Effect of intramolecular cross-linking between glutamine-41 and lysine-50 on actin structure and function

Actin's C-terminus coordinates actin structural changes and functions

Actin is essential to eukaryotic cellular processes. Actin's C-terminus appears to play a direct role in modulating actin's structure and properties, facilitating the binding and function of actin-binding proteins (ABPs). The structural and functional characterization of filamentous actin's C-terminus has been impeded by its inherent flexibility, as well as actin's resistance to crystallization for x-ray diffraction and the historical resolution constraints associated with electron microscopy. Many biochemical studies have established that actin's C-terminus must retain its flexibility and structural integrity to modulate actin's structure and functions. For example, C-terminal structural changes are known to affect nucleotide binding and exchange, as well as propagate actin structural changes throughout extensive allosteric networks, facilitating the binding and function of ABPs. Advances in electron microscopy have resulted in high-resolution structures of filamentous actin, providing insights into subtle structural changes that are mediated by actin's C-terminus. Here, we review existing knowledge establishing the importance of actin's C-terminus within actin structural changes and functions and discuss how modern structural characterization techniques provide the tools to understand the role of actin's C-terminus in cellular processes.

Impacts of soybean protein isolate hydrolysates produced at high hydrostatic pressure on gelling properties, structural characteristics, and molecular forces of myofibrillar protein

BACKGROUND

Soybean protein isolate hydrolysates (SPIHs) produced at high hydrostatic pressure have higher bioactivity. The aim of the study was to analyze the effects of different SPIH concentrations obtained under various pressures (0.1, 100, 200, and 300 MPa) on gelling properties, structural characteristics, and main forces of myofibrillar protein (MP) in MP-SPIH plural gels.

RESULTS

The MP-SPIH plural gel with 3% SPIH produced under 200 MPa had the maximum gel strength (0.42 N) and water holding capacity (53.69%). A decline in thermal stability and a rise in storage modulus (G') of MP-SPIH plural gels were found with increased SPIH pressure and concentration. Additionally, the addition of SPIHs increased the amounts of α-helix and β-sheet, decreased random coil structural content of MP in MP-SPIH plural gels, and facilitated the generation of a denser and uniform gels network. The molecular forces in MP-SPIH plural gels were mainly hydrophobic interaction and hydrogen bond.

CONCLUSION

This study showed that the interaction of MP with 3% SPIH obtained at 200 MPa improved the quality of plural gels. © 2022 Society of Chemical Industry.

Posttranslational modifications of the cytoskeleton

The cytoskeleton plays important roles in many essential processes at the cellular and organismal levels, including cell migration and motility, cell division, and the establishment and maintenance of cell and tissue architecture. In order to facilitate these varied functions, the main cytoskeletal components-microtubules, actin filaments, and intermediate filaments-must form highly diverse intracellular arrays in different subcellular areas and cell types. The question of how this diversity is conferred has been the focus of research for decades. One key mechanism is the addition of posttranslational modifications (PTMs) to the major cytoskeletal proteins. This posttranslational addition of various chemical groups dramatically increases the complexity of the cytoskeletal proteome and helps facilitate major global and local cytoskeletal functions. Cytoskeletal proteins undergo many PTMs, most of which are not well understood. Recent technological advances in proteomics and cell biology have allowed for the in-depth study of individual PTMs and their functions in the cytoskeleton. Here, we provide an overview of the major PTMs that occur on the main structural components of the three cytoskeletal systems-tubulin, actin, and intermediate filament proteins-and highlight the cellular function of these modifications.

