Trix, a novel Rac guanine-nucleotide exchange factor from Dictyostelium discoideum is an actin-binding protein and accumulates at endosomes

Small Rho family GTPases are involved in regulation of actin cytoskeleton dynamics. These molecular switches are themselves mainly controlled by specific GTPase-activating proteins (GAPs) and guanine-nucleotide exchange factors (GEFs). We have cloned and initially characterized a novel putative RhoGEF from Dictyostelium discoideum. The predicted 135-kDa protein displays a unique domain organization in its N-terminus by harboring two type3 calponin homology (CH) domains followed by a single type1 CH domain. The C-terminal region encompasses a diffuse B-cell lymphoma homology/pleckstrin homology tandem domain that is typically found in RhoGEFs. We therefore refer to this protein as Trix (triple CH-domain array exchange factor). A recombinant N-terminal region of Trix carrying all three CH domains binds to F-actin and bundles actin filaments. Trix-null mutants are viable and display only subtle defects when compared to wild-type cells with the exception of a substantial decrease in exocytosis of a fluid-phase marker. GFP fusions with the full-length protein or the N-terminal part containing all three CH domains revealed that Trix localizes to the cortical region and strongly accumulates on late endosomes. Our results suggest that Trix is specifically involved in a Rho GTPase-signaling pathway that is required for regulation of the actin cytoskeleton during exocytosis.     

Trix, a novel Rac guanine-nucleotide exchange factor from Dictyostelium discoideum is an actin-binding protein and accumulates at endosomes

Small Rho family GTPases are involved in regulation of actin cytoskeleton dynamics. These molecular switches are themselves mainly controlled by specific GTPase-activating proteins (GAPs) and guanine-nucleotide exchange factors (GEFs). We have cloned and initially characterized a novel putative RhoGEF from Dictyostelium discoideum. The predicted 135-kDa protein displays a unique domain organization in its N-terminus by harboring two type3 calponin homology (CH) domains followed by a single type1 CH domain. The C-terminal region encompasses a diffuse B-cell lymphoma homology/pleckstrin homology tandem domain that is typically found in RhoGEFs. We therefore refer to this protein as Trix (triple CH-domain array exchange factor). A recombinant N-terminal region of Trix carrying all three CH domains binds to F-actin and bundles actin filaments. Trix-null mutants are viable and display only subtle defects when compared to wild-type cells with the exception of a substantial decrease in exocytosis of a fluid-phase marker. GFP fusions with the full-length protein or the N-terminal part containing all three CH domains revealed that Trix localizes to the cortical region and strongly accumulates on late endosomes. Our results suggest that Trix is specifically involved in a Rho GTPase-signaling pathway that is required for regulation of the actin cytoskeleton during exocytosis.     

Green fluorescent protein fusions to Arabidopsis fimbrin 1 for spatio-temporal imaging of F-actin dynamics in roots

The visualization of green fluorescent protein (GFP) fusions with microtubule or actin filament (F-actin) binding proteins has provided new insights into the function of the cytoskeleton during plant development. For studies on actin, GFP fusions to talin have been the most generally used reporters. Although GFP-Talin has allowed in vivo F-actin imaging in a variety of plant cells, its utility in monitoring F-actin in stably transformed plants is limited particularly in developing roots where interesting actin dependent cell processes are occurring. In this study, we created a variety of GFP fusions to Arabidopsis Fimbrin 1 (AtFim1) to explore their utility for in vivo F-actin imaging in root cells and to better understand the actin binding properties of AtFim1 in living plant cells. Translational fusions of GFP to full-length AtFim1 or to some truncated variants of AtFim1 showed filamentous labeling in transient expression assays. One truncated fimbrin-GFP fusion was capable of labeling distinct filaments in stably transformed Arabidopsis roots. The filaments decorated by this construct were highly dynamic in growing root hairs and elongating root cells and were sensitive to actin disrupting drugs. Therefore, the fimbrin-GFP reporters we describe in this study provide additional tools for studying the actin cytoskeleton during root cell development. Moreover, the localization of AtFim1-GFP offers insights into the regulation of actin organization in developing roots by this class of actin cross-linking proteins.     

