The first seven amino acids encoded by the v-src oncogene act as a myristylation signal: lysine 7 is a critical determinant

Lipid-driven Src self-association modulates its transformation capacity

Src tyrosine kinase regulates cell growth and adhesion through membrane signaling, and its deregulation is associated with cancer. Although active Src is anchored to the plasma membrane, the role of membrane lipids in its regulation remains unclear. Here, we report that Src self-associates via a lysine cluster in its SH4 region, a process mediated by lipids in human cells and in vitro. Mutation of the lysine cluster to arginine alters Src self-association and modulates its transforming function in human cells. Lipid-anchored micron-sized condensates of full-length Src form in supported homogeneous lipid bilayers (i.e., independently of lipid phase separation). Condensates also arise from the purified Src N-terminal regulatory element, which includes the myristoylated SH4 domain, the intrinsically disordered Unique domain, and the globular SH3 domain. However, the isolated SH4 domain alone forms small protein-lipid clusters rather than micron-sized condensates. Our findings reveal lipid-mediated kinase self-association as an additional regulatory mechanism for Src. This mechanism may also apply to other membrane-associated signaling proteins containing similar lysine clusters in their unstructured regions.

Fyn Tyrosine Kinase as Harmonizing Factor in Neuronal Functions and Dysfunctions

Fyn is a non-receptor or cytoplasmatic tyrosine kinase (TK) belonging to the Src family kinases (SFKs) involved in multiple transduction pathways in the central nervous system (CNS) including synaptic transmission, myelination, axon guidance, and oligodendrocyte formation. Almost one hundred years after the original description of Fyn, this protein continues to attract extreme interest because of its multiplicity of actions in the molecular signaling pathways underlying neurodevelopmental as well as neuropathologic events. This review highlights and summarizes the most relevant recent findings pertinent to the role that Fyn exerts in the brain, emphasizing aspects related to neurodevelopment and synaptic plasticity. Fyn is a common factor in healthy and diseased brains that targets different proteins and shapes different transduction signals according to the neurological conditions. We will primarily focus on Fyn-mediated signaling pathways involved in neuronal differentiation and plasticity that have been subjected to considerable attention lately, opening the fascinating scenario to target Fyn TK for the development of potential therapeutic interventions for the treatment of CNS injuries and certain neurodegenerative disorders like Alzheimer’s disease.

Toxoplasma gondii immune mapped protein 1 is anchored to the inner leaflet of the plasma membrane and adopts a novel protein fold

The immune mapped protein 1 (IMP1) was first identified as a protective antigen in Eimeria maxima and described as vaccine candidate and invasion factor in Toxoplasma gondii. We show here that TgIMP1 localizes to the inner leaflet of plasma membrane (PM) via dual acylation. Mutations either in the N-terminal myristoylation or palmitoylation sites (G2 and C5) cause relocalization of TgIMP1 to the cytosol. The first 11 amino acids are sufficient for PM targeting and the presence of lysine (K7) is critical. Disruption of TgIMP1 gene by double homologous recombination revealed no invasion defect or any measurable alteration in the lytic cycle of tachyzoites. Following immunization with TgIMP1 DNA vaccine, mice challenged with either wild type or IMP1-ko parasites showed no significant difference in protection. The sequence analysis identified a structured C-terminal domain that is present in a broader family of IMP1-like proteins conserved across the members of Apicomplexa. We present the solution structure of this domain determined from NMR data and describe a new protein fold not seen before.

Domain Interaction Studies of Herpes Simplex Virus 1 Tegument Protein UL16 Reveal Its Interaction with Mitochondria

The UL16 tegument protein of herpes simplex virus 1 (HSV-1) is conserved among all herpesviruses and plays many roles during replication. This protein has an N-terminal domain (NTD) that has been shown to bind to several viral proteins, including UL11, VP22, and glycoprotein E, and these interactions are negatively regulated by a C-terminal domain (CTD). Thus, in pairwise transfections, UL16 binding is enabled only when the CTD is absent or altered. Based on these results, we hypothesized that direct interactions occur between the NTD and the CTD. Here we report that the separated and coexpressed functional domains of UL16 are mutually responsive to each other in transfected cells and form complexes that are stable enough to be captured in coimmunoprecipitation assays. Moreover, we found that the CTD can associate with itself. To our surprise, the CTD was also found to contain a novel and intrinsic ability to localize to specific spots on mitochondria in transfected cells. Subsequent analyses of HSV-infected cells by immunogold electron microscopy and live-cell confocal imaging revealed a population of UL16 that does not merely accumulate on mitochondria but in fact makes dynamic contacts with these organelles in a time-dependent manner. These findings suggest that the domain interactions of UL16 serve to regulate not just the interaction of this tegument protein with its viral binding partners but also its interactions with mitochondria. The purpose of this novel interaction remains to be determined.

IMPORTANCE

The HSV-1-encoded tegument protein UL16 is involved in multiple events of the virus replication cycle, ranging from virus assembly to cell-cell spread of the virus, and hence it can serve as an important drug target. Unfortunately, a lack of both structural and functional information limits our understanding of this protein. The discovery of domain interactions within UL16 and the novel ability of UL16 to interact with mitochondria in HSV-infected cells lays a foundational framework for future investigations aimed at deciphering the structure and function of not just UL16 of HSV-1 but also its homologs in other herpesviruses.