D-loop Dynamics and Near-Atomic-Resolution Cryo-EM Structure of Phalloidin-Bound F-Actin

Detailed molecular information on G-actin assembly into filaments (F-actin), and their structure, dynamics, and interactions, is essential for understanding their cellular functions. Previous studies indicate that a flexible DNase I binding loop (D-loop, residues 40-50) plays a major role in actin's conformational dynamics. Phalloidin, a "gold standard" for actin filament staining, stabilizes them and affects the D-loop. Using disulfide crosslinking in yeast actin D-loop mutant Q41C/V45C, light-scattering measurements, and cryoelectron microscopy reconstructions, we probed the constraints of D-loop dynamics and its contribution to F-actin formation/stability. Our data support a model of residues 41-45 distances that facilitate G- to F-actin transition. We report also a 3.3-Å resolution structure of phalloidin-bound F-actin in the ADP-Pi-like (ADP-BeFx) state. This shows the phalloidin-binding site on F-actin and how the relative movement between its two protofilaments is restricted by it. Together, our results provide molecular details of F-actin structure and D-loop dynamics.

Interactions of histatin-3 and histatin-5 with actin

Background

Histatins are histidine rich polypeptides produced in the parotid and submandibular gland and secreted into the saliva. Histatin-3 and −5 are the most important polycationic histatins. They possess antimicrobial activity against fungi such as Candida albicans. Histatin-5 has a higher antifungal activity than histatin-3 while histatin-3 is mostly involved in wound healing in the oral cavity. We found that these histatins, like other polycationic peptides and proteins, such as LL-37, lysozyme and histones, interact with extracellular actin.

Results

Histatin-3 and −5 polymerize globular actin (G-actin) to filamentous actin (F-actin) and bundle F-actin filaments. Both actin polymerization and bundling by histatins is pH sensitive due to the high histidine content of histatins. In spite of the equal number of net positive charges and histidine residues in histatin-3 and −5, less histatin-3 is needed than histatin-5 for polymerization and bundling of actin. The efficiency of actin polymerization and bundling by histatins greatly increases with decreasing pH. Histatin-3 and −5 induced actin bundles are dissociated by 100 and 50 mM NaCl, respectively. The relatively low NaCl concentration required to dissociate histatin-induced bundles implies that the actin-histatin filaments bind to each other mainly by electrostatic forces. The binding of histatin-3 to F-actin is stronger than that of histatin-5 showing that hydrophobic forces have also some role in histatin-3- actin interaction. Histatins affect the fluorescence of probes attached to the D-loop of G-actin indicating histatin induced changes in actin structure. Transglutaminase cross-links histatins to actin. Competition and limited proteolysis experiments indicate that the main histatin cross-linking site on actin is glutamine-49 on the D-loop of actin.

Conclusions

Both histatin-3 and −5 interacts with actin, however, histatin 3 binds stronger to actin and affects actin structure at lower concentration than histatin-5 due to the extra 8 amino acid sequence at the C-terminus of histatin-3. Extracellular actin might regulate histatin activity in the oral cavity, which should be the subject of further investigation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12858-017-0078-0) contains supplementary material, which is available to authorized users.

Actin enables the antimicrobial action of LL-37 peptide in the presence of microbial proteases

Host defense peptides play an important host-protective role by their microcidal action, immunomodulatory functions, and tissue repair activities. Proteolysis is a common strategy of pathogens used to neutralize host defense peptides. Here, we show that actin, the most abundant structural protein in eukaryotes, binds the LL-37 host defense peptide, protects it from degradation by the proteases of Pseudomonas aeruginosa and Porphyromonas gingivalis, and enables its antimicrobial activity despite the presence of the proteases. Co-localization of LL-37 with extracellular actin was observed in necrotized regions of samples from oral lesions. Competition assays, cross-linking experiments, limited proteolysis, and mass spectrometry revealed that LL-37 binds by specific hydrophobic interactions to the His-40-Lys-50 segment of actin, located in the DNase I binding loop. The integrity of the binding site of both LL-37 and actin is a prerequisite to the binding. Our results demonstrate that actin, presumably released by dead cells and abundant in infected sites, might be utilized by the immune system to enhance spatio-temporal immunity in an attempt to arrest infection and control inflammation.