Association of the leukocyte plasma membrane with the actin cytoskeleton through coiled coil-mediated trimeric coronin 1 molecules

Coronin 1 is a member of the coronin protein family specifically expressed in leukocytes and accumulates at sites of rearrangements of the F-actin cytoskeleton. Here, we describe that coronin 1 molecules are coiled coil-mediated homotrimeric complexes, which associate with the plasma membrane and with the cytoskeleton via two distinct domains. Association with the cytoskeleton was mediated by trimerization of a stretch of positively charged residues within a linker region between the N-terminal, WD repeat-containing domain and the C-terminal coiled coil. In contrast, neither the coiled coil nor the positively charged residues within the linker domain were required for plasma membrane binding, suggesting that the N-terminal, WD repeat-containing domain mediates membrane interaction. The capacity of coronin 1 to link the leukocyte cytoskeleton to the plasma membrane may serve to integrate outside-inside signaling with modulation of the cytoskeleton.     

Evidence that insertion of Tomato ringspot nepovirus NTB-VPg protein in endoplasmic reticulum membranes is directed by two domains: a C-terminal transmembrane helix and an N-terminal amphipathic helix

The NTB-VPg protein of Tomato ringspot nepovirus is an integral membrane protein found in association with endoplasmic reticulum (ER)-derived membranes active in virus replication. A transmembrane helix present in a hydrophobic region at the C terminus of the NTB domain was previously shown to traverse the membranes, resulting in the translocation of the VPg domain in the lumen. We have now conducted an in planta analysis of membrane-targeting domains within NTB-VPg using in-frame fusions to the green fluorescent protein (GFP). As expected, the entire NTB-VPg protein directed the GFP fluorescence to ER membranes. GFP fusion proteins containing the C-terminal 86 amino acids of NTB-VPg also associated with ER membranes, resulting in ER-specific glycosylation at a naturally occurring glycosylation site in the VPg domain. Deletion of the hydrophobic region prevented the membrane association. The N-terminal 80 amino acids of NTB were also sufficient to direct the GFP fluorescence to intracellular membranes. A putative amphipathic helix in this region was necessary and sufficient to promote membrane association of the fusion proteins. Using in vitro membrane association assays and glycosylation site mapping, we show that the N terminus of NTB can be translocated in the lumen at least in vitro. This translocation was dependent on the presence of the putative amphipathic helix, suggesting that oligomeric forms of this helix traverse the membrane. Taken together, our results suggest that at least two distinct elements play a key role in the insertion of NTB-VPg in the membranes: a C-terminal transmembrane helix and an N-terminal amphipathic helix. An updated model of the topology of the protein in the membrane is presented.     

The crystal structure of murine coronin-1: a regulator of actin cytoskeletal dynamics in lymphocytes

Mammalian coronin-1 is preferentially expressed in hematopoietic cells and plays a poorly understood role in the dynamic reorganization of the actin cytoskeleton. Sequence analysis of coronin-1 revealed five WD40 repeats that were predicted to form a beta propeller. They are followed by a 130 residue extension and a 30 residue leucine zipper domain that is responsible for multimerization of the protein. Here, we present the crystal structure of murine coronin-1 without the leucine zipper at 1.75 A resolution. Coronin-1 forms a seven-bladed beta propeller composed of the five predicted WD40 repeats and two additional blades that lack any homology to the canonical WD40 motif. The C-terminal extension adopts an extended conformation, packs tightly against the bottom surface of the propeller, and is likely to be required for the structural stability of the propeller. Analysis of charged and conserved surface residues delineate possible binding sites for F-actin on the beta propeller.     

