Conformation of thymosin beta 4 in water determined by NMR spectroscopy

Thymosin β4 and β10 Expression in Human Organs during Development: A Review

This review summarizes the results of a series of studies performed by our group with the aim to define the expression levels of thymosin β4 and thymosin β10 over time, starting from fetal development to different ages after birth, in different human organs and tissues. The first section describes the proteomics investigations performed on whole saliva from preterm newborns and gingival crevicular fluid, which revealed to us the importance of these acidic peptides and their multiple functions. These findings inspired us to start an in-depth investigation mainly based on immunochemistry to establish the distribution of thymosin β4 and thymosin β10 in different organs from adults and fetuses at different ages (after autopsy), and therefore to obtain suggestions on the functions of β-thymosins in health and disease. The functions of β-thymosins emerging from these studies, for instance, those performed during carcinogenesis, add significant details that could help to resolve the nowadays so-called "β-thymosin enigma", i.e., the potential molecular role played by these two pleiotropic peptides during human development.

Computer- and NMR-Aided Design of Small-Molecule Inhibitors of the Hub1 Protein

By binding to the spliceosomal protein Snu66, the human ubiquitin-like protein Hub1 is a modulator of the spliceosome performance and facilitates alternative splicing. Small molecules that bind to Hub1 would be of interest to study the protein-protein interaction of Hub1/Snu66, which is linked to several human pathologies, such as hypercholesterolemia, premature aging, neurodegenerative diseases, and cancer. To identify small molecule ligands for Hub1, we used the interface analysis, peptide modeling of the Hub1/Snu66 interaction and the fragment-based NMR screening. Fragment-based NMR screening has not proven sufficient to unambiguously search for fragments that bind to the Hub1 protein. This was because the Snu66 binding pocket of Hub1 is occupied by pH-sensitive residues, making it difficult to distinguish between pH-induced NMR shifts and actual binding events. The NMR analyses were therefore verified experimentally by microscale thermophoresis and by NMR pH titration experiments. Our study found two small peptides that showed binding to Hub1. These peptides are the first small-molecule ligands reported to interact with the Hub1 protein.

Moonlighting Proteins in the Fuzzy Logic of Cellular Metabolism

The numerous interconnected biochemical pathways that make up the metabolism of a living cell comprise a fuzzy logic system because of its high level of complexity and our inability to fully understand, predict, and model the many activities, how they interact, and their regulation. Each cell contains thousands of proteins with changing levels of expression, levels of activity, and patterns of interactions. Adding more layers of complexity is the number of proteins that have multiple functions. Moonlighting proteins include a wide variety of proteins where two or more functions are performed by one polypeptide chain. In this article, we discuss examples of proteins with variable functions that contribute to the fuzziness of cellular metabolism.

Molecular Motions and Interactions in Aqueous Solutions of Thymosin-β4 , Stabilin CTD and Their 1 : 1 Complex, Studied by 1 H-NMR Spectroscopy

Wide-line ¹ H NMR measurements were extended and all results were interpreted in a thermodynamics-based new approach on aqueous solutions of thymosin-β₄ (Tβ₄ ), stabilin cytoplasmic domain (CTD), and their 1 : 1 complex. Energy distributions of potential barriers controlling the motion of protein-bound water molecules were determined. Heterogeneous and homogeneous regions were found in the protein-water interface. The measure of heterogeneity of this interface gives quantitative value for the portion of disordered parts in the protein. Ordered structural elements were found extending up to ∼20 % of the individual whole proteins. About 40 % of the binding sites of free Tβ₄ get involved in bonds holding the complex together. The complex has the most heterogeneous solvent accessible surface (SAS) in terms of protein-water interactions. The complex is more disordered than Tβ₄ or stabilin CTD. The greater SAS area of the complex is interpreted as a clear sign of its open structure.

Molecular Motions and Interactions in Aqueous Solutions of Thymosin-β4 , Stabilin C-Terminal Domain (CTD) and Their 1:1 Complex Studied by 1 H NMR Spectroscopy

Wide-line ¹ H NMR measurements were extended and all results were reinterpreted in a new thermodynamics-based approach to study aqueous solutions of thymosin-β₄ (Tβ₄ ), stabilin C-terminal domain (CTD) and their 1:1 complex. The energy distributions of the potential barriers, which control motion of protein-bound water molecules, were determined. Heterogeneous and homogeneous regions were found at the protein-water interface. The measure of heterogeneity gives a quantitative value for the portion of disordered parts in the protein. Ordered structural elements were found extending up to 20 % of the whole proteins. About 40 % of the binding sites of free Tβ₄ become involved in bonds holding the complex together. The complex has the most heterogeneous solvent accessible surface (SAS) in terms of protein-water interactions. The complex is more disordered than Tβ₄ or stabilin CTD. The greater SAS area of the complex is interpreted as a clear sign of its open structure.

Intrinsic, Functional, and Structural Properties of β-Thymosins and β-Thymosin/WH2 Domains in the Regulation and Coordination of Actin Self-Assembly Dynamics and Cytoskeleton Remodeling

β-Thymosins are a family of heat-stable multifunctional polypeptides that are expressed as small proteins of about 5kDa (~45 amino acids) almost exclusively in multicellular animals. They were first isolated from the thymus. As full-length or truncated polypeptides, they appear to stimulate a broad range of extracellular activities in various signaling pathways, including tissue repair and regeneration, inflammation, cell migration, and immune defense. However, their cell surface receptors and structural mechanisms of regulations in these multiple pathways remain still poorly understood. Besides their extracellular activities, they belong to a larger family of small, intrinsically disordered actin-binding domains called WH2/β-thymosin domains that have been identified in more than 1800 multidomain proteins found in different taxonomic domains of life and involved in various actin-based motile processes including cell morphogenesis, motility, adhesions, tissue development, intracellular trafficking, or pathogen infections. This review briefly surveys the main recent findings to understand how these small, intrinsically disordered but functional domains can interact with many unrelated partners and can thus integrate and coordinate various intracellular activities in actin self-assembly dynamics and cell signaling pathways linked to their cytoskeleton remodeling.

