1 binds to cells and blocks entry, suggesting that L1 serves as the receptor-binding protein for entry. Our goal is to identify the structural domains of L1 which are essential for its functions in VACV entry. We hypothesized that the myristate and the conserved residues at the N terminus of L1 are critical for entry. To test our hypothesis, we generated mutants in the N terminus of L1 and used a complementation assay to evaluate their ability to rescue infectivity. We also assessed the myristoylation efficiency of the mutants and their ability to interact with the EFC. We found that the N terminus of L1 constitutes a region that is critical for the infectivity of VACV and for myristoylation. At the same time, the nonmyristoylated mutants were incorporated into mature virions, suggesting that the myristate is not required for the association of L1 with the viral membrane. Although some of the mutants exhibited altered structural conformations, two mutants with impaired infectivity were similar in conformation to wild-type L1. Importantly, these two mutants, with changes at A4 and A5, undergo myristoylation. Overall, our results imply dual differential roles for myristate and the amino acids at the N terminus of L1. We propose a myristoyl switch model to describe how L1 functions.

Functional evidence that Drosha overexpression in cervical squamous cell carcinoma affects cell phenotype and microRNA profiles

Although gain of chromosome 5p is one of the most frequent DNA copy-number imbalances in cervical squamous cell carcinoma (SCC), the genes that drive its selection remain poorly understood. In a previous cross-sectional clinical study, we showed that the microRNA processor Drosha (located on chromosome 5p) demonstrates frequent copy-number gain and overexpression in cervical SCC, associated with altered microRNA profiles. Here, we have conducted gene depletion/overexpression experiments to demonstrate the functional significance of up-regulated Drosha in cervical SCC cells. Drosha depletion by RNA interference (RNAi) produced significant, specific reductions in cell motility/invasiveness in vitro, with a silent RNAi-resistant Drosha mutation providing phenotype rescue. Unsupervised hierarchical clustering following global profiling of 319 microRNAs in 18 cervical SCC cell line specimens generated two groups according to Drosha expression levels. Altering Drosha levels in individual SCC lines changed the group into which the cells clustered, with gene depletion effects being rescued by the RNAi-resistant mutation. Forty-five microRNAs showed significant differential expression between the groups, including four of 14 that were differentially expressed in association with Drosha levels in clinical samples. miR-31 up-regulation in Drosha-overexpressing samples/cell lines was the highest-ranked change (by adjusted p value) in both analyses, an observation validated by northern blotting. These functional data support the role of Drosha as an oncogene in cervical SCC, by affecting expression of cancer-associated microRNAs that have the potential to regulate numerous protein-coding genes.

Subcellular and regional location of "brain" proteins BASP1 and MARCKS in kidney and testis

Proteins BASP1 and MARCKS are abundant in axonal endings of neurons. Similarly to brain-specific protein GAP-43, BASP1 and MARCKS are reversibly bound to the plasma membrane. These proteins control both actin polymerization and actin cytoskeleton binding to the membrane. Performing these functions, BASP1 and MARCKS take part in growth cone guidance during development and in neurotransmitter secretion in adults. These activities predetermine the pivotal role of BASP1 and MARCKS in learning and memory. BASP1 and MARCKS were also found in non-nerve tissues, in particular, in the kidney and testis. Evidently, the physiological roles of these proteins differ in different tissues. Correspondingly, their intracellular location and activities may not be similar to those in neurons. In this paper, we analyze subcellular fractions (cytoplasm and nuclei) of rat kidney and testis with the purpose of determining the intracellular location of BASP1 and MARCKS. Western blots demonstrated that in these tissues, as in the brain, both proteins are present in the cytoplasm of the cell. According to our immunohistochemical study, BASP1 and MARCKS are specifically distributed in the tissues studied. In kidney, both proteins are present in cells located in glomeruli. In the testicular tubules, BASP1 is mainly expressed at the late stage of spermatogenesis (in spermatids) and is preserved in mature spermatozoa, while MARCKS appears equally during all stages of spermatogenesis. MARCKS is not found in mature spermatozoa. The results indicate that study of functions of BASP1 and MARCKS in the kidney and in the reproduction system holds much promise.

Oligomeric structure of brain abundant proteins GAP-43 and BASP1

Brain abundant proteins GAP-43 and BASP1 participate in the regulation of actin cytoskeleton dynamics in neuronal axon terminals. The proposed mechanism suggests that the proteins sequester phosphatidylinositol-4,5-diphosphate (PIP(2)) in the inner leaflet of the plasma membrane. We found that model anionic phospholipid membranes in the form of liposomes induce rapid oligomerization of GAP-43 and BASP1 proteins. Multiply charged phosphoinositides produced the most potent effect. Anionic detergent sodium dodecyl sulfate (SDS) at submicellar concentration stimulated formation of similar oligomers in solution. BASP1, but not GAP-43, also formed oligomers at sufficiently high concentration in the absence of lipids and SDS. Electron microscopy study demonstrated that the oligomers have disk-shaped or annular structure of 10-30nm in diameter. BASP1 also formed higher aggregates of linear rod-like structure, with average length of about 100nm. In outward appearance, the oligomers and linear aggregates are reminiscent of oligomers and protofibrils of amyloid proteins. Both the synthetic N-terminal peptide GAP-43(1-40) and the brain-derived fragment GAP-43-3 preserved the ability to oligomerize under the action of acidic phospholipids and SDS. On the contrary, BASP1 fragment truncated by the short N-terminal myristoylated peptide was unable to form oligomers. GAP-43 and BASP1 oligomerization can be regulated by calmodulin, which disrupts the oligomers and displaces the proteins from the membrane. We suggest that in vivo, the role of membrane-bound GAP-43 and BASP1 oligomers consists in accumulation of PIP(2) in functional clusters, which become accessible for other PIP(2)-binding proteins after dissociation of the oligomers.