The side chain of glutamine 13 is the acyl-donor amino acid modified by type 2 transglutaminase in subunit T of the native rabbit skeletal muscle troponin complex

Subunit T of the native muscle troponin complex is a recognised substrate of transglutaminase both in vitro and in situ with formation of isopeptide bonds. Using a proteomic approach, we have now determined the precise site of in vitro labelling of the protein. A preparation of troponin purified from ether powder from mixed rabbit skeletal muscles was employed as transglutaminase substrate. The only isoform TnT2F present in our preparation was recognised as acyl-substrate by human type 2 transglutaminase which specifically modified glutamine 13 in the N-terminal region. During the reaction, the troponin protein complex was polymerized. Results are discussed in relation to the structure of the troponin T subunit, in the light of the role of troponins in skeletal and cardiac muscle diseases, and to the rules governing glutamine side chain selection by tissue transglutaminase.

Effects of post-translational modifications catalysed by pollen transglutaminase on the functional properties of microtubules and actin filaments

TGases (transglutaminases) are a class of calcium-dependent enzymes that catalyse the interactions between acyl acceptor glutamyl residues and amine donors, potentially making cross-links between proteins. To assess the activity of apple (Malus domestica) pollen TGase on the functional properties of actin and tubulin, TGase was prepared from apple pollen by hydrophobic- interaction chromatography and assayed on actin and tubulin purified from the same cell type. The enzyme catalysed the incorporation of putrescine into the cytoskeleton monomers. When tested on actin filaments, pollen TGase induced the formation of high-molecular-mass aggregates of actin. Use of fluorescein-cadaverine showed that the labelled polyamine was incorporated into actin by pollen TGase, similar to with guinea pig liver TGase. The pollen TGase also reduced the enzyme activity and the binding of myosin to TGase-treated actin filaments. Polymerization of tubulin in the presence of pollen TGase also yielded the formation of high-molecular-mass aggregates. Furthermore, the pollen TGase also affected the binding of kinesin to microtubules and reduced the motility of microtubules along kinesin-coated slides. These results indicate that the pollen TGase can control different properties of the pollen tube cytoskeleton (including the ability of actin and tubulin to assemble and their interaction with motor proteins) and consequently regulate the development of pollen tubes.

Tyrosinase-aided protein cross-linking: effects on gel formation of chicken breast myofibrils and texture and water-holding of chicken breast meat homogenate gels

The effects of Trichoderma reesei tyrosinase-catalyzed cross-linking of isolated chicken breast myofibril proteins as a simplified model system were studied with special emphasis on the thermal stability and gel formation of myofibrillar proteins. In addition, tyrosinase-catalyzed cross-linking was utilized to modify the firmness, water-holding capacity (WHC), and microstructure of cooked chicken breast meat homogenate gels. According to SDS-PAGE, the myosin heavy chain (MHC) and troponin T were the most sensitive proteins to the action of tyrosinase, whereas actin was not affected to the same extent. Calorimetric enthalpy (DeltaH) of the major thermal transition associated with myosin denaturation was reduced and with actin denaturation increased in the presence of tyrosinase. Low-amplitude viscoelastic measurements at constant temperatures of 25 degrees C and 40 degrees C showed that tyrosinase substantially increased the storage modulus (G') of the 4% myofibrillar protein suspension in the 0.35 M NaCl concentration. The effect was the most pronounced with high-enzyme dosages and at 40 degrees C. Without tyrosinase, the G' increase was low. Tyrosinase increased the firmness of the cooked phosphate-free and low-meat chicken breast meat homogenate gels compared to the corresponding controls. Tyrosinase maintained gel firmness at the control level of the low-salt homogenate gel and weakened it when both salt and phosphate levels were low. Tyrosinase improved the WHC of the low-meat and low-salt homogenate gels and maintained it at the level of the corresponding controls of phosphate-free and low-salt/low-phosphate homogenate gels. Microstructural characterization showed that a collagen network was formed in the presence of tyrosinase.

KEYWORDS

Chicken myofibrillar proteins; protein modification; cross-linking; tyrosinase; gelation; thermal stability; texture; water-holding capacity; microstructure.