Actin-Induced Structure in the Beta-Thymosin Family of Intrinsically Disordered Proteins

Thymosin β4 (Tβ4) is a 43-amino acid signature motif peptide that defines the beta-thymosin (βT) family of proteins. βTs are intrinsically unstructured in their free states and undergo disorder-to-order transitions in carrying out their biological functions. This property poses challenges in determining their 3D structures, mainly favoring structural studies on the complexes formed between βTs and their interaction partners. One of the βTs' primary binding partners is monomeric actin, a major component of the cytoskeleton in eukaryotic cells. Tβ4's role in this system is to maintain the highly concentrated pool of monomeric actin that can be accessed through profilin by actin filament nucleating machineries. Here, we give an account of the structures of βTs that have been illuminated by nuclear magnetic resonance (NMR) and X-ray crystallography. NMR has been the method of choice for probing regions that have intrinsic conformational preference within the largely disordered βTs in their native states in solution. X-ray crystallography has demonstrated at atomic detail how βTs interact with actin. Detailed analysis of these structures highlights the disorder-to-order transition of Tβ4 in binding to actin and its isoform specificity.

Development of the intein-mediated method for production of recombinant thymosin β4 from the acetylated in vivo fusion protein

Thymosin β4 is a 43 amino acid long peptide with an acetylated N-terminal serin that has a high potential as a remedy for healing ulcers, wounds and burns. Although protein biosynthesis offers attractive opportunities in terms of a large-scale production, currently thymosin β4 is mainly produced by chemical synthesis. The problems that hinder the successful commercialization of the biotechnological approach are associated with the small peptides expression and N-terminal acetylation. This work presents an innovative biotechnological method for thymosin β4 production that employs the peptide acetylation in vivo. A genetically engineered construct was created, where the Tβ4 coding sequence fused with the intein Mxe GyrA sequence and chitin-binding domain was combined with the acetyltransferase coding sequence to form a polycistronic construct under a stringent control of T7 promoter. This plasmid construct provided for the expression of the Tβ4-intein fusion protein. In the process of the post-translational modification in vivo formyl methionine was completely removed from the target peptide N-terminus and followed by the Tβ4 precursor N-terminal acetylation. The use of the intein-mediated expression system made it possible to extract thymosin β4 in only 2 chromatographic runs. The method is straightforward to implement and scale up.

Structural basis of thymosin-β4/profilin exchange leading to actin filament polymerization

Thymosin-β4 (Tβ4) and profilin are the two major sequestering proteins that maintain the pool of monomeric actin (G-actin) within cells of higher eukaryotes. Tβ4 prevents G-actin from joining a filament, whereas profilin:actin only supports barbed-end elongation. Here, we report two Tβ4:actin structures. The first structure shows that Tβ4 has two helices that bind at the barbed and pointed faces of G-actin, preventing the incorporation of the bound G-actin into a filament. The second structure displays a more open nucleotide binding cleft on G-actin, which is typical of profilin:actin structures, with a concomitant disruption of the Tβ4 C-terminal helix interaction. These structures, combined with biochemical assays and molecular dynamics simulations, show that the exchange of bound actin between Tβ4 and profilin involves both steric and allosteric components. The sensitivity of profilin to the conformational state of actin indicates a similar allosteric mechanism for the dissociation of profilin during filament elongation.

High-resolution HPLC-ESI-MS characterization of the contact sites of the actin-thymosin β(4) complex by chemical and enzymatic cross-linking

Thymosin β4 sequesters actin by formation of a 1:1 complex. This transient binding in the complex was stabilized by formation of covalent bonds using the cross-linking agents 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and a microbial transglutaminase. The localization of cross-linking sites was determined after separating the products using SDS-PAGE by tryptic in-gel digestion and high-resolution HPLC-ESI-MS. Three cross-linked fragments were identified after chemical cross-linking, indicating three contact sites. Because the cross-linked fragments were detected simultaneously with the corresponding non-cross-linked fragments, the three contact sites were not formed in parallel. K3 of thymosin β4 was cross-linked to E167 of actin, K18 or K19 of thymosin β4 to one of the first three amino acids of actin (DDE), and S43 of thymosin β4 to H40 of actin. The imidazole ring of histidine was proven to be an acyl acceptor for carbodiimide-mediated cross-linking. Molecular modeling proved an extended conformation of thymosin β4 along the subdomains 1 to 3 of actin. The enzymatic cross-linking using a microbial transglutaminase led to the formation of three cross-linking sites. Q41 of actin was cross-linked to K19 of thymosin β4, and K61 of actin to Q39 of thymosin β4. The third cross-linking site was identified between Q41 of actin and Q39 of thymosin β4, which are simultaneously cross-linked to K16, K18, or K19 of thymosin β4. When both cross-linking reactions are taken together, the complex formation of actin by thymosin β4 is more likely to be flexible than rigid and is localized along the subdomains 1 to 3 of actin.

Bacterial expression, purification and angiogenesis-promoting activity of human thymosin β4

Thymosin β4 (Tβ4) is an actin-binding peptide involved in tissue regeneration and angiogenesis. This 43-amino acid peptide is chemically synthesized for research or clinical trials. To overcome the high costs of solid phase synthesis, we developed a genetic engineering procedure of Tβ4 expression in a protease-deficient host strain, Escherichia coli BL21(DE3), transformed with different expression vectors (pRSETA, pET-15b and pEcoli-Cterm6 × HN). The recombinant, non-glycosylated peptide was overexpressed in soluble form and purified by two-step immobilized metal ion affinity chromatography. Use of the pET vector expression system allowed for easy removal of the polyhistidine tag by thrombin. Functional studies revealed that recombinant Tβ4 stimulated angiogenesis via activation of the endothelial proteolytic systems, inhibition of endothelial cell adhesion, promotion of migration and capillary tube formation in Matrigel, and that its activity was similar to that observed for the synthetic peptide. The presented study comprises the first evidence that recombinant Tβ4 promotes angiogenesis in an in vitro endothelial cell model.