Improved imaging of actin filaments in transgenic Arabidopsis plants expressing a green fluorescent protein fusion to the C- and N-termini of the fimbrin actin-binding domain 2

The role of the actin cytoskeleton in plant development is intimately linked to its dynamic behavior. Therefore it is essential to continue refining methods for studying actin organization in living plant cells. The discovery of green fluorescent protein (GFP) has popularized the use of translational fusions of GFP with actin filament (F-actin) side-binding proteins to visualize in vivo actin organization in plants. The most recent of these live cell F-actin reporters are GFP fusions to the actin-binding domain 2 (ABD2) of Arabidopsis fimbrin 1 (ABD2-GFP). To improve ABD2-GFP fluorescence for enhanced in vivo F-actin imaging, transgenic Arabidopsis plants were generated expressing a construct with GFP fused to both the C- and N-termini of ABD2 under the control of the CaMV 35S promoter (35S::GFP-ABD2-GFP). The 35S::GFP-ABD2-GFP lines had significantly increased fluorescence compared with the original 35S::ABD2-GFP lines. The enhanced fluorescence of the 35S::GFP-ABD2-GFP-expressing lines allowed the acquisition of highly resolved images of F-actin in different plant organs and stages of development because of the reduced confocal microscope excitation settings needed for data collection. This simple modification to the ABD2-GFP construct presents an important tool for studying actin function during plant development.     

Arabidopsis Acyl-CoA-Binding Protein ACBP2 Interacts With an Ethylene-Responsive Element-Binding Protein,AtEBP, via its Ankyrin Repeats

Cytosolic acyl-CoA-binding proteins (ACBP) bind long-chain acyl-CoAs and act as intracellular acyl-CoA transporters and maintain acyl-CoA pools. Arabidopsis thaliana ACBP2 shows conservation at the acyl-CoA-binding domain to cytosolic ACBPs but is distinct by the presence of an N-terminal transmembrane domain and C-terminal ankyrin repeats. The function of the acyl-CoA-binding domain in ACBP2 has been confirmed by site-directed mutagenesis and four conserved residues crucial for palmitoyl-CoA binding have been identified. Results from ACBP2:GFP fusions transiently expressed in onion epidermal cells have demonstrated that the transmembrane domain functions in plasma membrane targeting, suggesting that ACBP2 transfers acyl-CoA esters to this membrane. In this study, we investigated the significance of its ankyrin repeats in mediating protein-protein interactions by yeast two-hybrid analysis and in vitro protein-binding assays; we showed that ACBP2 interacts with the A. thaliana ethylene-responsive element-binding protein AtEBP via its ankyrin repeats. This interaction was lacking in yeast two-hybrid analysis upon removal of the ankyrin repeats. When the subcellular localizations of ACBP2 and AtEBP were further investigated using autofluorescent protein fusions in transient expression by agroinfiltration of tobacco leaves, the DsRed:ACBP2 fusion protein was localized to the plasma membrane while the GFP:AtEBP fusion protein was targeted to the nucleus and plasma membrane. Co-expression of DsRed:ACBP2 and GFP:AtEBP showed a common localization of both proteins at the plasma membrane, suggesting that ACBP2 likely interacts with AtEBP at the plasma membrane.     

Cloning, Characterization, and Chromosomal Localization of Human Supervillin (SVIL)

