The ternary complex of DNase I, actin and thymosin beta4

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.

Gelsolin activity controls efficient early HIV-1 infection

Background

HIV-1 entry into target lymphocytes requires the activity of actin adaptors that stabilize and reorganize cortical F-actin, like moesin and filamin-A. These alterations are necessary for the redistribution of CD4-CXCR4/CCR5 to one pole of the cell, a process that increases the probability of HIV-1 Envelope (Env)-CD4/co-receptor interactions and that generates the tension at the plasma membrane necessary to potentiate fusion pore formation, thereby favouring early HIV-1 infection. However, it remains unclear whether the dynamic processing of F-actin and the amount of cortical actin available during the initial virus-cell contact are required to such events.

Results

Here we show that gelsolin restructures cortical F-actin during HIV-1 Env-gp120-mediated signalling, without affecting cell-surface expression of receptors or viral co-receptor signalling. Remarkably, efficient HIV-1 Env-mediated membrane fusion and infection of permissive lymphocytes were impaired when gelsolin was either overexpressed or silenced, which led to a loss or gain of cortical actin, respectively. Indeed, HIV-1 Env-gp120-induced F-actin reorganization and viral receptor capping were impaired under these experimental conditions. Moreover, gelsolin knockdown promoted HIV-1 Env-gp120-mediated aberrant pseudopodia formation. These perturbed-actin events are responsible for the inhibition of early HIV-1 infection.

Conclusions

For the first time we provide evidence that through its severing of cortical actin, and by controlling the amount of actin available for reorganization during HIV-1 Env-mediated viral fusion, entry and infection, gelsolin can constitute a barrier that restricts HIV-1 infection of CD4+ lymphocytes in a pre-fusion step. These findings provide important insights into the complex molecular and actin-associated dynamics events that underlie early viral infection. Thus, we propose that gelsolin is a new factor that can limit HIV-1 infection acting at a pre-fusion step, and accordingly, cell-signals that regulate gelsolin expression and/or its actin-severing activity may be crucial to combat HIV-1 infection.

New aspects of the spontaneous polymerization of actin in the presence of salts

The mechanism of salt-induced actin polymerization involves the energetically unfavorable nucleation step, followed by filament elongation by the addition of monomers. The use of a bifunctional cross-linker, N,N'-(1,4-phenylene)dimaleimide, revealed rapid formation of the so-called lower dimers (LD) in which actin monomers are arranged in an antiparallel fashion. The filament elongation phase is characterized by a gradual LD decay and an increase in the yield of "upper dimers" (UD) characteristic of F-actin. Here we have used 90 degrees light scattering, electron microscopy, and N, N'-(1,4-phenylene)dimaleimide cross-linking to reinvestigate relationships between changes in filament morphology, LD decay, and increase in the yield of UD during filament growth in a wide range of conditions influencing the rate of the nucleation reaction. The results show irregularity and instability of filaments at early stages of polymerization under all conditions used, and suggest that an earlier documented coassembling of LD with monomeric actin contributes to the initial disordering of the filaments rather than to the nucleation of polymerization. The effects of the type of G-actin-bound divalent cation (Ca2+/Mg2+), nucleotide (ATP/ADP), and polymerizing salt on the relation between changes in filament morphology and progress in G-actin-to-F-actin transformation show that ligand-dependent alterations in G-actin conformation determine not only the nucleation rate but also the kinetics of ordering of the filament structure in the elongation phase. The time courses of changes in the yield of UD suggest that filament maturation involves cooperative propagation of "proper" interprotomer contacts. Acceleration of this process by the initially bound MgATP supports the view that the filament-destabilizing conformational changes triggered by ATP hydrolysis and Pi liberation during polymerization are constrained by the intermolecular contacts established between MgATP monomers prior to ATP hydrolysis. An important role of contacts involving the DNase-I-binding loop and the C-terminus of actin is proposed.

Models of the Actin-Bound Forms of the beta-Thymosins

In recent years two structures have been reported that demonstrate how the two halves of a beta-thymosin repeat bind to actin monomers. Here we assess the validity of these structures and construct minimally biased models of the beta-thymosin:actin complexes. The models reveal that the beta-thymosins interact with actin throughout their length and that all the conserved residues are functional in this interface. These models are judged to be in excellent agreement with published biochemical and functional data. In particular, the models are consistent with the actin monomer sequestering and actin filament binding properties of beta-thymosins. The models also correctly predict competition between thymosin-beta4 with DNase I or profilin in binding actin while allowing ternary complexes at higher concentrations.

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.

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.

ATPase activity and conformational changes in the regulation of actin

The eukaryotic microfilament system is regulated in part through the nucleotide- and cation-dependent conformation of the actin molecule. In this review, recent literature on the crystal and solution structures of actin and other actin-superfamily proteins is summarized. Furthermore, the structure of the nucleotide binding cleft is discussed in terms of the mechanism of ATP hydrolysis and P(i) release. Two distinct domain movements are suggested to participate in the regulation of actin. (1) High-affinity binding of Mg(2+) to actin induces a rearrangement of side chains in the nucleotide binding site leading to an increased ATPase activity and polymerizability, as well as a rotation of subdomain 2 which is mediated by the hydroxyl of serine-14. (2) Hydrolysis of ATP and subsequent release of inorganic phosphate lead to a butterfly-like opening of the actin molecule brought about by a shearing in the interdomain helix 135-150. These domain rearrangements modulate the interaction of actin with a variety of different proteins, and conversely, protein binding to actin can restrict these conformational changes, with ultimate effects on the assembly state of the microfilament system.

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.

Mapping the binding site of thymosin beta4 on actin by competition with G-actin binding proteins indicates negative co-operativity between binding sites located on opposite subdomains of actin

The beta-thymosins are small monomeric (G-)actin-binding proteins of 5 kDa that are supposed to act intracellularly as actin-sequestering factors stabilizing the cytoplasmic monomeric pool of actin. The binding region of thymosin beta4 was determined by analysing the binding of thymosin beta4 to actin complexed with DNase I, gelsolin or gelsolin segment 1. Binding was analysed by determining the increase in the critical concentration of actin polymerization by native gel electrophoresis or chemical cross-linking. The formation of a ternary complex including thymosin beta4 should indicate that the actin-binding proteins attach to different sites on actin. Competition would be indicative of binding to identical or overlapping sites on actin or of a negative co-operative linkage between the two binding sites. Competition of thymosin beta4 for actin binding was observed in the presence of intact gelsolin or the N-terminal gelsolin fragment, segment 1, indicating that thymosin beta4 binds to a site close to or identical with the gelsolin segment 1-binding site. The ternary complex of actin-DNase I-thymosin beta4 was obtained only when using the chemically cross-linked actin-thymosin beta4 complex, indicating that thymosin beta4 is dissociated by the binding of DNase I to actin. It is suggested that the dissociation of thymosin beta4 by DNase I binding to actin is caused by negative co-operativity between their spatially separated binding sites on actin. A similar negative co-operativity was observed between DNase I and gelsolin segment 1 binding to actin. The results therefore indicate that the respective binding sites for DNase I and segment 1 on subdomains 1 and 2 of actin are linked in a negative co-operative manner.