Ablation of cyclase-associated protein 2 (CAP2) leads to cardiomyopathy

Cyclase-associated proteins are highly conserved proteins that have a role in the regulation of actin dynamics. Higher eukaryotes have two isoforms, CAP1 and CAP2. To study the in vivo function of CAP2, we generated mice in which the CAP2 gene was inactivated by a gene-trap approach. Mutant mice showed a decrease in body weight and had a decreased survival rate. Further, they developed a severe cardiac defect marked by dilated cardiomyopathy (DCM) associated with drastic reduction in basal heart rate and prolongations in atrial and ventricular conduction times. Moreover, CAP2-deficient myofibrils exhibited reduced cooperativity of calcium-regulated force development. At the microscopic level, we observed disarrayed sarcomeres with development of fibrosis. We analyzed CAP2's role in actin assembly and found that it sequesters G-actin and efficiently fragments filaments. This activity resides completely in its WASP homology domain. Thus CAP2 is an essential component of the myocardial sarcomere and is essential for physiological functioning of the cardiac system, and a deficiency leads to DCM and various cardiac defects.

Molecular architecture of the Spire-actin nucleus and its implication for actin filament assembly

The Spire protein is a multifunctional regulator of actin assembly. We studied the structures and properties of Spire-actin complexes by X-ray scattering, X-ray crystallography, total internal reflection fluorescence microscopy, and actin polymerization assays. We show that Spire-actin complexes in solution assume a unique, longitudinal-like shape, in which Wiskott-Aldrich syndrome protein homology 2 domains (WH2), in an extended configuration, line up actins along the long axis of the core of the Spire-actin particle. In the complex, the kinase noncatalytic C-lobe domain is positioned at the side of the first N-terminal Spire-actin module. In addition, we find that preformed, isolated Spire-actin complexes are very efficient nucleators of polymerization and afterward dissociate from the growing filament. However, under certain conditions, all Spire constructs--even a single WH2 repeat--sequester actin and disrupt existing filaments. This molecular and structural mechanism of actin polymerization by Spire should apply to other actin-binding proteins that contain WH2 domains in tandem.

Thymosin beta4: structure, function, and biological properties supporting current and future clinical applications

Published studies have described a number of physiological properties and cellular functions of thymosin beta4 (Tbeta4), the major G-actin-sequestering molecule in mammalian cells. Those activities include the promotion of cell migration, blood vessel formation, cell survival, stem cell differentiation, the modulation of cytokines, chemokines, and specific proteases, the upregulation of matrix molecules and gene expression, and the downregulation of a major nuclear transcription factor. Such properties have provided the scientific rationale for a number of ongoing and planned dermal, corneal, cardiac clinical trials evaluating the tissue protective, regenerative and repair potential of Tbeta4, and direction for future clinical applications in the treatment of diseases of the central nervous system, lung inflammatory disease, and sepsis. A special emphasis is placed on the development of Tbeta4 in the treatment of patients with ST elevation myocardial infarction in combination with percutaneous coronary intervention.

Structures of actin-bound Wiskott-Aldrich syndrome protein homology 2 (WH2) domains of Spire and the implication for filament nucleation

Three classes of proteins are known to nucleate new filaments: the Arp2/3 complex, formins, and the third group of proteins that contain ca. 25 amino acid long actin-binding Wiskott-Aldrich syndrome protein homology 2 domains, called the WH2 repeats. Crystal structures of the complexes between the actin-binding WH2 repeats of the Spire protein and actin were determined for the Spire single WH2 domain D, the double (SpirCD), triple (SpirBCD), quadruple (SpirABCD) domains, and an artificial Spire WH2 construct comprising three identical D repeats (SpirDDD). SpirCD represents the minimal functional core of Spire that can nucleate actin filaments. Packing in the crystals of the actin complexes with SpirCD, SpirBCD, SpirABCD, and SpirDDD shows the presence of two types of assemblies, "side-to-side" and "straight-longitudinal," which can serve as actin filament nuclei. The principal feature of these structures is their loose, open conformations, in which the sides of actins that normally constitute the inner interface core of a filament are flipped inside out. These Spire structures are distant from those seen in the filamentous nuclei of Arp2/3, formins, and in the F-actin filament.

Lifeact: a versatile marker to visualize F-actin

Live imaging of the actin cytoskeleton is crucial for the study of many fundamental biological processes, but current approaches to visualize actin have several limitations. Here we describe Lifeact, a 17-amino-acid peptide, which stained filamentous actin (F-actin) structures in eukaryotic cells and tissues. Lifeact did not interfere with actin dynamics in vitro and in vivo and in its chemically modified peptide form allowed visualization of actin dynamics in nontransfectable cells.

Fesselin is a natively unfolded protein

Fesselin is a heat stable proline-rich actin binding protein. The stability, amino acid composition, and ability to bind to several proteins suggested that fesselin may be unfolded under native conditions. While the complete sequence of fesselin is unknown an analysis of a closely related protein, synaptopodin 2 from Gallus gallus, indicates that fesselin consists of a series of unstructured regions interspersed between short folded regions. To determine if fesselin is natively unfolded, we compared fesselin to a known globular protein (myosin S1) and a known unfolded protein Cad22 (the COOH terminal 22 kDa fragment of caldesmon). Fesselin, and Cad22, had larger Stokes radii than globular proteins of equivalent mass. The environments of tryptophan residues of fesselin and Cad22 were the same in the presence and absence of 6 M guanidine hydrochloride. Fesselin had a circular dichroism spectrum that was primarily random coil. Changes in pH over the range of 1.5-11.5 did not alter that spectrum. Increasing the temperature to 85 degrees C caused an increase in the degree of secondary structure. Calmodulin binding to fesselin altered the environment of the tryptophan residues so that they became less sensitive to the quencher acrylamide. These results show that fesselin is a natively unfolded protein.