Supervillin is a 205-kDa F-actin binding protein originally isolated from bovine neutrophils. This protein is tightly associated with both actin filaments and plasma membranes, suggesting that it forms a high-affinity link between the actin cytoskeleton and the membrane. Human supervillin cDNAs cloned from normal human kidney and from the cervical carcinoma HeLa S3 predict a bipartite structure with three potential nuclear localization signals in the NH₂-terminus and three potential actin-binding sequences in the COOH-terminus. In fact, throughout its length, the COOH-terminal half of supervillin is similar to segments 2–6 plus the COOH-terminal “headpiece” of villin, an actin-binding protein in intestinal microvilli. A comparison of the bovine and human sequences indicates that supervillin is highly conserved at the amino acid level, with 79.2% identity of the NH₂-terminus and conservation of three of the four nuclear localization signals found in bovine supervillin. The COOH-terminus is even more highly conserved, with 95.1% amino acid identity overall and 100% conservation of the villin-like headpiece. Supervillin mRNAs are expressed in all human tissues tested, but are most abundant in muscle, bone marrow, thyroid gland, and salivary gland; comparatively little message is found in brain. Human supervillin mRNA is ∼7.5 kb; this message is especially abundant in HeLa S3 cervical carcinoma, SW480 adenocarcinoma, and A549 lung carcinoma cell lines. The human supervillin gene (SVIL) is localized to a single chromosomal locus at 10p11.2, a region that is deleted in some prostate tumors.     

A noncanonical WD-repeat protein from the cyanobacterium Synechocystis PCC6803: structural and functional study

SYNECHOCYSTIS: PCC6803 possesses several open reading frames encoding putative WD-repeat proteins. One, the Hat protein, is involved in the control of a high-affinity transport system for inorganic carbon that is active when the cells are grown under a limiting concentration of this carbon substrate. The protein is composed of two major domains separated by a hydrophobic linker region of 20 amino acid residues. The N-terminal domain of Hat has no homolog in standard databases and does not display any particular structural features. Eleven WD repeats have been identified in the C-terminal moiety. The region encompassing the four terminal WD repeats is essential for growth under a limiting inorganic carbon regime. The region encompassing the two most terminal WD repeats is required for the activity of the high-affinity transport system. However, because the Hat protein is located in the thylakoids, it should not be itself an element of the transport system. The structural organization of the WD-containing domain of Hat was modeled from the crystal structure of the G protein beta subunit (with seven WD repeats) and of hemopexin (a structural analog with four blades). Functional and structural data argue in favor of an organization of the Hat WD moiety in two subdomains of seven and four WD repeats. The C-terminal 4-mer subdomain might interact with another, yet unknown, protein/peptide. This interaction could be essential in modulating the stability of the 4-mer structure and, thus, the accessibility of this subdomain, or at least of the region encompassing the last two WD repeats.     

Isolation and chromosomal assignment of a novel human gene, CORO1C, homologous to coronin-like actin-binding proteins

We have isolated a gene, termed CORO1C (human coronin-like actin-binding protein 1C), that encodes a new member of the coronin-like family of proteins. The cDNA consists of 3,857 nucleotides, with an open reading frame of 1,422 bp encoding a 474 amino acid protein. The deduced amino acid sequence shared 65% identity with p57 (human coronin-like actin-binding protein), as well as 46% identity with coronin, a protein first isolated from the slime mold Dictyostelium discoideum. Computer analysis predicted that the product of the CORO1C gene would contain five WD repeats in its N-terminal region and a coiled-coil motif in its C-terminal region, both of which are conserved among coronin-like proteins. CORO1C was ubiquitously expressed in all human tissues examined, in contrast to other known coronin-like molecules, each of which is expressed in a tissue-specific manner. Immunocytochemical staining demonstrated that CORO1C was co-localized with F-actin; therefore, the gene product is likely to be important in cytokinesis, motility, and signal transduction, as are the other members of this molecular family. We assigned this novel gene to chromosome 12q24.1 by fluorescence in situ hybridization.     