Identification of avian leukosis virus subgroup J-associated acutely transforming viruses carrying the v-src oncogene in layer chickens

To elucidate the molecular basis for the rapid oncogenicity of an acutely transforming avian leukosis virus (ALV), isolated from fibrosarcomas in Hy-Line Brown commercial layer chickens infected with ALV subgroup J (ALV-J), the complete genomic structure of the provirus was determined. In addition to ALV-J replication-complete virus SDAU1102, five proviral DNA genomes, named SJ-1, SJ-2, SJ-3, SJ-4 and SJ-5, carrying different lengths of the v-src oncogene were amplified from original tumours and chicken embryo fibroblasts (CEFs) infected with viral stocks. The genomic sequences of the SJ-1-SJ-5 provirus were closely related to that of SDAU1102 but were defective. The results of Western blot analysis and immunohistochemical staining also showed overexpression of the p60v-src protein in infected CEFs and tumour tissue. To the best of our knowledge, this is the first report of the isolation and identification of acutely transforming viruses carrying the v-src oncogene with ALV-J as the helper virus. It also offers insight into the generation of acutely transforming ALVs carrying the v-src oncogene.

MXS-Chaining: A Highly Efficient Cloning Platform for Imaging and Flow Cytometry Approaches in Mammalian Systems

The continuous improvement of imaging technologies has driven the development of sophisticated reporters to monitor biological processes. Such constructs should ideally be assembled in a flexible enough way to allow for their optimization. Here we describe a highly reliable cloning method to efficiently assemble constructs for imaging or flow cytometry applications in mammalian cell culture systems. We bioinformatically identified a list of restriction enzymes whose sites are rarely found in human and mouse cDNA libraries. From the best candidates, we chose an enzyme combination (MluI, XhoI and SalI: MXS) that enables iterative chaining of individual building blocks. The ligation scar resulting from the compatible XhoI- and SalI-sticky ends can be translated and hence enables easy in-frame cloning of coding sequences. The robustness of the MXS-chaining approach was validated by assembling constructs up to 20 kb long and comprising up to 34 individual building blocks. By assessing the success rate of 400 ligation reactions, we determined cloning efficiency to be 90% on average. Large polycistronic constructs for single-cell imaging or flow cytometry applications were generated to demonstrate the versatility of the MXS-chaining approach. We devised several constructs that fluorescently label subcellular structures, an adapted version of FUCCI (fluorescent, ubiquitination-based cell cycle indicator) optimized to visualize cell cycle progression in mouse embryonic stem cells and an array of artificial promoters enabling dosage of doxycyline-inducible transgene expression. We made publicly available through the Addgene repository a comprehensive set of MXS-building blocks comprising custom vectors, a set of fluorescent proteins, constitutive promoters, polyadenylation signals, selection cassettes and tools for inducible gene expression. Finally, detailed guidelines describe how to chain together prebuilt MXS-building blocks and how to generate new customized MXS-building blocks.

Cis- and cell-type-dependent trans-requirements for Lassa virus-like particle production

Lassa virus (LASV) small zinc-finger protein (Z), which contains two L-domain motifs, plays a central role in virus budding. Here, we report that co-expression of glycoprotein (GPC) altered the requirements for cholesterol but not the L-domains and host factor, Tsg101, for Z-induced virus-like particle (VLP) production. In particular, the cholesterol requirement for VLP production was cell-type-dependent. In addition, GPC was found to be important for co-localization of Z with CD63, a late endosomal marker. We also found that the N-terminal region (aa 3-10) of Z was critical for its myristoylation and VLP production. These findings will contribute to our understanding of LASV assembly and budding.

Src protein-tyrosine kinase structure, mechanism, and small molecule inhibitors

The physiological Src proto-oncogene is a protein-tyrosine kinase that plays key roles in cell growth, division, migration, and survival signaling pathways. From the N- to C-terminus, Src contains a unique domain, an SH3 domain, an SH2 domain, a protein-tyrosine kinase domain, and a regulatory tail. The chief phosphorylation sites of human Src include an activating pTyr419 that results from phosphorylation in the kinase domain by an adjacent Src molecule and an inhibitory pTyr530 in the regulatory tail that results from phosphorylation by C-terminal Src kinase (Csk) or Chk (Csk homologous kinase). The oncogenic Rous sarcoma viral protein lacks the equivalent of Tyr530 and is constitutively activated. Inactive Src is stabilized by SH2 and SH3 domains on the rear of the kinase domain where they form an immobilizing and inhibitory clamp. Protein kinases including Src contain hydrophobic regulatory and catalytic spines and collateral shell residues that are required to assemble the active enzyme. In the inactive enzyme, the regulatory spine contains a kink or a discontinuity with a structure that is incompatible with catalysis. The conversion of inactive to active Src is accompanied by electrostatic exchanges involving the breaking and making of distinct sets of kinase domain salt bridges and hydrogen bonds. Src-catalyzed protein phosphorylation requires the participation of two Mg(2+) ions. Although nearly all protein kinases possess a common K/E/D/D signature, each enzyme exhibits its unique variations of the protein-kinase reaction template. Bosutinib, dasatinib, and ponatinib are Src/multikinase inhibitors that are approved by the FDA for the treatment of chronic myelogenous leukemia and vandetanib is approved for the treatment of medullary thyroid cancer. The Src and BCR-Abl inhibitors saracatinib and AZD0424, along with the previous four drugs, are in clinical trials for a variety of solid tumors including breast and lung cancers. Both ATP and targeted therapeutic Src protein kinase inhibitors such as dasatinib and ponatinib make hydrophobic contacts with catalytic spine residues and form hydrogen bonds with hinge residues connecting the small and large kinase lobes.