N-myristoylated proteins, key components in intracellular signal transduction systems enabling rapid and flexible cell responses

N-myristoylation, one of the co- or post-translational modifications of proteins, has so far been regarded as necessary for anchoring of proteins to membranes. Recently, we have revealed that N(alpha)-myristoylation of several brain proteins unambiguously regulates certain protein-protein interactions that may affect signaling pathways in brain. Comparison of the amino acid sequences of myristoylated proteins including those in other organs suggests that this regulation is involved in signaling pathways not only in brain but also in other organs. Thus, it has been shown that myristoylated proteins in cells regulate the signal transduction between membranes and cytoplasmic fractions. An algorithm we have developed to identify myristoylated proteins in cells predicts the presence of hundreds of myristoylated proteins. Interestingly, a large portion of the myristoylated proteins thought to take part in signal transduction between membranes and cytoplasmic fractions are included in the predicted myristoylated proteins. If the proteins functionally regulated by myristoylation, a posttranslational protein modification, were understood as cross-talk points within the intracellular signal transduction system, known signaling pathways could thus be linked to each other, and a novel map of this intracellular network could be constructed. On the basis of our recent results, this review will highlight the multifunctional aspects of protein N-myristoylation in brain.

Myelin basic protein as a "PI(4,5)P2-modulin": a new biological function for a major central nervous system protein

The 18.5 kDa isoform of myelin basic protein (MBP) is multifunctional and has previously been shown to have structural and phenomenological similarities with domains of other membrane- and cytoskeleton-associated proteins such as MARCKS (myristoylated alanine-rich C kinase substrate). Here, we have investigated whether 18.5 kDa MBP can sequester phosphatidylinositol-(4,5)-bis-phosphate (PI(4,5)P 2) in membranes, like MARCKS and other "PIPmodulins" do. Using fluorescence-quenching and electron paramagnetic resonance (EPR) spectroscopy, and model membranes containing BODIPY-FL- or proxyl-labeled PI(4,5)P 2, respectively, we have demonstrated that MBP laterally sequesters PI(4,5)P 2. The MBP-PI(4,5)P 2 interactions are electrostatic, partially cholesterol-dependent, and sensitive to phosphorylation, deimination, and Ca (2+)-CaM binding. Confocal microscopy of cultured oligodendrocytes also revealed patched colocalization of MBP and PI(4,5)P 2, indicating the spatial clustering of PI(4,5)P 2 in the plasma membrane. On the basis of these findings as well as the overwhelming convergence of functional properties, modifying enzymes, and interaction partners, we propose that MBP is mechanistically related to GAP-43, MARCKS, and CAP-23. During myelinogenesis, it may mediate calcium and phosphorylation-sensitive plasma membrane availability of PI(4,5)P 2. This regulation of PI(4,5)P 2 availability at the cell cortex may be coupled to the elaboration and outgrowth of the membranous cellular processes by oligodendrocytes.

Suppression of Calpain-dependent Cleavage of the CDK5 Activator p35 to p25 by Site-specific Phosphorylation

Cdk5 is a proline-directed Ser/Thr protein kinase predominantly expressed in postmitotic neurons together with its activator, p35. N-terminal truncation of p35 to p25 by calpain results in deregulation of Cdk5 and contributes to neuronal cell death associated with several neurodegenerative diseases. Previously we reported that p35 occurred as a phosphoprotein, phospho-p35 levels changed with neuronal maturation, and that phosphorylation of p35 affected its vulnerability to calpain cleavage. Here, we identify the p35 residues Ser(8) and Thr(138) as the major sites of phosphorylation by Cdk5. Mutagenesis of these sites to unphosphorylatable Ala increased susceptibility to calpain in cultured cells and neurons while changing them to phosphomimetic glutamate-attenuated cleavage. Furthermore, phosphorylation state-specific antibodies to these sites revealed that Thr(138) was dephosphorylated in adult rat, although both Ser(8) and Thr(138) were phosphorylated in prenatal brains. In cultured neurons, inhibition of protein phosphatases converted phosho-Ser(8) p35 to dual phospho-Ser(8)/Thr(138) p35 and conferred resistance to calpain cleavage. These results suggest phosphorylation of Thr(138) predominantly defines the susceptibility of p35 to calpain-dependent cleavage and that dephosphorylation of this site is a critical determinant of Cdk5-p25-induced cell death associated with neurodegeneration.