Enaptin, a giant actin-binding protein, is an element of the nuclear membrane and the actin cytoskeleton

Enaptin belongs to a family of recently identified giant proteins that associate with the F-actin cytoskeleton as well as the nuclear membrane. It is composed of an N-terminal alpha-actinin type actin-binding domain (ABD) followed by a long coiled coil rod and a transmembrane domain at the C-terminus. The ABD binds to F-actin in vivo and in vitro and leads to bundle formation. The human Enaptin gene spreads over 515 kb and gives rise to several splicing isoforms (Nesprin-1, Myne-1, Syne-1, CPG2). The longest assembled cDNA encompasses 27,669 bp and predicts a 1014 kDa protein. Antibodies against the ABD of Enaptin localise the protein at F-actin-rich structures throughout the cell and in focal contacts as well as at the nuclear envelope. In COS7 cells, the protein is also present within the nuclear compartment. With the discovery of the actin-binding properties of Enaptin and the highly homologous Nuance, we define a family of proteins that integrate the cytoskeleton with the nucleoskeleton.     

N-Terminal Protein Acylation Confers Localization to Cholesterol, Sphingolipid-enriched Membranes But Not to Lipid Rafts/Caveolae

When variably fatty acylated N-terminal amino acid sequences were appended to a green fluorescent reporter protein (GFP), chimeric GFPs were localized to different membranes in a fatty acylation-dependent manner. To explore the mechanism of localization, the properties of acceptor membranes and their interaction with acylated chimeric GFPs were analyzed in COS-7 cells. Myristoylated GFPs containing a palmitoylated or polybasic region colocalized with cholesterol and ganglioside GM(1), but not with caveolin, at the plasma membrane and endosomes. A dipalmitoylated GFP chimera colocalized with cholesterol and GM(1) at the plasma membrane and with caveolin in the Golgi region. Acylated GFP chimeras did not cofractionate with low-density caveolin-rich lipid rafts prepared with Triton X-100 or detergent-free methods. All GFP chimeras, but not full-length p62(c-yes) and caveolin, were readily solubilized from membranes with various detergents. These data suggest that, although N-terminal acylation can bring GFP to cholesterol and sphingolipid-enriched membranes, protein-protein interactions are required to localize a given protein to detergent-resistant membranes or caveolin-rich membranes. In addition to restricting acceptor membrane localization, N-terminal fatty acylation could represent an efficient means to enrich the concentration of signaling proteins in the vicinity of detergent-resistant membranes and facilitate protein-protein interactions mediating transfer to a detergent-resistant lipid raft core.     

Rapid protein-folding assay using green fluorescent protein.

Formation of the chromophore of green fluorescent protein (GFP) depends on the correct folding of the protein. We constructed a "folding reporter" vector, in which a test protein is expressed as an N-terminal fusion with GFP. Using a test panel of 20 proteins, we demonstrated that the fluorescence of Escherichia coli cells expressing such GFP fusions is related to the productive folding of the upstream protein domains expressed alone. We used this fluorescent indicator of protein folding to evolve proteins that are normally prone to aggregation during expression in E. coli into closely related proteins that fold robustly and are fully soluble and functional. This approach to improving protein folding does not require functional assays for the protein of interest and provides a simple route to improving protein folding and expression by directed evolution.     

烟草丝束蛋白ABD2与微丝的体内体外相互作用

微丝骨架在细胞的生命活动中具有重要的功能,而其动态的解聚聚合特性是其实现功能的前提. 丝束蛋白（fimbrin/plastin）做为微丝结合蛋白质,是微丝骨架的重要调控因子之一,含有2个肌动蛋白结合结构域,目前对其结合微丝的机制并不清楚. 本文以烟草丝束蛋白的肌动蛋白结合结构域2(NtFAbd2) 为研究对象,通过原核细胞表达纯化NtFAbd2,利用体外沉淀法分析发现,NtFAbd2能够与微丝结合;利用激光共聚焦扫描显微镜分析发现,在烟草BY-2悬浮细胞内,NtAbd2-GFP与微丝共分布,这些结果为深入分析植物丝束蛋白的作用机制提供了新的数据.     