The Structural Basis of Actin Interaction with Multiple WH2/β-Thymosin Motif-Containing Proteins

Participation of actin in cellular processes relies on the dynamics of filament assembly. Filament elongation is fed by monomeric actin in complex with either profilin or a Wiscott-Aldrich syndrome protein (WASP) homology domain 2 (WH2)/beta-thymosin (betaT) domain. WH2/betaT motif repetition (typified by ciboulot) or combination with nonrelated domains (as found in N-WASP) results in proteins that yield their actin to filament elongation. Here, we report the crystal structures of actin bound hybrid proteins, constructed between gelsolin and WH2/betaT domains from ciboulot or N-WASP. We observe the C-terminal half of ciboulot domain 2 bound to actin. In solution, we show that cibolout domains 2 and 3 bind to both G- and F-actin, and that whole ciboulot forms a complex with two actin monomers. In contrast, the analogous portion of N-WASP WH2 domain 2 is detached from actin, indicating that the C-terminal halves of the betaT and WH2 motifs are not functionally analogous.

Nuclear localisation of the G-actin sequestering peptide thymosin β4

Thymosin β4 is regarded as the main G-actin sequestering peptide in the cytoplasm of mammalian cells. It is also thought to be involved in cellular events like cancerogenesis, apoptosis, angiogenesis, blood coagulation and wound healing. Thymosin β4 has been previously reported to localise intracellularly to the cytoplasm as detected by immunofluorescence. It can be selectively labelled at two of its glutamine-residues with fluorescent Oregon Green cadaverine using transglutaminase; however, this labelling does not interfere with its interaction with G-actin. Here we show that after microinjection into intact cells, fluorescently labelled thymosin β4 has a diffuse cytoplasmic and a pronounced nuclear staining. Enzymatic cleavage of fluorescently labelled thymosin β4 with AsnC-endoproteinase yielded two mono-labelled fragments of the peptide. After microinjection of these fragments, only the larger N-terminal fragment, containing the proposed actin-binding sequence exhibited nuclear localisation, whereas the smaller C-terminal fragment remained confined to the cytoplasm. We further showed that in digitonin permeabilised and extracted cells, fluorescent thymosin β4 was solely localised within the cytoplasm, whereas it was found concentrated within the cell nuclei after an additional Triton X100 extraction. Therefore, we conclude that thymosin β4 is specifically translocated into the cell nucleus by an active transport mechanism, requiring an unidentified soluble cytoplasmic factor. Our data furthermore suggest that this peptide may also serve as a G-actin sequestering peptide in the nucleus, although additional nuclear functions cannot be excluded.

Structural basis of actin sequestration by thymosin-beta4: implications for WH2 proteins

The WH2 (Wiscott-Aldridge syndrome protein homology domain 2) repeat is an actin interacting motif found in monomer sequestering and filament assembly proteins. We have stabilized the prototypical WH2 family member, thymosin-beta4 (Tbeta4), with respect to actin, by creating a hybrid between gelsolin domain 1 and the C-terminal half of Tbeta4 (G1-Tbeta4). This hybrid protein sequesters actin monomers, severs actin filaments and acts as a leaky barbed end cap. Here, we present the structure of the G1-Tbeta4:actin complex at 2 A resolution. The structure reveals that Tbeta4 sequesters by capping both ends of the actin monomer, and that exchange of actin between Tbeta4 and profilin is mediated by a minor overlap in binding sites. The structure implies that multiple WH2 motif-containing proteins will associate longitudinally with actin filaments. Finally, we discuss the role of the WH2 motif in arp2/3 activation.

Regulation of cytoskeletal dynamics by actin-monomer-binding proteins

The actin cytoskeleton is a vital component of several key cellular and developmental processes in eukaryotes. Many proteins that interact with filamentous and/or monomeric actin regulate the structure and dynamics of the actin cytoskeleton. Actin-filament-binding proteins control the nucleation, assembly, disassembly and crosslinking of actin filaments, whereas actin-monomer-binding proteins regulate the size, localization and dynamics of the large pool of unpolymerized actin in cells. In this article, we focus on recent advances in understanding how the six evolutionarily conserved actin-monomer-binding proteins - profilin, ADF/cofilin, twinfilin, Srv2/CAP, WASP/WAVE and verprolin/WIP - interact with actin monomers and regulate their incorporation into filament ends. We also present a model of how, together, these ubiquitous actin-monomer-binding proteins contribute to cytoskeletal dynamics and actin-dependent cellular processes.

Coupling of Folding and Binding of Thymosin β4 upon Interaction with Monomeric Actin Monitored by Nuclear Magnetic Resonance

Thymosin beta4 is a major actin-sequestering protein, yet the structural basis for its biological function is still unknown. This study provides insight regarding the way this 43-amino acid peptide, mostly unstructured in solution, binds to monomeric actin and prevents its assembly in filaments. We show here that the whole backbone of thymosin beta4 is highly affected upon binding to G-actin. The assignment of all amide protons and nitrogens of thymosin in the bound state, obtained using a combination of NMR experiments and selective labelings, shows that thymosin folds completely upon binding and displays a central extended region flanked by two N- and C-terminal helices. The cleavage of actin by subtilisin in the DNase I binding loop does not modify the structure of thymosin beta4 in the complex, showing that the backbone of the peptide is not in close proximity to segment 42-47 of actin. The combination of our NMR results and previously published mutation and cross-link data allows a better characterization of the binding mode of thymosins on G-actin.

The β-Thymosin/WH2 Domain

The widespread beta-thymosin/WH2 actin binding domain has versatile regulatory properties in actin dynamics and motility. beta-thymosins (isolated WH2 domain) maintain monomeric actin in a "sequestered" nonpolymerizable form. In contrast, when repeated in tandem or inserted in modular proteins, the beta-thymosin/WH2 domain promotes actin assembly at filament barbed ends, like profilin. The structural basis for these opposite functions is addressed using ciboulot, a three beta-thymosin repeat protein. Only the first repeat binds actin and possesses the function of ciboulot. The region that shows the strongest interaction with actin is an amphipathic N-terminal alpha helix, present in all beta-thymosin/WH2 domains, which recognizes the ATP bound actin structure and uses the shear motion of actin linked to ATP hydrolysis to control polymerization. Crystallographic ((1)H, (15)N), NMR, and mutagenetic data reveal that the weaker interaction of the C-terminal region of beta-thymosin/WH2 domain with actin accounts for the switch in function from inhibition to promotion of actin assembly.