Lipid binding by the Unique and SH3 domains of c-Src suggests a new regulatory mechanism

c-Src is a non-receptor tyrosine kinase involved in numerous signal transduction pathways. The kinase, SH3 and SH2 domains of c-Src are attached to the membrane-anchoring SH4 domain through the flexible Unique domain. Here we show intra- and intermolecular interactions involving the Unique and SH3 domains suggesting the presence of a previously unrecognized additional regulation layer in c-Src. We have characterized lipid binding by the Unique and SH3 domains, their intramolecular interaction and its allosteric modulation by a SH3-binding peptide or by Calcium-loaded calmodulin binding to the Unique domain. We also show reduced lipid binding following phosphorylation at conserved sites of the Unique domain. Finally, we show that injection of full-length c-Src with mutations that abolish lipid binding by the Unique domain causes a strong in vivo phenotype distinct from that of wild-type c-Src in a Xenopus oocyte model system, confirming the functional role of the Unique domain in c-Src regulation.

A dual role for caveolin-1 in the regulation of fibronectin matrix assembly by uPAR

The relationship between the plasminogen activator system and integrin function is well documented but incompletely understood. The mechanism of uPAR-mediated signaling across the membrane and the molecular basis of uPAR-dependent activation of integrins remain important issues. The present study was undertaken to identify the molecular intermediates involved in the uPAR signaling pathway controlling alpha5beta1-integrin activation and fibronectin polymerization. Disruption of lipid rafts with MbetaCD or depletion of caveolin-1 by siRNA led to the inhibition of uPAR-dependent integrin activation and stimulation of fibronectin polymerization in human dermal fibroblasts. The data indicate a dual role for caveolin-1 in the uPAR signaling pathway, leading to integrin activation. Caveolin-1 functions initially as a membrane adaptor or scaffold to mediate uPAR-dependent activation of Src and EGFR. Subsequently, in its phosphorylated form, caveolin-1 acts as an accessory molecule to direct trafficking of activated EGFR to focal adhesions. These studies provide a novel paradigm for the regulation of crosstalk among integrins, growth-factor receptors and uPAR.

SH4-domain-induced plasma membrane dynamization promotes bleb-associated cell motility

SH4 domains provide bipartite membrane-targeting signals for oncogenic Src family kinases. Here we report the induction of non-apoptotic plasma membrane (PM) blebbing as a novel and conserved activity of SH4 domains derived from the prototypic Src kinases Src, Fyn, Yes and Lck as well as the HASPB protein of Leishmania parasites. SH4-domain-induced blebbing is highly dynamic, with bleb formation and collapse displaying distinct kinetics. These reorganizations of the PM are controlled by Rho but not Rac or Cdc42 GTPase signalling pathways. SH4-induced membrane blebbing requires the membrane association of the SH4 domain, is regulated by the activities of Rock kinase and myosin II ATPase, and depends on the integrity of F-actin as well as microtubules. Endogenous Src kinase activity is crucial for PM blebbing in SH4-domain-expressing cells, active Src and Rock kinases are enriched in SH4-domain-induced PM blebs, and PM blebbing correlates with enhanced cell invasion in 3D matrices. These results establish a novel link between SH4 domains, Src activity and Rho signalling, and implicate SH4-domain-mediated PM dynamization as a mechanism that influences invasiveness of cells transformed by SH4-domain-containing oncoproteins.

Src tyrosine kinase as a chemotherapeutic target: is there a clinical case?

Src tyrosine kinase was the first protooncogene described. It has been found to be overexpressed and activated in a large number of different cancers. Cellular Src has been shown to activate a number of different effectors that are involved in different aspects of cancer biology such as metastasis, cell cycle regulation and cell survival. Despite this, Src inhibitors have not entered the regular arsenal of chemotherapeutics. This article reviews some of the biology, rationale, in vitro and in vivo preclinical evidence, and some very early clinical trials demonstrating efficacy of Src inhibitors.

VHY, a Novel Myristoylated Testis-restricted Dual Specificity Protein Phosphatase Related to VHX

The human DUSP15 gene encodes an uncharacterized 235-amino acid member of the subfamily of small dual specificity protein phosphatases related to the Vaccinia virus VH1 phosphatase. Similar to VHR-related MKPX (VHX) (DUSP22), the predicted protein has an N-terminal myristoylation recognition sequence, and we show here that both are indeed modified by the attachment of a myristate to Gly-2. In recognition of this relatedness to VHX, we refer to the DUSP15-encoded protein as VH1-related member Y (VHY). We report that VHY is expressed at high levels in the testis and barely detectable levels in the brain, spinal cord, and thyroid. A VHY-specific antiserum detected a protein with an apparent molecular mass of 26 kDa, and histochemical analysis showed that VHY was readily detectable in pachytene spermatocytes (midstage of meiotic division I) and round spermatids and weakly in Leydig cells (somatic cells outside of the seminiferous tubules). When expressed in 293T or NIH-3T3 cells, VHY was concentrated at the plasma membrane with some staining of vesicular structures in the Golgi region. Mutation of the myristoylation site Gly-2 abrogated membrane location. Finally, we demonstrate that VHY is an active phosphatase in vitro. We conclude that VHY is a new member of a subgroup of myristoylated VH1-like small dual specificity phosphatases.

Inhibitors of JAKs/STATs and the kinases: a possible new cluster of drugs

JAK(s)/STAT(s) relay cytokine signals through tyrosine site-specific phosphorylation of the proteins involved in cellular responses for the activation and proliferation of bone marrow-derived cells. In recent years, the constitutive or elevated expression of JAK/STAT has been found in cancer cells and oncogene transfected cells, and has been shown to be involved in the immune rejection of allografts and the inflammatory processes of autoimmune diseases. This review discusses the strategies for screening and rational design of selective, potent JAK/STAT and kinase inhibitors that are either ATP-competitive or non-ATP competitive, naturally derived or synthetic, as well as other unique inhibitors and analogues for different therapeutic indications.