HIV-1 Nef upregulates CCL2/MCP-1 expression in astrocytes in a myristoylation- and calmodulin-dependent manner

HIV-associated dementia (HAD) correlates with infiltration of monocytes into the brain. The accessory HIV-1 negative factor (Nef) protein, which modulates several signaling pathways, is constitutively present in persistently infected astroctyes. We demonstrated that monocytes responded with chemotaxis when subjected to cell culture supernatants of nef-expressing astrocytic U251MG cells. Using a protein array, we identified CC chemokine ligand 2/monocyte chemotactic protein-1 (CCL2/MCP-1) as a potential chemotactic factor mediating this phenomenon. CCL2/MCP-1 upregulation by Nef was further confirmed by ribonuclease protection assay, RT-PCR and ELISA. By applying neutralizing antibodies against CCL2/MCP-1 and using CCR2-deficient monocytes, we confirmed CCL2/MCP-1 as the exclusive factor secreted by nef-expressing astrocytes capable of attracting monocytes. Additionally, we showed that Nef-induced CCL2/MCP-1 expression depends on the myristoylation moiety of Nef and requires functional calmodulin. In summary, we suggest that Nef-induced CCL2/MCP-1 expression in astrocytes contributes to infiltration of monocytes into the brain, and thereby to progression of HAD.

Involvement of axonal guidance proteins and their signaling partners in the developing mouse mammary gland

Mammary morphogenesis in the mouse is driven by specialized structures at the ends of the developing ducts, the terminal end buds (TEB). The mechanisms controlling the precise branching and spacing of the ducts are, as yet, unknown. To identify genes that are associated with migration of TEB and differentiation of the subtending ducts, we developed a novel method of isolating TEB and ducts free of stroma, and compared the gene expression profiles of these two isolates using oligonucleotide microarrays. Ninety one genes were upregulated in TEB compared to ducts. Three of these genes, Sprr1A, Sema3B, and BASP1, are associated with axonal growth and guidance. Two additional members of the Sprr family, Sprr2A and 2B, not previously associated with axonal growth, were also highly expressed in TEB. Expression of these genes was confirmed by RT-PCR and Western blotting, and the cellular distribution of Sprr1A and BASP1 was demonstrated by immunohistochemistry. Other semaphorins, including Sema3C, 4A, 4F and the cancer invasion associated Sema 4D were also expressed in the mouse mammary gland along with the semaphorin receptors, Plexins A2, A3, B2, and D1, and Neuropilins 1 and 2. These results are discussed in the context of other proteins expressed in the developing gland that are known to be downstream effectors of these signaling molecules. We suggest that these genes may influence ductal growth and morphogenesis in the developing mammary gland.

Reversible intracellular translocation of KRas but not HRas in hippocampal neurons regulated by Ca2+/calmodulin

The Ras/MAPK pathway regulates synaptic plasticity and cell survival in neurons of the central nervous system. Here, we show that KRas, but not HRas, acutely translocates from the plasma membrane (PM) to the Golgi complex and early/recycling endosomes in response to neuronal activity. Translocation is reversible and mediated by the polybasic-prenyl membrane targeting motif of KRas. We provide evidence that KRas translocation occurs through sequestration of the polybasic-prenyl motif by Ca²⁺/calmodulin (Ca²⁺/CaM) and subsequent release of KRas from the PM, in a process reminiscent of GDP dissociation inhibitor–mediated membrane recycling of Rab and Rho GTPases. KRas translocation was accompanied by partial intracellular redistribution of its activity. We conclude that the polybasic-prenyl motif acts as a Ca²⁺/CaM-regulated molecular switch that controls PM concentration of KRas and redistributes its activity to internal sites. Our data thus define a novel signaling mechanism that differentially regulates KRas and HRas localization and activity in neurons.

Myristoyl moiety of HIV Nef is involved in regulation of the interaction with calmodulin in vivo

Human immunodeficiency virus Nef is a myristoylated protein expressed early in infection by HIV. In addition to the well known down-regulation of the cell surface receptors CD4 and MHCI, Nef is able to alter T-cell signaling pathways. The ability to alter the cellular signaling pathways suggests that Nef can associate with signaling proteins. In the present report, we show that Nef can interact with calmodulin, the major intracellular receptor for calcium. Coimmunoprecipitation analyses with lysates from the NIH3T3 cell line constitutively expressing the native HIV-1 Nef protein revealed the presence of a stable Nef-calmodulin complex. When lysates from NIH3T3 cells were incubated with calmodulin-agarose beads in the presence of CaCl(2) or EGTA, calcium ion drastically enhanced the interaction between Nef and calmodulin, suggesting that the binding is under the influence of Ca(2+) signaling. Glutathione S-transferase-Nef fusion protein bound directly to calmodulin with high affinity. Using synthetic peptides based on the N-terminal sequence of Nef, we determined that within a 20-amino-acid N-terminal basic domain was sufficient for calmodulin binding. Furthermore, the myristoylated peptide bound to calmodulin with higher affinity than nonmyris-toylated form. Thus, the N-terminal myristoylation domain of Nef plays an important role in interacting with calmodulin. This domain is highly conserved in several HIV-1 Nef variants and resembles the N-terminal domain of NAP-22/CAP23, a myristoylated calmodulin-binder. These results for the interaction between HIV Nef and calmodulin in the cells suggested that the Nef might interfere with intracellular Ca(2+) signaling through calmodulin-mediated interactions in infected cells.