Thymosin beta 4 interactions

Thymosin beta 4 is a small, 5-kDa protein with a diverse range of activities, including its function as an actin monomer sequestering protein, an antiinflammatory agent, and an inhibitor of bone marrow stem cell proliferation. Only the effects of thymosin beta 4 on the actin cytoskeleton have an explanation based on identified molecular interactions. Thymosin beta 4 is largely unfolded or perhaps completely unfolded in solution. Based on the paradigm introduced by Wright and Dyson (1999) that unfolded proteins may have multiple functions based on their ability to recognize numerous ligands, the flexible structure of thymosin beta 4 may facilitate the recognition of a variety of molecular targets, thus explaining the plethora of functions attributed to thymosin beta 4. Furthermore, if multiple ligands bind to thymosin beta 4, then it is possible that thymosin beta 4 has a unique integrative function that links the actin cytoskeleton to important immune and cell growth-signaling cascades.

The thymosins. Prothymosin alpha, parathymosin, and beta-thymosins: structure and function

The studies on thymosins were initiated in 1965, when the group of A. White searched for thymic factors responsible for the physiological functions of thymus. To restore thymic functions in thymic-deprived or immunodeprived animals, as well as in humans with primary immuno-deficiency diseases and in immunosuppressed patients, a standardized extract from bovine thymus gland called thymosin fraction 5 was prepared. Thymosin fraction 5 indeed improved immune response. It turned out that thymosin fraction 5 consists of a mixture of small polypeptides. Later on, several of these peptides (polypeptide beta 1, thymosin alpha 1, prothymosin alpha, parathymosin, and thymosin beta 4) were isolated and tested for their biological activity. The research of many groups has indicated that none of the isolated peptides is really a thymic hormone; nevertheless, they are biologically important peptides with diverse intracellular and extracellular functions. Studies on these functions are still in progress. The current status of knowledge of structure and functions of the thymosins is discussed in this review.

A phage display-based method for determination of relative affinities of mutants. Application of the actin-binding motifs in thymosin beta 4 and the villin headpiece

We propose phage display combined with enzyme-linked immunosorbent assay as a tool for the systematic analysis of protein-protein interactions by investigating the binding behavior of variants to a partner protein. Via enzyme-linked immunosorbent assay we determine both the amount of fusion protein presented at the phage surface and the amount of complex formed, the ratio of which is proportional to the affinity. Hence this method enables us to calculate the relative affinities of a large number of mutants. As model systems, we investigated actin-binding motifs conserved in a number of proteins binding monomeric or filamentous actin. The hexapeptide motifs LKKTET, present in thymosin beta4, and LKKEKG, present in the villin headpiece, were mutated, and the variants were analyzed. Study of the positional tolerance allows postulating that the motifs, although similar in primary structures adopt different conformations when bound to actin. In addition, our data show that the second and the fourth amino acid of the thymosin beta4 motif and the first three residues of the villin headpiece motif are most important for actin binding. The latter result challenges the charged crown hypothesis for the villin headpiece filamentous actin interaction.

Actin binding proteins: regulation of cytoskeletal microfilaments

The actin cytoskeleton is a complex structure that performs a wide range of cellular functions. In 2001, significant advances were made to our understanding of the structure and function of actin monomers. Many of these are likely to help us understand and distinguish between the structural models of actin microfilaments. In particular, 1) the structure of actin was resolved from crystals in the absence of cocrystallized actin binding proteins (ABPs), 2) the prokaryotic ancestral gene of actin was crystallized and its function as a bacterial cytoskeleton was revealed, and 3) the structure of the Arp2/3 complex was described for the first time. In this review we selected several ABPs (ADF/cofilin, profilin, gelsolin, thymosin beta4, DNase I, CapZ, tropomodulin, and Arp2/3) that regulate actin-driven assembly, i.e., movement that is independent of motor proteins. They were chosen because 1) they represent a family of related proteins, 2) they are widely distributed in nature, 3) an atomic structure (or at least a plausible model) is available for each of them, and 4) each is expressed in significant quantities in cells. These ABPs perform the following cellular functions: 1) they maintain the population of unassembled but assembly-ready actin monomers (profilin), 2) they regulate the state of polymerization of filaments (ADF/cofilin, profilin), 3) they bind to and block the growing ends of actin filaments (gelsolin), 4) they nucleate actin assembly (gelsolin, Arp2/3, cofilin), 5) they sever actin filaments (gelsolin, ADF/cofilin), 6) they bind to the sides of actin filaments (gelsolin, Arp2/3), and 7) they cross-link actin filaments (Arp2/3). Some of these ABPs are essential, whereas others may form regulatory ternary complexes. Some play crucial roles in human disorders, and for all of them, there are good reasons why investigations into their structures and functions should continue.

WH2 domain: a small, versatile adapter for actin monomers

The actin cytoskeleton plays a central role in many cell biological processes. The structure and dynamics of the actin cytoskeleton are regulated by numerous actin-binding proteins that usually contain one of the few known actin-binding motifs. WH2 domain (WASP homology domain-2) is a approximately 35 residue actin monomer-binding motif, that is found in many different regulators of the actin cytoskeleton, including the beta-thymosins, ciboulot, WASP (Wiskott Aldrich syndrome protein), verprolin/WIP (WASP-interacting protein), Srv2/CAP (adenylyl cyclase-associated protein) and several uncharacterized proteins. The most highly conserved residues in the WH2 domain are important in beta-thymosin's interactions with actin monomers, suggesting that all WH2 domains may interact with actin monomers through similar interfaces. Our sequence database searches did not reveal any WH2 domain-containing proteins in plants. However, we found three classes of these proteins: WASP, Srv2/CAP and verprolin/WIP in yeast and animals. This suggests that the WH2 domain is an ancient actin monomer-binding motif that existed before the divergence of fungal and animal lineages.