The N-terminal SH4 region of the Src family kinase Fyn is modified by methylation and heterogeneous fatty acylation: role in membrane targeting, cell adhesion, and spreading

The N-terminal SH4 domain of Src family kinases is responsible for promoting membrane binding and plasma membrane targeting. Most Src family kinases contain an N-terminal Met-Gly-Cys consensus sequence that undergoes dual acylation with myristate and palmitate after removal of methionine. Previous studies of Src family kinase fatty acylation have relied on radiolabeling of cells with radioactive fatty acids. Although this method is useful for verifying that a given fatty acid is attached to a protein, it does not reveal whether other fatty acids or other modifying groups are attached to the protein. Here we use matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry to identify fatty acylated species of the Src family kinase Fyn. Our results reveal that Fyn is efficiently myristoylated and that some of the myristoylated proteins are also heterogeneously S-acylated with palmitate, palmitoleate, stearate, or oleate. Furthermore, we show for the first time that Fyn is trimethylated at lysine residues 7 and/or 9 within its N-terminal region. Both myristoylation and palmitoylation were required for methylation of Fyn. However, a general methylation inhibitor had no inhibitory effect on myristoylation and palmitoylation of Fyn, suggesting that methylation occurs after myristoylation and palmitoylation. Lysine mutants of Fyn that could not be methylated failed to promote cell adhesion and spreading, suggesting that methylation is important for Fyn function.

Lipid-dependent recruitment of neuronal Src to lipid rafts in the brain

Although most Src family tyrosine kinases are modified by palmitoylation as well as myristoylation, Src itself is only myristoylated. Dual acylation is important for attachment to liquid-ordered microdomains or lipid rafts. Accordingly, Src is excluded from lipid rafts in fibroblasts. Evidence of partial genetic redundancy between Src and Fyn for brain-specific targets suggests that these two kinases may occupy overlapping subcellular locations. Neuronal Src (NSrc), an alternative isoform of Src with a 6-amino acid insert in the Src homology 3 domain, is highly expressed in neurons. We investigated whether this structural difference in NSrc allows it to associate with lipid rafts. We found that perinatal mouse brains express predominantly NSrc, which is partly (10-20%) in a lipid raft fraction from brain but not fibroblasts. The association of Src with brain lipid rafts does not depend on the NSrc insert but depends on the amino-terminal myristoylation signal. A crude lipid fraction from brain promotes NSrc entry into rafts in vitro. Moreover, lipid raft-localized NSrc is more catalytically active than NSrc from the soluble fraction, possibly because raft localization alters access to other tyrosine kinases and phosphatases. These findings suggest that NSrc may be involved in signaling from lipid rafts in mouse brain.

Src-family kinase signaling modulates the adhesion of Plasmodium falciparum on human microvascular endothelium under flow

The pathogenicity of Plasmodium falciparum is due to the unique ability of infected erythrocytes (IRBCs) to adhere to vascular endothelium. We investigated whether adhesion of IRBCs to CD36, the major cytoadherence receptor on human dermal microvascular endothelial cells (HDMECs), induces intracellular signaling and regulates adhesion. A recombinant peptide corresponding to the minimal CD36-binding domain from P falciparum erythrocyte membrane protein 1 (PfEMP1), as well as an anti-CD36 monoclonal antibody (mAb) that inhibits IRBC binding, activated the mitogen-activated protein (MAP) kinase pathway that was dependent on Src-family kinase activity. Treatment of HDMECs with a Src-family kinase-selective inhibitor (PP1) inhibited adhesion of IRBCs in a flow-chamber assay by 72% (P <.001). More importantly, Src-family kinase activity was also required for cytoadherence to intact human microvessels in a human/severe combined immunodeficient (SCID) mouse model in vivo. The effect of PP1 could be mimicked by levamisole, a specific alkaline-phosphatase inhibitor. Firm adhesion to PP1-treated endothelium was restored by exogenous alkaline phosphatase. In contrast, inhibition of the extracellular signal-regulated kinase 1/2 (ERK 1/2) and p38 MAP kinase pathways had no immediate effect on IRBC adhesion. These results suggest a novel mechanism for the modulation of cytoadherence under flow conditions through a signaling pathway involving CD36, Src-family kinases, and an ectoalkaline phosphatase. Targeting endothelial ectoalkaline phosphatases and/or signaling molecules may constitute a novel therapeutic strategy against severe falciparum malaria.

Selected glimpses into the activation and function of Src kinase

Since the discovery of the v-src and c-src genes and their products, much progress has been made in the elucidation of the structure, regulation, localization, and function of the Src protein. Src is a non-receptor protein tyrosine kinase that transduces signals that are involved in the control of a variety of cellular processes such as proliferation, differentiation, motility, and adhesion. Src is normally maintained in an inactive state, but can be activated transiently during cellular events such as mitosis, or constitutively by abnormal events such as mutation (i.e. v-Src and some human cancers). Activation of Src occurs as a result of disruption of the negative regulatory processes that normally suppress Src activity, and understanding the various mechanisms behind Src activation has been a target of intense study. Src associates with cellular membranes, in particular the plasma membrane, and endosomal membranes. Studies indicate that the different subcellular localizations of Src could be important for the regulation of specific cellular processes such as mitogenesis, cytoskeletal organization, and/or membrane trafficking. This review will discuss the history behind the discovery and initial characterization of Src and the regulatory mechanisms of Src activation, in particular, regulation by modification of the carboxy-terminal regulatory tyrosine by phosphatases and kinases. Its focus will then turn to the different subcellular localizations of Src and the possible roles of nuclear and perinuclear targets of Src. Finally, a brief section will review some of our present knowledge regarding Src involvement in human cancers.