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.

Motor, sensory and autonomic nerve terminals containing NAP-22 immunoreactivity in the rat muscle

Neuron-enriched acidic protein having a molecular mass of 22 kDa, NAP-22, is a Ca(2+)-dependent calmodulin-binding protein and is phosphorylated with protein kinase C (PKC). This protein is localized to the biological membrane via myristoylation and found in the membrane fraction of the brain and in the synaptic vesicle fraction. Recent studies showed that NAP-22 is localized in the membrane raft domain in a cholesterol-dependent manner and suggest a role for NAP-22 in maturation and/or maintenance of nerve terminals by controlling cholesterol-dependent membrane dynamics. The present study revealed the immunohistochemical distribution of NAP-22 in the peripheral nerves in rat muscles. In all examined muscles, nerve terminals in the motor endplates showed NAP-22 immunoreactivity associated with the membranes of synaptic vesicles and nerve terminals. In the muscle spindles, annulospiral endings, which made spirals around the intrafusal muscles, showed intense NAP-22 immunoreactivity. Autonomic nerve fibers around the intramuscular blood vessels also showed the immunoreactivity for NAP-22. NAP-22 immunoreactivity in these peripheral nerves was observed from birth to adulthood (100 days after birth). Though growth-associated protein-43 (GAP-43) immunoreactivity in these nerves was observed from birth, this immunoreactivity decreased from 20 days after birth. These findings suggest that NAP-22 is distributed and regulates functions in the motor, sensory and autonomic nerve terminals in the peripheral nervous system.

Myristoylation-regulated Direct Interaction Between Calcium-bound Calmodulin and N-terminal Region of pp60v-src

pp60v-src tyrosine protein kinase was suggested to interact with Ca2+-bound calmodulin (Ca2+/CaM) through the N-terminal region based on its structural similarities to CAP-23/NAP-22, a myristoylated neuron-specific protein, whose myristoyl group is essential for interaction with Ca2+/CaM; (1) the N terminus of pp60v-src is myristoylated like CAP-23/NAP-22; (2) both lysine residues are required for the myristoylation-dependent interaction and serine residues that are thought to regulate the interaction through the phosphorylations located in the N-terminal region of pp60v-src. To verify this possibility, we investigated the direct interaction between pp60v-src and Ca2+/CaM using a myristoylated peptide corresponding to the N-terminal region of pp60v-src. The binding assay indicated that only the myristoylated peptide binds to Ca2+/CaM, and the non-myristoylated peptide is not able to bind to Ca2+/CaM. Analyses of the binding kinetics revealed two independent reactions with the dissociation constants (KD) of 2.07 x 10(-9)M (KD1) and 3.93 x 10(-6)M (KD2), respectively. Two serine residues near the myristoyl moiety of the peptide (Ser2, Ser11) were phosphorylated by protein kinase C in vitro, and the phosphorylation drastically reduced the interaction. NMR experiments indicated that two molecules of the myristoylated peptide were bound around the hydrophobic clefts of a Ca2+/CaM molecule. The small-angle X-ray scattering analyses showed that the size of the peptide-Ca2+/CaM complex is 2-3A smaller than that of the known Ca2+/CaM-target molecule complexes. These results demonstrate clearly the direct interaction between pp60v-src and Ca2+/CaM in a novel manner different from that of known Ca2+/CaM, the target molecules, interactions.

Crystal structure of a myristoylated CAP-23/NAP-22 N-terminal domain complexed with Ca2+/calmodulin

A variety of viral and signal transduction proteins are known to be myristoylated. Although the role of myristoylation in protein-lipid interaction is well established, the involvement of myristoylation in protein-protein interactions is less well understood. CAP-23/NAP-22 is a brain-specific protein kinase C substrate protein that is involved in axon regeneration. Although the protein lacks any canonical calmodulin (CaM)-binding domain, it binds CaM with high affinity. The binding of CAP-23/NAP-22 to CaM is myristoylation dependent and the N-terminal myristoyl group is directly involved in the protein-protein interaction. Here we show the crystal structure of Ca2+-CaM bound to a myristoylated peptide corresponding to the N-terminal domain of CAP-23/NAP-22. The myristoyl moiety of the peptide goes through a hydrophobic tunnel created by the hydrophobic pockets in the N- and C-terminal domains of CaM. In addition to the myristoyl group, several amino-acid residues in the peptide are important for CaM binding. This is a novel mode of binding and is very different from the mechanism of binding in other CaM-target complexes.