Polymerisation of chemically cross-linked actin:thymosin beta(4) complex to filamentous actin: alteration in helical parameters and visualisation of thymosin beta(4) binding on F-actin

The beta-thymosins are intracellular monomeric (G-)actin sequestering proteins forming 1:1 complexes with G-actin. Here, we analysed the interaction of thymosin beta(4) with F-actin. Thymosin beta(4) at 200 microM was chemically cross-linked to F-actin. In the presence of phalloidin, the chemically cross-linked actin:thymosin beta(4) complex was incorporated into F-actin. These mixed filaments were of normal appearance when inspected by conventional transmission electron microscopy after negative staining. We purified the chemically cross-linked actin:thymosin beta(4) complex, which polymerised only when phalloidin and the gelsolin:2-actin complex were present simultaneously. Using scanning transmission electron microscopy, the mass-per-length of control and actin:thymosin beta(4) filaments was found to be 16.0(+/-0.8) kDa/nm and 18.0(+/-0.9) kDa/nm, respectively, indicating an increase in subunit mass of 5.4 kDa. Analysis of the helical parameters revealed an increase of the crossover spacing of the two right-handed long-pitch helical strands from 36.0 to 40.5 nm. Difference map analysis of 3-D helical reconstruction of control and actin:thymosin beta(4) filaments yielded an elongated extra mass. Qualitatively, the overall size and shape of the difference mass were compatible with published data of the atomic structure of thymosin beta(4). The deduced binding sites of thymosin beta(4) to actin were in agreement with those identified previously. However, parts of the difference map might represent subtle conformational changes of both proteins occurring upon complex formation.

Formation and implications of a ternary complex of profilin, thymosin beta 4, and actin

Data from affinity chromatography, analytical ultracentrifugation, covalent cross-linking, and fluorescence anisotropy show that profilin, thymosin beta(4), and actin form a ternary complex. In contrast, steady-state assays measuring F-actin concentration are insensitive to the formation of such a complex. Experiments using a peptide that corresponds to the N terminus of thymosin beta(4) (residues 6-22) confirm the presence of an extensive binding surface between actin and thymosin beta(4), and explain why thymosin beta(4) and profilin can bind simultaneously to actin. Surprisingly, despite much lower affinity, the N-terminal thymosin beta(4) peptide has a very slow dissociation rate constant relative to the intact protein, consistent with a catalytic effect of the C terminus on conformational change occurring at the N terminus of thymosin beta(4). Intracellular concentrations of thymosin beta(4) and profilin may greatly exceed the equilibrium dissociation constant of the ternary complex, inconsistent with models showing sequential formation of complexes of profilin-actin or thymosin beta(4)-actin during dynamic remodeling of the actin cytoskeleton. The formation of a ternary complex results in a very large amplification mechanism by which profilin and thymosin beta(4) can sequester much more actin than is possible for either protein acting alone, providing an explanation for significant sequestration even if molecular crowding results in a very low critical concentration of actin in vivo.

beta-Thymosins, small acidic peptides with multiple functions

The beta-thymosins are a family of highly conserved polar 5 kDa peptides originally thought to be thymic hormones. About 10 years ago, thymosin beta(4) as well as other members of this ubiquitous peptide family were identified as the main intracellular G-actin sequestering peptides, being present in high concentrations in almost every cell. beta-Thymosins bind monomeric actin in a 1:1 complex and act as actin buffers, preventing polymerization into actin filaments but supplying a pool of actin monomers when the cell needs filaments. Changes in the expression of beta-thymosins appear to be related to the differentiation of cells. Increased expression of beta-thymosins or even the synthesis of a beta-thymosin normally not expressed might promote metastasis possibly by increasing mobility of the cells. Thymosin beta(4) is detected outside of cells in blood plasma or in wound fluid. Several biological effects are attributed to thymosin beta(4), oxidized thymosin beta(4), or to the fragment, acSDKP, possibly generated from thymosin beta(4). Among the effects are induction of metallo-proteinases, chemotaxis, angiogenesis and inhibition of inflammation as well as the inhibition of bone marrow stem cell proliferation. However, nothing is known about the molecular mechanisms mediating the effects attributed to extracellular beta-thymosins.

Structural requirements for thymosin beta4 in its contact with actin. An NMR-analysis of thymosin beta4 mutants in solution and correlation with their biological activity

We examined the conformational preferences of mutants of thymosin beta4, an actin monomer sequestering protein by NMR spectroscopy in 60% (v/v) trifluoroethanol. Under these conditions, the wild-type thymosin beta4 conformation consists of an alpha-helix (helix I) extending from residues 5-16 with a more stable fragment from lysine 11 to lysine 16 and a second alpha-helix (helix II) encompassing residues 31-39. The point mutations studied here are located in helix I or in the LKKTET segment (residues 17-22) that form the two main entities of interaction with the actin molecule. The alpha-1H conformational shifts allow us to investigate the helicity of the polypeptides at the residue level and to correlate these structures with their biological activity. We determine that an extension of helix I at its C-terminal end over the LKK-segment results in loss of activity. The correct termination of this helix is connected to a specific orientation of the polypeptide essential for a cooperative action of the thymosin beta4 binding entities required for full activity.

Thymosin-beta(4) changes the conformation and dynamics of actin monomers

Thymosin-beta(4) (Tbeta(4)) binds actin monomers stoichiometrically and maintains the bulk of the actin monomer pool in metazoan cells. Tbeta(4) binding quenches the fluorescence of N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine (AEDANS) conjugated to Cys(374) of actin monomers. The K(d) of the actin-Tbeta(4) complex depends on the cation and nucleotide bound to actin but is not affected by the AEDANS probe. The different stabilities are determined primarily by the rates of dissociation. At 25 degrees C, the free energy of Tbeta(4) binding MgATP-actin is primarily enthalpic in origin but entropic for CaATP-actin. Binding is coupled to the dissociation of bound water molecules, which is greater for CaATP-actin than MgATP-actin monomers. Proteolysis of MgATP-actin, but not CaATP-actin, at Gly(46) on subdomain 2 is >12 times faster when Tbeta(4) is bound. The C terminus of Tbeta(4) contacts actin near this cleavage site, at His(40). By tritium exchange, Tbeta(4) slows the exchange rate of approximately eight rapidly exchanging amide protons on actin. We conclude that Tbeta(4) changes the conformation and structural dynamics ("breathing") of actin monomers. The conformational change may reflect the unique ability of Tbeta(4) to sequester actin monomers and inhibit nucleotide exchange.