Serine 6 of Lck tyrosine kinase: a critical site for Lck myristoylation, membrane localization, and function in T lymphocytes

Lck is a member of the Src family kinases expressed predominantly in T cells, and plays a pivotal role in TCR-mediated signal transduction. Myristoylation of glysine 2 in the N-terminal Src homology 4 (SH4) domain of Lck is essential for membrane localization and function. In this study, we examined a site within the SH4 domain of Lck regulating myristoylation, membrane localization, and function of Lck. A Lck mutant in which serine 6 (Ser6) was substituted by an alanine was almost completely cytosolic in COS-7 cells, and this change of localization was associated with a drastic inhibition of myristoylation in this mutant. To assess the role of Ser6 of Lck in T cell function, we established stable transfectants expressing various Lck mutants using Lck-negative JCaM1 cells. The Lck mutant of Ser6 to alanine, most of which did not target to the plasma membrane, was not able to reconstitute TCR-mediated signaling events in JCaM1 cells, as analyzed by tyrosine phosphorylation of intracellular proteins and CD69 expression. These results demonstrate that Ser6 is a critical factor for Lck myristoylation, membrane localization, and function in T cells, presumably because the residue is important for N-myristoyl transferase recognition.

Multiple roles for Src in a PDGF-stimulated cell

The observation that platelet-derived growth factor (PDGF) increases the catalytic activity of Src family members (Src) suggests that they contribute to PDGF-dependent responses. The role of Src in PDGF-dependent cell cycle progression, phosphorylation of proteins, and chemotaxis has been tested by investigators using a variety of cell types and approaches, and it appears that the contribution of Src is highly variable. This idea is perhaps best illustrated by the finding that Src plays radically different roles downstream of the PDGF alpha- and beta-receptor subunits. Hence, Src is a versatile signal relay enzyme, whose contribution to a signaling cascade depends on variables such as the nature of the receptor via which the cell is activated, as well as the cell type itself.

Tec family of protein-tyrosine kinases: an overview of their structure and function

The Tec family is a recently emerging subfamily of non-receptor protein-tyrosine kinases (PTKs) represented by its first member, Tec. This family is composed of five members, namely Tec, Btk. Itk/Emt/Tsk, Bmx and Txk/Rlk. The most characteristic feature of this family is the presence of a pleckstrin homology (PH) domain in their protein structure. The PH domain is known to bind phosphoinositides; on this basis, Tec family PTKs may act as merge points of phosphotyrosine-mediated and phospholipid-mediated signaling systems. Many Tec family proteins are abundantly expressed in hematopoietic tissues, and are presumed to play important roles in the growth and differentiation processes of blood cells. Supporting this, mutations in the Btk gene cause X chromosome-linked agammaglobulinemia (XLA) in humans and X chromosome-linked immunodeficiency (Xid) in mice, indicating that Btk activity is indispensable for B-cell ontogeny. In addition, Tec family kinases have been shown to be involved in the intracellular signaling mechanisms of cytokine receptors, lymphocyte surface antigens, heterotrimeric G-protein-coupled receptors and integrin molecules. Efforts are being made to identify molecules which interact with Tec kinases to transfer Tec-mediated signals in vivo. Candidates for such second messengers include PLC-gamma2, guanine nucleotide exchange factors for RhoA and TFII-I/BAP-135. This review summarizes current knowledge concerning the input and output factors affecting the Tec kinases.

DAPP1: a dual adaptor for phosphotyrosine and 3-phosphoinositides

We have identified a novel 280 amino acid protein which contains a putative myristoylation site at its N-terminus followed by an Src homology (SH2) domain and a pleckstrin homology (PH) domain at its C-terminus. It has been termed dual adaptor for phosphotyrosine and 3-phosphoinositides (DAPP1). DAPP1 is widely expressed and exhibits high-affinity interactions with PtdIns(3,4,5)P(3) and PtdIns(3,4)P(2), but not with other phospholipids tested. These observations predict that DAPP1 will interact with both tyrosine phosphorylated proteins and 3-phosphoinositides and may therefore play a role in regulating the location and/or activity of such proteins(s) in response to agonists that elevate PtdIns(3,4,5)P(3) and PtdIns(3,4)P(2).

Dual fatty acylation of p59(Fyn) is required for association with the T cell receptor zeta chain through phosphotyrosine-Src homology domain-2 interactions

The first 10 residues within the Src homology domain (SH)-4 domain of the Src family kinase Fyn are required for binding to the immune receptor tyrosine-based activation motif (ITAM) of T cell receptor (TCR) subunits. Recently, mutation of glycine 2, cysteine 3, and lysines 7 and 9 was shown to block binding of Fyn to TCR zeta chain ITAMs, prompting the designation of these residues as an ITAM recognition motif (Gauen, L.K.T., M.E. Linder, and A.S. Shaw. 1996. J. Cell Biol. 133:1007-1015). Here we show that these residues do not mediate direct interactions with TCR ITAMs, but rather are required for efficient myristoylation and palmitoylation of Fyn. Specifically, coexpression of a K7,9A-Fyn mutant with N-myristoyltransferase restored myristoylation, membrane binding, and association with the cytoplasmic tail of TCR zeta fused to CD8. Conversely, treatment of cells with 2-hydroxymyristate, a myristoylation inhibitor, blocked association of wild-type Fyn with zeta. The Fyn NH2 terminus was necessary but not sufficient for interaction with zeta and both Fyn kinase and SH2 domains were required, directing phosphorylation of zeta ITAM tyrosines and binding to zeta ITAM phosphotyrosines. Fyn/zeta interaction was sensitive to octylglucoside and filipin, agents that disrupt membrane rafts. Moreover, a plasma membrane bound, farnesylated Fyn construct, G2A,C3S-FynKRas, was not enriched in the detergent insoluble fraction and did not associate with zeta. We conclude that the Fyn SH4 domain provides the signals for fatty acylation and specific plasma membrane localization, stabilizing the interactions between the Fyn SH2 domain and phosphotyrosines in TCR zeta chain ITAMs.