MYRbase: analysis of genome-wide glycine myristoylation enlarges the functional spectrum of eukaryotic myristoylated proteins

We evaluated the evolutionary conservation of glycine myristoylation within eukaryotic sequences. Our large-scale cross-genome analyses, available as MYRbase, show that the functional spectrum of myristoylated proteins is currently largely underestimated. We give experimental evidence for in vitro myristoylation of selected predictions. Furthermore, we classify five membrane-attachment factors that occur most frequently in combination with, or even replacing, myristoyl anchors, as some protein family examples show.

Indolicidin, a 13-residue basic antimicrobial peptide rich in tryptophan and proline, interacts with Ca(2+)-calmodulin

Indolicidin, ILPWKWPWWPWRR-NH(2), a short 13-residue antimicrobial and cytolytic peptide characterized from bovine neutrophils, has the calmodulin-recognition 1-5-10 hydrophobic pattern (indicated by amino acids in bold), is cationic, and thereby fulfills the requirements to interact with calmodulin. Hence, we have investigated the calmodulin-binding properties of indolicidin. Indolicidin interacted with calmodulin with fairly high affinity in a Ca(2+)-dependent manner. However, when bound, the peptide did not adopt helical conformation. Indolicidin also inhibited calmodulin-stimulated phosphodiesterase activity with IC(50) values in the nanomolar range. Replacement of either the proline residues of indolicidin with alanines or tryptophan residues with phenylalanines did not affect binding to calmodulin. However, these replacements had distinctive effects on the conformations of the bound peptides. While the alanine analog of indolicidin adopted predominantly alpha-helical conformation, the phenylalanine analog remained largely unordered. Differences in the ability of these analogs to inhibit the calmodulin-stimulated phosphodiesterase activity were observed. While the alanine analog was capable of inhibiting the activity with IC(50) values comparable to that of indolicidin, the phenylalanine analog did not inhibit the activity. Our results indicate that ability to adopt amphiphilic alpha-helical structure is not a prerequisite for binding to calmodulin and also binding does not necessarily result in inhibition of calmodulin-stimulated enzyme activities.

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.

Quaternary structure of the neuronal protein NAP-22 in aqueous solution

NAP-22, a myristoylated, anionic protein, is a major protein component of the detergent-insoluble fraction of neurons. After extraction from the membrane, it is readily soluble in water. NAP-22 will partition only into membranes with specific lipid compositions. The lipid specificity is not expected for a monomeric myristoylated protein. We have studied the self-association of NAP-22 in solution. Sedimentation velocity experiments indicated that the protein is largely associated. The low concentration limiting s value is approximately 1.3 S, indicating a highly asymmetric monomer. In contrast, a nonmyristoylated form of the protein shows no evidence of oligomerization by velocity sedimentation and has an s value corresponding to the smallest component of NAP-22, but without the presence of higher oligomers. Sedimentation equilibrium runs indicate that there is a rapidly reversible equilibrium between monomeric and oligomeric forms of the protein followed by a slower, more irreversible association into larger aggregates. In situ atomic force microscopy of the protein deposited on mica from freshly prepared dilute solution revealed dimers on the mica surface. The values of the association constants obtained from the sedimentation equilibrium data suggest that the weight concentration of the monomer exceeds that of the dimer below a total protein concentration of 0.04 mg/ml. Since the concentration of NAP-22 in the neurons of the developing brain is approximately 0.6 mg/ml, if the protein were in solution, it would be in oligomeric form and bind specifically to cholesterol-rich domains. We demonstrate, using fluorescence resonance energy transfer, that at low concentrations, NAP-22 labeled with Texas Red binds equally well to liposomes of phosphatidylcholine either with or without the addition of 40 mol% cholesterol. Thus, oligomerization of NAP-22 contributes to its lipid selectivity during membrane binding.

Natural N-terminal fragments of brain abundant myristoylated protein BASP1

BASP1 (also known as CAP-23 and NAP-22) is a novel myristoylated calmodulin-binding protein, abundant in nerve terminals. It is considered as a signal protein participating in neurite outgrowth and synaptic plasticity. BASP1 is also present in significant amounts in kidney, testis, and lymphoid tissues. In this study, we show that BASP1 is accompanied by at least six BASP1 immunologically related proteins (BIRPs), which are present in all animal species studied (rat, bovine, human, chicken). BIRPs have lower molecular masses than that of BASP1. Similarly to BASP1, they are myristoylated. Peptide mapping and partial sequencing have shown that BIRPs represent a set of BASP1 N-terminal fragments devoid of C-terminal parts of different length. In a definite species, the same set of BASP1 fragments is present in both brain and other tissues. The sum amount of the fragments is about 50% of the BASP1 amount in a tissue. Obligatory accompanying of BASP1 by a set of specific fragments indicates that these fragments are of physiological significance.