C-terminal variations in beta-thymosin family members specify functional differences in actin-binding properties

Mammalian cells express several isoforms of beta-thymosin, a major actin monomer sequestering factor, including thymosins beta4, beta10, and beta15. Differences in actin-binding properties of different beta-thymosin family members have not been investigated. We find that thymosin beta15 binds actin with a 2.4-fold higher affinity than does thymosin beta4. Mutational analysis was performed to determine the amino acid differences in thymosin beta15 that specify its increased actin-affinity. Previous work with thymosin beta4 identified an alpha-helical domain, as well as a conserved central motif, as crucial for actin binding. Mutational analysis confirms that these domains are also vital for actin binding in thymosin beta15, but that differences in these domains are not responsible for the variation in actin-binding properties between thymosins beta4 and beta15. Truncation of the unique C-terminal residues in thymosin beta15 inhibits actin binding, suggesting that this domain also has an important role in mediating actin-binding affinity. Replacement of the 10 C-terminal amino acids of thymosin beta15 with those of thymosin beta4 did, however, reduce the actin-binding affinity of the hybrid relative to thymosin beta15. Similarly, replacement of the thymosin beta4 C-terminal amino acids with those of thymosin beta15 led to increased actin binding. We conclude that functional differences between closely related beta-thymosin family members are, in part, specified by the C-terminal variability between these isoforms. Such differences may have consequences for situations where beta-thymosins are differentially expressed as in embryonic development and in cancer.

Structural modules in actin-binding proteins: towards a new classification

The number of actin binding proteins for which (part of) the three-dimensional structure is known, is steadily increasing. This has led to a picture in which defined structural modules with actin binding capacity are shared between different actin binding proteins. A classification of these based on their common three-dimensional modules appears a logical future step and in this review we provide an initial list starting from the currently known structures. The discussed cases illustrate that a comparison of the similarities and variations within the common structural actin binding unit of different members of a particular class may ultimately provide shortcuts for defining their actin target site and for understanding their effect on actin dynamics. Within this concept, the multitude of possible interactions by an extensive, and still increasing, list of actin binding proteins becomes manageable because they can be presented as variations upon a limited number of structural themes. We discuss the possible evolutionary routes that may have produced the present array of actin binding modules.

Structure determination of the three disulfide bond isomers of alpha-conotoxin GI: a model for the role of disulfide bonds in structural stability

The three possible disulfide bonded isomers of alpha-conotoxin GI have been selectively synthesised and their structures determined by 1H NMR spectroscopy. alpha-Conotoxin GI derives from the venom of Conus geographus and is a useful neuropharmacological tool as it selectively binds to the nicotinic acetylcholine receptor (nAChR), a ligand-gated ion channel involved in nerve signal transmission. The peptide has the sequence ECCNPACGRHYSC-NH2, and the three disulfide bonded isomers are referred to as GI(2-7;3-13), GI(2-13;3-7) and GI(2-3;7-13). The NMR structure for the native isomer GI(2-7;3-13) is of excellent quality, with a backbone pairwise RMSD of 0.16 A for a family of 35 structures, and comprises primarily a distorted 310 helix between residues 5 to 11. The two non-native isomers exhibit multiple conformers in solution, with the major populated forms being different in structure both from each other and from the native form. Structure-activity relationships for the native GI(2-7;3-13) as well as the role of the disulfide bonds on folding and stability of the three isomers are examined. It is concluded that the disulfide bonds in alpha-conotoxin GI play a crucial part in determining both the structure and stability of the peptide. A trend for increased conformational heterogeneity was observed in the order of GI(2-7;3-13)<GI(2-13;3-7)<GI(2-3;7-13). It was found that the peptide bond joining Cys2 to Cys3 in GI(2-3;7-13) is predominantly trans, rather than cis as theoretically predicted. These structural data are used to interpret the varying nAChR binding of the non-native forms.A model for the binding of native GI(2-7;3-13) to the mammalian nAChR is proposed, with an alpha-subunit binding face made up of Cys2, Asn4, Pro5, Ala6 and Cys7 and a selectivity face, comprised of Arg9 and His10. These two faces orient the molecule between the alpha and delta subunits of the receptor. The structure of the CCNPAC sequence of the native GI(2-7;3-13) is compared to the structure of the identical sequence from the toxic domain of heat-stable enterotoxins, which forms part of the receptor binding region of the enterotoxins, but which has a different disulfide connectivity.

Thymosin beta 4 binds actin in an extended conformation and contacts both the barbed and pointed ends

The beta-thymosins are a family of highly polar peptides which serve in vivo to maintain a reservoir of unpolymerized actin monomers. In vitro, beta-thymosins form 1:1 complexes with actin monomers and inhibit both polymerization and exchange of the bound nucleotide. Circular dichroism data indicate that free thymosin beta 4 is predominantly unstructured, containing at most six residues of alpha-helix, and that up to six additional residues may adopt an alpha-helical conformation upon binding actin. NMR data indicate that many parts of thymosin beta 4 are not in tight contact with actin. Contacts between specific residues in actin and thymosin beta 4 were identified by zero-length cross-linking followed by isolation and sequencing of cross-linked peptides. After carbodiimide-mediated cross-linking, Lys-3 of thymosin beta 4 was cross-linked to Glu-167 of actin, and Lys-18 of thymosin beta 4 was cross-linked to one of the the N-terminal acidic residues of actin (Asp-1-Glu-4); the cross-linked actin residues lie within subdomains 3 and 1, respectively. These two contacts flank the alpha-helical region of thymosin beta 4 and place it on the barbed end; thymosin beta 4 can thus block actin polymerization sterically. After transglutaminase-mediated cross-linking, Lys-38 of thymosin beta 4 was cross-linked to Gln-41 of actin, placing the C-terminal region of thymosin beta 4 in contact with subdomain 2 on the pointed end; thymosin beta 4 may sterically block actin polymerization at the pointed end as well as the barbed end of the monomer. The distance between the pointed-end and barbed-end contacts requires that the C-terminal half of thymosin beta 4 be in a predominantly extended conformation.