The role of v-Fgr myristoylation and the Gag domain in membrane binding and cellular transformation

The v-fgr oncogene encodes a chimeric oncoprotein composed of feline sarcoma virus (FeSV)-derived gag and cellular-derived actin and c-Fgr sequences. v-Fgr is myristoylated and membrane bound, two criteria which must be met for src kinases to induce cellular transformation. Although inhibition of myristoylation resulted in a decreased ability of v-Fgr to sediment with membranes from an NIH-3T3 P100 fraction, deletion of the gag domain caused nearly all of the protein to remain unbound and cytosolic. Systematic deletions within gag indicate that while amino acids 3 through 9 are critical determinants of myristoylation and/or define a domain which directs membrane localization, these residues cooperate with additional gag sequences when anchoring the protein to the plasma membrane. Furthermore, nonmyristoylated and/or cytoplasmic variants of v-Fgr failed to induce anchorage-independent growth of NIH-3T3 cells, indicating that proper subcellular localization of v-Fgr is a key factor in its ability to induce transformation.

Substitution of leucine 8 in the simian immunodeficiency virus matrix protein impairs particle formation without affecting N-myristylation of the Gag precursor

In the late stages of replication of the simian immunodeficiency viruses (SIV), the matrix protein (MA) plays a central role in the transport of Pr55gag to the plasma membrane, assembly of virus particles, and incorporation of the envelope glycoprotein into particles. Targeting of Pr55gag to the plasma membrane is mediated by two motifs within the MA protein: the N-terminal myristate and a cluster of positively charged amino acids. In this report, we characterized the assembly phenotype of an SIV Gag mutant (L8Q) carrying the single amino acid substitution of glutamine for leucine at position 8 in the MA domain. The hydropathic profile of the mutated MA protein indicated that the L8Q amino acid change disrupts the hydrophobic character of the region comprising the first 10 residues of the protein. Expression of mutant L8Q Gag protein in CV-1 cells, by means of the vaccinia virus vector system, resulted in efficient synthesis and N-myristylation of Pr55gag. However, this mutation severely impaired particle production, as inferred from both biochemical and electron microscopy analyses. Cellular fractionation assays revealed that in cells expressing mutant L8Q, the proportion of cytosol-associated Pr55gag was significantly increased compared to that observed upon expression of wild-type Gag. Furthermore, mutant L8Q Gag partitioned onto cytosol and membrane fractions in a manner similar to nonmyristylated Gag polyprotein. Taken together, these results indicate that the L8Q mutation reduces the membrane-binding capacity of the Gag precursor. It is therefore likely that in the SIV MA, in addition to the N-myristate group, the hydrophobicity of the neighboring region is important for efficient association of Pr55gag with the plasma membrane.

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.

Regulation, substrates and functions of src

Src is the best understood member of a family of 9 tyrosine kinases that regulates cellular responses to extracellular stimuli. Activated mutants of Src are oncogenic. Using Src as an example, and referring to other Src family members where appropriate, this review describes the structure of Src, the functions of the individual domains, the regulation of Src kinase activity in the cell, the selection of substrates, and the biological functions of Src. The review concentrates on developments in the last 6-7 years, and cites data resulting from the isolation and characterization of Src mutants, crystallographic studies of the structures of SH2, SH3 and tyrosine kinase domains, biochemical studies of Src kinase activity and binding properties, and the biology of transgenic and knockout mouse strains.

A screen for genes that function downstream of Ras1 during Drosophila eye development

Cell-fate specification of the R7 photoreceptor cell is controlled by the sevenless receptor tyrosine kinase (SevRTK) and Ras1, the Drosophila homologue of mammalian H-ras, K-ras and N-ras oncogenes. An activated form of Ras1 expressed under control of the sevenless enhancer/promoter (sev-Ras1V12) induces production of supernumerary R7 photoreceptor cells, which causes the eye to become rough in appearance. To isolate mutations in genes functioning downstream of Ras1, we carried out a screen for dominant suppressors and enhancers of this rough eye phenotype. Approximately 850,000 mutagenized flies were screened, and 282 dominant suppressors and 577 dominant enhancers were isolated. Mutations in the Drosophila homologues of Raf, MEK, MAPK, type I Geranylgeranyl Transferase and Protein Phosphatase 2A were isolated, as were mutations in several novel signaling genes. Some of these mutant genes appear to be general signaling factors that function in other Ras1 pathways, while one seems to be more specific for photoreceptor development. At least two suppressors appear to function either between Ras1 and Raf or in parallel to Raf.

High expression of the tec gene product in murine testicular germ cells and erythroblasts

Tec is a novel non-receptor-type protein tyrosine kinase that was originally identified from a murine liver cDNA library. While the function of Tec remains unknown, it was shown recently that two Tec-related kinases are involved directly in the growth and differentiation of bone marrow stem cells. As the localization of Tec protein has not been reported yet, immunohistochemical and immunochemical studies of various murine organs were conducted in the present study to clarify which cells express this kinase protein. An intense immunohistologic reaction was observed in neonatal and adult testicular germ cells, and neonatal and fetal hepatic erythroblasts. In addition, a clear immunostaining was noted in neonatal and adult tubal epithelial cells, hepatocytes, basal cells of the non-glandular stomach, foveolar epithelium of the glandular stomach, sebaceous cells of the skin and fetal cartilage. The immune reaction of germ cells and erythroblasts was observed in the cell membrane, although this protein does not have a transmembrane domain. Supportive western blotting of testis, adult liver, spleen and heart of adult C.B-17 mice with the use of anti-Tec antibody demonstrated a heavy 70 kDa band in the liver and testis, and a much weaker, small band in the heart and spleen. These results suggest that Tec protein has a specific role in testicular germ cells and erythroblasts.