Molecular characterization of the detergent-insoluble cholesterol-rich membrane microdomain (raft) of the central nervous system

Many fundamental neurological issues such as neuronal polarity, the formation and remodeling of synapses, synaptic transmission, and the pathogenesis of the neuronal cell death are closely related to the membrane dynamics. The elucidation of functional roles of a detergent-insoluble cholesterol-rich domain (raft) could therefore provide good clues to the molecular understanding of these important phenomena, for the participation of the raft in the fundamental cell functions, such as signal transduction and selective transport of lipids and proteins, has been elucidated in nonneural cells. Interestingly, the brain is rich in raft and the brain-derived raft differs in its lipid and protein components from other tissue-derived rafts. Since many excellent reviews are written on the membrane lipid dynamics of this microdomain, signal transduction, and neuronal glycolipids, we review on the characterization of the raft proteins recovered in the detergent-insoluble low-density fraction from rat brain. Special focus is addressed on the biochemical characterization of a neuronal enriched protein, NAP-22, for the lipid organizing activity of this protein has become increasingly clear.

The arrangement of cholesterol in membranes and binding of NAP-22

Cholesterol forms crystals when the mol fraction of sterol in a membrane bilayer exceeds a certain value. The solubility limit of cholesterol is very dependent on the nature of the phospholipid with which it is mixed. NMR methods have proven useful in quantifying the amount of cholesterol monohydrate crystals present in mixtures with phospholipids. A protein, NAP-22, present in high abundance in the synaptic cell membrane and synaptic vesicle, promotes the formation of cholesterol crystallites in lipid mixtures in which cholesterol would be completely dissolved in the membrane in the absence of protein. This finding, along with effects of the protein on the phase transitions of mixtures of phosphatidylcholine (PC) and cholesterol indicate that NAP-22 facilitates the formation of cholesterol-rich domains. This protein will bind only to membranes of PC that contain either cholesterol or phosphatidylethanolamine (PE). The process requires the presence of a myristoyl group on the N-terminus of NAP-22. The phenomenon also does not occur with a 19 amino acid myristoylated peptide corresponding to the amino terminal segment of NAP-22. The basis of the selectivity of NAP-22 for interacting with membranes of specific composition is suggested to be due to the accessibility of the myristoyl group.

Lipid binding activity of a neuron-specific protein NAP-22 studied in vivo and in vitro

There exists a microdomain called "raft" in the cell membrane. The enrichment of cholesterol and sphingolipids in its outer leaflet is well recognized. In contrast, little is known of the lipid composition of the inner leaflet of raft, where many acylated signal-transducing molecules, such as trimeric G proteins and protein tyrosine kinases, associate. NAP-22 is a neuronal protein localized on the inner leaflet of raft domain. This protein was found to bind cholesterol in the liposome. In this study, we further analyze the lipid binding activity of NAP-22 using eukaryotic and bacterial expression systems. In addition to cholesterol, NAP-22 showed a phosphatidylethanolamine (PE)- and polyphosphoinositide-dependent membrane binding in the liposome assay. The N-terminal myristoylation was essential for the liposome binding. The C-terminal deletion up to D61 showed little effect on the binding. The lipid binding region was hence judged to be in the N-terminal 60-amino-acid sequence. NAP-22 was then expressed in COS7 cells, and the intracellular localization was studied. Biochemical analysis showed the localization of NAP-22 in a Triton-insoluble low-density fraction. Cell staining analysis showed colocalization patterns of NAP-22 with PE and cholesterol in the membrane.

Reversible translocation and activity-dependent localization of the calcium-myristoyl switch protein VILIP-1 to different membrane compartments in living hippocampal neurons

Visinin-like protein-1 (VILIP-1) belongs to the family of neuronal calcium sensor (NCS) proteins, a neuronal subfamily of EF-hand [corrected] calcium-binding proteins that are myristoylated at their N termini. NCS proteins are discussed to play roles in calcium-dependent signal transduction of physiological and pathological processes in the CNS. The calcium-dependent membrane association, the so-called calcium-myristoyl switch, localizes NCS proteins to a distinct cellular signaling compartment and thus may be a critical mechanism for the coordinated regulation of signaling cascades. To study whether the biochemically defined calcium-myristoyl switch of NCS proteins can occur in living neuronal cells, the reversible and stimulus-dependent translocation of green fluorescent protein (GFP)-tagged VILIP-1 to subcellular targets was examined by fluorescence microscopy in transfected cell lines and hippocampal primary neurons. In transiently transfected NG108-15 and COS-7 cells, a translocation of diffusely distributed VILIP-1-GFP but not of myristoylation-deficient VILIP-1-GFP to the plasma membrane and to intracellular targets, such as Golgi membranes, occurred after raising the intracellular calcium concentration with a calcium ionophore. The observed calcium-dependent localization was completely reversed after depletion of intracellular calcium by EGTA. Interestingly, a fast and reversible translocation of VILIP-1-GFP and translocation of endogenous VILIP-1 to specialized membrane structures was also observed after a depolarizing stimulus or activation of glutamate receptors in hippocampal neurons. These results show for the first time the reversibility and stimulus-dependent occurrence of the calcium-myristoyl switch in living neurons, suggesting a physiological role as a signaling mechanism of NCS proteins, enabling them to activate specific targets localized in distinct membrane compartments.