Coronin proteins as multifunctional regulators of the cytoskeleton and membrane trafficking

Coronins constitute an evolutionarily conserved family of WD-repeat actin-binding proteins, which can be clearly classified into two distinct groups based on their structural features. All coronins possess a conserved basic N-terminal motif and three to ten WD repeats clustered in one or two core domains. Dictyostelium and mammalian coronins are important regulators of the actin cytoskeleton, while the fly Dpod1 and the yeast coronin proteins crosslink both actin and microtubules. Apart from that, several coronins have been shown to be involved in vesicular transport. C. elegans POD-1 and Drosophila coro regulate the actin cytoskeleton, but also govern vesicular trafficking as indicated by mutant phenotypes. In both organisms, defects in cytoskeleton and trafficking lead to severe developmental defects ranging from abnormal cell division to aberrant formation of morphogen gradients. Finally, mammalian coronin 7 appears not to execute any cytoskeleton-related functions, but rather participates in regulating Golgi trafficking. Here, we review recent data providing more insight into molecular mechanisms underlying the regulation of F-actin structures, cytoskeletal rearrangements and intracellular membrane transport by coronin proteins and the way that they might link cytoskeleton with trafficking in development and disease.     

Dual targeting to mitochondria and plastids of AtBT1 and ZmBT1, two members of the mitochondrial carrier family

Zea mays and Arabidopsis thaliana Brittle 1 (ZmBT1 and AtBT1, respectively) are members of the mitochondrial carrier family. Although they are presumed to be exclusively localized in the envelope membranes of plastids, confocal fluorescence microscopy analyses of potato, Arabidopsis and maize plants stably expressing green fluorescent protein (GFP) fusions of ZmBT1 and AtBT1 revealed that the two proteins have dual localization to plastids and mitochondria. The patterns of GFP fluorescence distribution observed in plants stably expressing GFP fusions of ZmBT1 and AtBT1 N-terminal extensions were fully congruent with that of plants expressing a plastidial marker fused to GFP. Furthermore, the patterns of GFP fluorescence distribution and motility observed in plants expressing the mature proteins fused to GFP were identical to those observed in plants expressing a mitochondrial marker fused to GFP. Electron microscopic immunocytochemical analyses of maize endosperms using anti-ZmBT1 antibodies further confirmed that ZmBT1 occurs in both plastids and mitochondria. The overall data showed that (i) ZmBT1 and AtBT1 are dually targeted to mitochondria and plastids; (ii) AtBT1 and ZmBT1 N-terminal extensions comprise targeting sequences exclusively recognized by the plastidial compartment; and (iii) targeting sequences to mitochondria are localized within the mature part of the BT1 proteins.     

The carboxy terminal WD domain of the pre-mRNA splicing factor Prp17p is critical for function

In Saccharomyces cerevisiae, Prp17p is required for the efficient completion of the second step of pre-mRNA splicing. The function and interacting factors for this protein have not been elucidated. We have performed a mutational analysis of yPrp17p to identify protein domains critical for function. A series of deletions were made throughout the region spanning the N-terminal 158 amino acids of the protein, which do not contain any identified structural motifs. The C-terminal portion (amino acids 160-455) contains a WD domain containing seven WD repeats. We determined that a minimal functional Prp17p consists of the WD domain and 40 amino acids N-terminal to it. We generated a three-dimensional model of the WD repeats in Prp17p based on the crystal structure of the beta-transducin WD domain. This model was used to identify potentially important amino acids for in vivo functional characterization. Through analysis of mutations in four different loops of Prp17p that lie between beta strands in the WD repeats, we have identified four amino acids, 235TETG238, that are critical for function. These amino acids are predicted to be surface exposed and may be involved in interactions that are important for splicing. Temperature-sensitive prp17 alleles with mutations of these four amino acids are defective for the second step of splicing and are synthetically lethal with a U5 snRNA loop I mutation, which is also required for the second step of splicing. These data reinforce the functional significance of this region within the WD domain of Prp17p in the second step of splicing.