Myristoylation of endothelial cell nitric oxide synthase is important for extracellular release of nitric oxide

Endothelial cell nitric oxide synthase (NOS) is known to have a N-myristoylation consensus sequence. Such a consensus sequence is not evident in the macrophage, smooth muscle and neuronal NOS. A functional role for this N-terminal myristoylation is not clear yet. In the present study, we examined the effect of N-terminal myristoylation on the NOS activity determined by the conversion of L-[3H]arginine to L-[3H]citrulline and extracellular NO release determined by nitrite production in the conditioned medium from the COS-7 cells transfected with wild-type bovine aortic endothelial cell (BAEC) NOS cDNA or non-myristoylated BAEC-NOS mutant cDNA. NOS activity of wild type BAEC-NOS in COS-7 cells was localized in the particulate fraction and that of mutant NOS was in the cytosolic fraction. In contrast, nitrite production from COS-7 cells transfected with wild type BAEC-NOS cDNA was greater than that of mutant cDNA in a time dependent and a concentration dependent manner. These results suggest that membrane localization of NOS with myristoylation facilitates extracellular transport of NO and leads to enhanced NO signaling on the vascular smooth muscle cells and the intravascular blood cells including neutrophils, macrophages and platelets.

Structural and functional studies of the intracellular tyrosine kinase MATK gene and its translated product

We recently cloned the cDNA which encodes a novel megakaryocyte-associated tyrosine kinase termed MATK. In this study, we have cloned and characterized the human MATK gene as well as the murine homolog of human MATK cDNA and performed functional studies of its translated product. Comparison of the deduced amino acid sequences of human and murine MATK cDNAs revealed 85% homology, indicating that MATK is highly conserved in mouse and human. The human gene consists of 13 exons interrupted by 12 introns. The genetic units which encode the SH3 and SH2 domains are located on separate exons. The putative ATP binding site (GXGXXG) is localized on exon 7, and the entire catalytic domain is subdivided into seven exons (7-13). Somatic cell hybrid analysis indicated that human MATK gene is located on chromosome 19 while the murine Matk gene is located on chromosome 10. The immediate 5'-flanking region was highly rich in GC sequences, and potential cis-acting elements were identified including several SP1, GATA-1, APRE, and APRE1. Antisense oligonucleotides directed against MATK mRNA sequences significantly inhibited megakaryocyte progenitor proliferation. Functional studies indicated that MATK can phosphorylate the carboxyl-terminal conserved tyrosine of the Src protein. These results support the notion that MATK acts as a regulator of p60c-src in megakaryocytic cells and participates in the pathways regulating growth of cells of this lineage.

Identification of Rlk, a novel protein tyrosine kinase with predominant expression in the T cell lineage

The control of phosphorylation by protein tyrosine kinases represents an important regulatory mechanism in T cell growth, function, and differentiation. We have identified a 62-kDa murine protein tyrosine kinase predominantly expressed within the T cell lineage, which we have termed Rlk (for Resting lymphocyte kinase). rlk mRNA was found to be expressed in the fetal thymus as early as day 13 of embryonic development as well as in adult thymus and mature resting peripheral T cells. The sequence of rlk showed that it is most closely related to the subfamily of cytoplasmic tyrosine kinases that includes the Btk, Itk, and Tec proteins. However, Rlk differs from these kinases by virtue of its unique aminoterminal domain, which lacks a region of pleckstrin homology common to the other members of this protein subfamily. Examination of rlk abundance within different T cell subpopulations revealed preferential expression in Th1 relative to Th2 T cell clones, suggesting a possible role in signal transduction pathways that selectively regulate cytokine production in mature CD4+ T cell subsets. Rlk thus represents a novel cytoplasmic tyrosine kinase with potential functions in intrathymic T cell development and mature T cell signaling.

SH3 domains specifically regulate kinase activity of expressed Src family proteins

The Src homology 2 (SH2) and Src homology 3 (SH3) domain are approximately 50% conserved in various Src family kinase members. Several lines of evidence suggest that in Src these domains are sequence motifs that direct substrate recognition, regulate kinase activity, or control subcellular localization. We sought to investigate the function of the homology domains in human Lyn, and to determine whether the differences between various SH3 domains affect function. To do this, we generated variant forms of Lyn lacking SH2 and SH3 domains, and created chimeras in which the SH3 domains in human c-Src and Lyn were replaced with SH3 domains from other family members. In contrast to similar deletions in Src, forms of Lyn lacking SH2 or SH3 had decreased kinase activity. The SH3 chimeras all had individual characteristics. Insertion of the Blk SH3 domain into Lyn restored kinase activity, while insertion of the Fyn or Src SH3 into Lyn enhanced the kinase activity 2-3-fold. Insertion of the Lyn SH3 into Src also doubled kinase activity. Expression of the Lyn-Src SH3 chimera in mammalian cells induced cell transformation. This study 1) demonstrates that the regulation of Lyn is different than Src, and 2) provides new evidence that despite their homology, there are important functional differences between the SH3 domains of the various Src family members.