Conformation of thymosin beta 9 in water/fluoroalcohol solution determined by NMR spectroscopy

The conformation of thymosin beta 9 in solution of 40% (v/v) 1,1,1,3,3,3-hexafluoro-2-propanol-d2 in water has been investigated by two-dimensional 1H-nmr spectroscopy. Under this condition thymosin beta 9 adopts an ordered structure. The determination of the conformation of the peptide was based on a set of 304 approximate interproton distance constraints derived from nuclear Overhauser enhancement measurements. The conformation of thymosin beta 9 includes two helical regions from residues 4 to 27 and 32 to 41. The two helices are separated by a poorly defined loop region between amino acids 28 and 31; the N-terminus of thymosin beta 9 shows random-coil structure only.

Characterization of CD4-gp120 activation intermediates during human immunodeficiency virus type 1 syncytium formation

The mechanism by which cells expressing HIV envelope glycoproteins progress from binding CD4+ cells to syncytia formation is not entirely understood. The purpose of these investigations was to use physical and biochemical tools (temperature shifts, soluble CD4, protease inhibitors, and a battery of anti-CD4 monoclonal antibodies) to isolate discrete steps during syncytia formation. Previously (Fu et al., J Virol 1993;67:3818), we found that preincubation of cells stably expressing HIV-1 gp 160 (TF228.1.16) with CD4+ SupT1 cells at 16 degrees C, a temperature that is nonpermissive for syncytia formation, resulted in an increased rate of syncytia formation when the cocultures were shifted to the syncytia-permissive temperature of 37 degrees C. We have since found that syncytia formation is further enhanced by shifting the cocultures from 16 to 4 degrees C prior to incubation at 37 degrees C. Together, these data suggest that two discrete states, which we term the first and second activation intermediates (FAI and SAI), are involved in syncytia formation. We have found that acquisition of the FAI (by preincubation at 16 degree C) is sensitive to some serine protease inhibitors (PI), soluble CD4 (sCD4), shedding of gp120, and anti-CD4 monoclonal antibodies (MAb) directed toward the CDR-1/2 and CDR-3 regions of domain 1 on CD4. Expression of the FAI (formation of syncytia by shifting from 16 to 37 degrees C) remains sensitive to sCD4, shedding of gp120, and MAb directed toward CDR-1/2 but is less sensitive to MAb that bind CDR-3 and is insensitive to PI. Similarly, acquisition of the SAI (shifting cocultures from 16 to 4 degrees C), is sensitive to sCD4, shedding of gp120, and MAb directed toward CDR-1/2. In contrast, expression of the SAI (shifting cocultures from 16 to 4 to 37 degrees C) is sensitive only to MAb directed toward CDR-1/2 and cannot be blocked by sCD4, shedding of gp120, or PI. These data allow us to propose that syncytia formation, mediated by HIV-1 envelope glycoproteins, proceeds by a multistep cascade.

Identification of contact sites in the actin-thymosin beta 4 complex by distance-dependent thiol cross-linking

Binding sites of actin and thymosin beta 4 were investigated using a set of bifunctional thiol-specific reagents, which allowed the insertion of cross-linkers of defined lengths between cysteine residues of the complexed proteins. After the cross-linkers were attached to actin specifically at either Cys10, Cys374, or the sulfur atom of the ATP analog adenosine 5'-O-(thiotriphosphate) (ATP gamma S), the actin derivatives were reacted with synthetic thymosin beta 4 analogs containing a cysteine at one of the positions 6, 17, 28, 34, and 40. Immediate cross-linking as followed by UV spectroscopy was found for Cys374 of actin and Cys6 of thymosin beta 4, indicating that the N terminus of thymosin beta 4 is in close proximity (< or = 9.2 A) to the C terminus of actin. In contrast, only insignificant reactivity was measured for all thymosin beta 4 analogs when the cross-linkers were anchored at Cys10 of actin. A second contact site was identified by cross-linking of Cys17 and Cys28 in thymosin beta 4 with the ATP gamma S derivative bound to actin, indicating that the hexamotif of thymosin beta 4 (positions 17-22) is in close proximity (< or = 9.2 A) to the nucleotide. The importance of the amino acids 17 and 28 in thymosin beta 4 for the interaction with actin was emphasized by the finding that thymosin analogs containing cysteine in these positions exhibited strongly reduced abilities to inhibit actin polymerization.

Correlations between biological activity and structural properties for two short homologous sequences in thymosin beta4 and gelsolin

Gelsolin and thymosin beta4 appear to be two important actin-associated proteins involved in the regulation of actin polymerization. It has been widely demonstrated that thymosin is the major cellular actin-sequestering factor shifting the polymerization equilibrium of actin towards a monomeric state. At the same time gelsolin, a Ca2+ and inositol phosphate sensitive protein, regulates actin filament length. The interactions of these two proteins with actin are rather complex and require the participation of several complementary peptide sequences. We have identified a common motif, (I, V)EKFD, in the two proteins in the functional sequences so far examined. Gelsolin- and thymosin beta4-related peptides including the common motif were synthesized and their structural and functional properties studied. These two sequences exert a major inhibitory effect on salt-induced actin polymerization. We used circular dichroism and Fourier-transform infrared spectroscopy to show that the two synthetic peptides present some secondary structure in solution. As far as the peptide derived from the thymosin sequence was concerned, alpha-helical structure was induced by trifluoroethanol as observed with the full-length molecule. These experiments underscore the importance of the conformational state of peptide fragments in their biological activities. ELISA and fluorescence measurements have been used to identify the binding regions of these fragments to a C-terminal region (subdomain 1) of the actin sequence. Our results also emphasize the relationship between the propensity of small sequences to form secondary structures and their propensity for biological activity as related to actin interaction and inhibition of actin polymerization.