Nef of HIV-1 interacts directly with calcium-bound calmodulin

It was recently found that the myristoyl group of CAP-23/NAP-22, a neuron-specific protein kinase C substrate, is essential for the interaction between the protein and Ca(2+)-bound calmodulin (Ca(2+)/CaM). Based on the N-terminal amino acid sequence alignment of CAP-23/NAP-22 and other myristoylated proteins, including the Nef protein from human immunodeficiency virus (HIV), we proposed a new hypothesis that the protein myristoylation plays important roles in protein-calmodulin interactions. To investigate the possibility of direct interaction between Nef and calmodulin, we performed structural studies of Ca(2+)/CaM in the presence of a myristoylated peptide corresponding to the N-terminal region of Nef. The dissociation constant between Ca(2+)/CaM and the myristoylated Nef peptide was determined to be 13.7 nM by fluorescence spectroscopy analyses. The NMR experiments indicated that the chemical shifts of some residues on and around the hydrophobic clefts of Ca(2+)/CaM changed markedly in the Ca(2+)/CaM-Nef peptide complex with the molar ratio of 1:2. Correspondingly, the radius of gyration determined by the small angle X-ray scattering measurements is 2-3 A smaller that of Ca(2+)/CaM alone. These results demonstrate clearly that Nef interacts directly with Ca(2+)/CaM.

An Arabidopsis calcium-dependent protein kinase is associated with the endoplasmic reticulum

Arabidopsis contains 34 genes that are predicted to encode calcium-dependent protein kinases (CDPKs). CDPK enzymatic activity previously has been detected in many locations in plant cells, including the cytosol, the cytoskeleton, and the membrane fraction. However, little is known about the subcellular locations of individual CDPKs or the mechanisms involved in targeting them to those locations. We investigated the subcellular location of one Arabidopsis CDPK, AtCPK2, in detail. Membrane-associated AtCPK2 did not partition with the plasma membrane in a two-phase system. Sucrose gradient fractionation of microsomes demonstrated that AtCPK2 was associated with the endoplasmic reticulum (ER). AtCPK2 does not contain transmembrane domains or known ER-targeting signals, but does have predicted amino-terminal acylation sites. AtCPK2 was myristoylated in a cell-free extract and myristoylation was prevented by converting the glycine at the proposed site of myristate attachment to alanine (G2A). In plants, the G2A mutation decreased AtCPK2 membrane association by approximately 50%. A recombinant protein, consisting of the first 10 amino acids of AtCPK2 fused to the amino-terminus of beta-glucuronidase, was also targeted to the ER, indicating that the amino terminus of AtCPK2 can specify ER localization of a soluble protein. These results indicate that AtCPK2 is localized to the ER, that myristoylation is likely to be involved in the membrane association of AtCPK2, and that the amino terminal region of AtCPK2 is sufficient for correct membrane targeting.

Cross-talk unfolded: MARCKS proteins

The proteins of the MARCKS (myristoylated alanine-rich C kinase substrate) family were first identified as prominent substrates of protein kinase C (PKC). Since then, these proteins have been implicated in the regulation of brain development and postnatal survival, cellular migration and adhesion, as well as endo-, exo- and phago-cytosis, and neurosecretion. The effector domain of MARCKS proteins is phosphorylated by PKC, binds to calmodulin and contributes to membrane binding. This multitude of mutually exclusive interactions allows cross-talk between the signal transduction pathways involving PKC and calmodulin. This review focuses on recent, mostly biophysical and biochemical results renewing interest in this protein family. MARCKS membrane binding is now understood at the molecular level. From a structural point of view, there is a consensus emerging that MARCKS proteins are "natively unfolded". Interestingly, domains similar to the effector domain have been discovered in other proteins. Furthermore, since the effector domain enhances the polymerization of actin in vitro, MARCKS proteins have been proposed to mediate regulation of the actin cytoskeleton. However, the recent observations that MARCKS might serve to sequester phosphatidylinositol 4,5-bisphosphate in the plasma membrane of unstimulated cells suggest an alternative model for the control of the actin cytoskeleton. While myristoylation is classically considered to be a co-translational, irreversible event, new reports on MARCKS proteins suggest a more dynamic picture of this protein modification. Finally, studies with mice lacking MARCKS proteins have investigated the functions of these proteins during embryonic development in the intact organism.