Significance of kinetic degrees of freedom in operation of the actomyosin motor

The actomyosin motor as a principal functional component of cell motility is highly coordinated in regulating the participating molecular components. At the same time, it has to be flexible and plastic enough to accommodate itself to a wide variety of operational conditions. We prepared two different types of actomyosin systems. One is a natural intact actomyosin system with no artificial constraint on the kinetic degrees of freedom of the actin filaments, and the other is a regulated one with actin filaments supplemented by intra- and intermolecular crosslinking to suppress the kinetic degrees of freedom to a certain extent. Crosslinked actomyosin systems were found to remain almost insensitive to calcium regulation even when intact troponin-tropomyosin regulatory component was incorporated. Both the ATPase and the motile activities of the actin filaments sliding on myosin molecules were markedly lowered by the crosslinking. In contrast, once the crosslinking was cleaved, both properties returned to the normal as with intact actomyosin systems.

Actin cross-linking and inhibition of the actomyosin motor

Intrastrand cross-linking of actin filaments by ANP, N-(4-azido-2-nitrophenyl) putrescine, between Gln-41 in subdomain 2 and Cys-374 at the C-terminus, was shown to inhibit force generation with myosin in the in vitro motility assays [Kim et al. (1998) Biochemistry 37, 17801-17809]. To clarify the immobilization of which of these two sites inhibits the actomyosin motor, the properties of actins with partially overlapping cross-linked sites were examined. pPDM (N,N'-p-phenylenedimaleimide) and ABP [N-(4-azidobenzoyl) putrescine] were used to obtain actin filaments cross-linked ( approximately 50%) between Cys-374 and Lys-191 (interstrand) and Gln-41 and Lys-113 (intrastrand), respectively. ANP, ABP, and pPDM cross-linked filaments showed similar inhibition of their sliding speeds and force generation with myosin ( approximately 25%) in the in vitro motility assays. In analogy to ANP cross-linking of actin, pPDM and ABP cross-linkings did not change the strong S1 binding to actin and the V(max) and K(m) parameters of actomyosin ATPase. The similar effects of these three cross-linkings reveal the tight coupling between structural elements of the subdomain 2/subdomain 1 interface and show the importance of its dynamic flexibility to force generation with myosin. The possibility that actin cross-linkings inhibit rate-limiting steps in motion and force generation during myosin cross-bridge cycle was tested in stopped-flow experiments. Measurements of the rates of mantADP release from actoS1 and ATP-induced dissociation of actoS1 did not reveal any differences between un-cross-linked and ANP cross-linked actin in these complexes. These findings are discussed in terms of the uncoupling between force generation and other aspects of actomyosin interactions due to a constrained dynamic flexibility of the subdomain 2/subdomain 1 interface in cross-linked actin filaments.

Probing the structure of F-actin: cross-links constrain atomic models and modify actin dynamics

Cross-links between protomers in F-actin can be used as a very sensitive probe of both the dynamics and structure of F-actin. We have characterized filaments formed from a previously described yeast actin Q41C mutant, where disulfide bonds can be formed between the Cys41 that is introduced into subdomain-2 and Cys374 on an adjacent protomer. We find that the distribution of cross-linked n-mers shows no cooperativity and corresponds to a random probability cross-linking reaction. The random distribution suggests that disulfide formation does not cause a significant perturbation of the F-actin structure. Consistent with this lack of perturbation, three-dimensional reconstructions of extensively cross-linked filaments, using a new approach to helical image analysis, show very small structural changes with respect to uncross-linked filaments. This finding is in conflict with refined models but in agreement with the original Holmes et al. model for F-actin. Under conditions where 94 % of the protomers are linked by disulfide bonds, the distribution of filament twist becomes more heterogeneous with respect to control filaments. A molecular model suggests that strain, introduced by the disulfide, is relieved by increasing the twist of the long-pitch actin helices. Disulfide formation makes yeast actin filaments approximately three times less flexible in terms of bending and similar, in this respect, to vertebrate skeletal muscle F-actin. These observations support previous reports that the rigidity of F-actin can be controlled by the position of subdomain-2, and that this region is more flexible in yeast F-actin than in skeletal muscle F-actin.