Cloning of FRK, a novel human intracellular SRC-like tyrosine kinase-encoding gene

We report the cloning of a novel tyrosine kinase (TyK)-encoding gene (TYK) from the human hepatoma cell line Hep3B. Using the polymerase chain reaction (PCR) and oligodeoxyribonucleotide primers based on conserved TYK motifs, a 180-bp fragment was cloned and used to obtain full-length cDNA clones of 2.9 kb, with an open reading frame of 505 amino acids (aa). Restricted expression was detected by Northern blotting or reverse-transcribed PCR in a broad range of cell lines. The predicted aa sequence contains characteristic TyK motifs without a transmembrane region, suggesting an intracellular localization. There was 49% aa sequence identity with human FYN product and 47% with human SRC product; however, several structural differences distinguish this clone from other SRC subfamily members. This clone, FYN-related kinase or FRK, is a novel member of the intracellular TYK gene family.

Deficient expression of a B cell cytoplasmic tyrosine kinase in human X-linked agammaglobulinemia

We describe a novel cytoplasmic tyrosine kinase, termed BPK (B cell progenitor kinase), which is expressed in all stages of the B lineage and in myeloid cells. BPK has classic SH1, SH2, and SH3 domains, but lacks myristylation signals and a regulatory phosphorylation site corresponding to tyrosine 527 of c-src. BPK has a long, basic amino-terminal region upstream of the SH3 domain. BPK was evaluated as a candidate for human X-linked agammaglobulinemia (XLA), an inherited immunodeficiency characterized by a severe deficit of B and plasma cells and profound hypogammaglobulinemia. BPK mapped to within 100 kb of a probe defining the polymorphism most closely linked to XLA at DXS178. Reduction in or the absence of BPK mRNA, protein expression, and kinase activity was observed in XLA pre-B and B cell lines. BPK is likely the XLA gene and functions in pathways critical to B cell expansion.

Endothelial nitric oxide synthase is myristylated

The enzyme responsible for the synthesis of endothelium-derived relaxing factor and/or nitric oxide in the endothelium has been described as a particulate enzyme, whereas other isoforms of nitric oxide synthase are soluble enzymes. Here we are reporting that endothelial cells metabolically incorporate myristate (C14), but not palmitate (C16), into nitric oxide synthase. We are postulating that the endothelial-derived nitric oxide synthase is a particulate enzyme because of the fatty acid acylation of the protein which 'anchors' the enzyme into the membrane either directly or via another membrane-bound protein.

Studies on processing, particle formation, and immunogenicity of the HIV-1 gag gene product: a possible component of a HIV vaccine

Antigens in a particulate conformation were shown to be highly immunogenic in mammals. For this reason, the particle forming capacity of derivatives of the HIV-1 group specific core antigen p55 gag was assayed and compared dependent on various expression systems: recombinant bacteria, vaccinia- and baculoviruses were established encoding the entire core protein p55 either in its authentic sequence or lacking the myristylation consensus signal. Moreover, p55 gag was expressed in combination with the protease (p55-PR) or with the entire polymerase (p55-pol), respectively. Budding of 100-160 nm p55 core particles, resembling immature HIV-virions, was observed in the eucaryotic expression systems only. In comparison to the vaccinia virus driven expression of p55 in mammalian cells, considerably higher yields of particulate core antigen were obtained by infection of Spodoptera frugiperda (Sf9) insect cells with the recombinant Autographa californica nuclear polyhedrosis (AcMNPV) baculovirus. Mutation of the NH2-terminal myristylation signal sequence prevented budding of the immature core particles. Expression of the HIV p55-PR gene construct by recombinant baculovirus resulted in complete processing of the p55 gag precursor molecule in this system. The introduction of an artificial frameshift near the natural frameshift site resulted in constitutive expression of the viral protease and complete processing of p55, both in Escherichia coli and in vaccinia virus infected cells. Interestingly, significant processing of p55 resembling that of HIV infected H9 cells could also be achieved in the vaccinia system by fusing the entire pol gene to the gag gene. Moreover, processing was not found to be dependent on amino-terminal myristylation of the gag procursor molecule, which is in contrast to observations with type C and type D retrovirus. However, complete processing of p55 into p24, p17, p9 and p6 abolished particle formation. Purified immature HIV-virus like particles were highly immunogenic in rabbits, leading to a strong humoral immune response after immunization. Empty immature p55 gag particles represent a noninfectious and attractive candidate for a basic vaccine component.

Analysis of protein transport through the Golgi in a reconstituted cell-free system

The processes which transport membrane proteins between compartments of the Golgi apparatus have been reconstituted in vitro using isolated Golgi fractions. This cell-free system allows a detailed analysis of protein transport not possible in intact cells. Transport of the membrane glycoprotein (G protein) of vesicular stomatitis virus (VSV) is measured from a "donor" to an "acceptor" Golgi fraction. The donor Golgi fraction is prepared from VSV-infected Chinese hamster ovary (CHO) mutant cells deficient in the glycosylation enzyme N-acetylglucosamine transferase I. "Acceptor" is prepared from uninfected wild-type CHO cells. Transport is measured by the addition of N-acetylglucosamine to G protein, which can occur only upon movement of G protein from donor to acceptor. Transport requires physiological pH and osmolarity, is dependent on nucleotide triphosphates, and is mediated by proteins both from cytosol and on the Golgi membranes. Protein movement is inhibited by the non-hydrolyzable GTP analogue, GTP gamma S. The process of transport proceeds through the budding, pinching off, targeting, and fusion of transport vesicles. In this system these vesicles are initially coated with a non-clathrin coat and are targeted with this coat intact. Several of the proteins which mediate transport have been characterized, and isolated to homogeneity. The successful development of this assay has led to the formulation of cell free assays for protein transport between other compartments. Comparison of these systems indicates that some common mechanisms of vesicular movement are used in transport between a variety of membrane compartments.