Protein-induced formation of cholesterol-rich domains

A major protein of neuronal rafts, NAP-22, binds specifically to cholesterol. We demonstrate by circular dichroism that NAP-22 contains a significant amount of beta-structure that is not sensitive to binding of the protein to membranes, suggesting that a major portion of the protein does not insert deeply into the membrane. The free energy of binding of NAP-22 to liposomes of dioleoylphosphatidylcholine with 40% cholesterol is -7.3 +/- 0.5 kcal/mol. NAP-22 mixed with dipalmitoylphosphatidylcholine and 40% cholesterol partitions into the detergent insoluble fraction in the presence of 1% Triton X-100. NAP-22 also causes this insoluble fraction to become enriched in cholesterol relative to phospholipid, again demonstrating the ability of this protein to segregate cholesterol and phospholipids into domains. Differential scanning calorimetry results demonstrate that NAP-22 promotes domain formation in liposomes composed of cholesterol and phosphatidylcholine. This is shown by NAP-22-promoted changes in the shape and enthalpy of the phase transition of phosphatidylcholine as well as by the appearance of cholesterol crystallite transitions in membranes composed of phosphatidylcholine with either saturated or unsaturated acyl chains. In situ atomic force microscopy revealed a marked change in the surface morphology of a supported bilayer of dioleoylphosphatidylcholine with 0.4 mole fraction of cholesterol upon addition of NAP-22. Prior to the addition of the protein, the bilayer appears to be a molecularly smooth structure with uniform thickness. Addition of NAP-22 resulted in the rapid formation of localized raised bilayer domains. Remarkably, there was no gross disruption or erosion of the bilayer but rather simply an apparent rearrangement of the lipid bilayer structure due to the interaction of NAP-22 with the lipid. Our results demonstrate that NAP-22 can induce the formation of cholesterol-rich domains in membranes. This is likely to be relevant in neuronal membrane domains that are rich in NAP-22.

The binding of myristoylated N-terminal nonapeptide from neuro-specific protein CAP-23/NAP-22 to calmodulin does not induce the globular structure observed for the calmodulin-nonmyristylated peptide complex

CAP-23/NAP-22, a neuron-specific protein kinase C substrate, is Nalpha-myristoylated and interacts with calmodulin (CaM) in the presence of Ca2+ ions. Takasaki et al. (1999, J Biol Chem 274:11848-11853) have recently found that the myristoylated N-terminal nonapeptide of CAP-23/NAP-22 (mC/N9) binds to Ca2+ -bound CaM (Ca2+/CaM). In the present study, small-angle X-ray scattering was used to investigate structural changes of Ca2+/CaM induced by its binding to mC/N9 in solution. The binding of one mC/N9 molecule induced an insignificant structural change in Ca2+/CaM. The 1:1 complex appeared to retain the extended conformation much like that of Ca2+/CaM in isolation. However, it could be seen that the binding of two mC/N9 molecules induced a drastic structural change in Ca2+/CaM, followed by a slight structural change by the binding of more than two but less than four mC/N9 molecules. Under the saturated condition (the molar ratio of 1:4), the radius of gyration (Rg) for the Ca2+/CaM-mC/N9 complex was 19.8 +/- 0.3 A. This value was significantly smaller than that of Ca2+/CaM (21.9 +/- 0.3 A), which adopted a dumbbell structure and was conversely 2-3 A larger than those of the complexes of Ca2+/CaM with the nonmyristoylated target peptides of myosin light chain kinase or CaM kinase II, which adopted a compact globular structure. The pair distance distribution function had no shoulder peak at around 40 A, which was mainly due to the dumbbell structure. These results suggest that Ca2+/CaM interacts with Nalpha-myristoylated CAP-23/NAP-22 differently than it does with other nonmyristoylated target proteins. The N-terminal amino acid sequence alignment of CAP-23/NAP-22 and other myristoylated proteins suggests that the protein myristoylation plays important roles not only in the binding of CAP-23/NAP-22 to Ca2+/CaM, but also in the protein-protein interactions related to other myristoylated proteins.

Analogous structural motifs in myelin basic protein and in MARCKS

Myelin basic protein (MBP) and myristoylated alanine-rich C-kinase substrate (MARCKS) are similar in terms of having extended conformations regulated by their environment (i.e., solubilised or lipid-associated), N-terminal modifications, a dual nature of interactions with lipids, binding to actin and Ca2+-calmodulin, and being substrates for different kinds of protein kinases. The further sequence similarities of segments of MBP with lipid effector regions of MARCKS, and numerous reports in the literature, support the thesis that some developmental isoform of MBP functions in signal transduction.

Mapping the interface between calmodulin and MARCKS-related protein by fluorescence spectroscopy

MARCKS-related protein (MRP) is a myristoylated protein kinase C substrate that binds calmodulin (CaM) with nanomolar affinity. To obtain structural information on this protein, we have engineered 10 tryptophan residues between positions 89 and 104 in the effector domain, a 24-residue-long amphipathic segment that mediates binding of MRP to CaM. We show that the effector domain is in a polar environment in free MRP, suggesting exposure to water, in agreement with a rod-shaped structure of the protein. The effector domain participates in the binding of MRP to CaM, as judged by the dramatic changes observed in the fluorescent properties of the mutants on complex formation. Intermolecular quenching of the fluorescence emission of the tryptophan residues in MRP by selenomethionine residues engineered in CaM reveals that the N-terminal side of the effector domain contacts the C-terminal domain of CaM, whereas the C-terminal side of the effector domain contacts the N-terminal domain of CaM. Finally, a comparison of the fluorescent properties of the myristoylated and unmyristoylated forms of a construct in which a tryptophan residue was introduced at position 4 close to the myristoylated N terminus of MRP suggests that the lipid moiety is also involved in the interaction of MRP with CaM.