Ligand-protein interactions in lysozyme investigated through a dual-resolution model.

A fully atomistic (AT) modeling of biological macromolecules at relevant length- and time-scales is often cumbersome or not even desirable, both in terms of computational effort required and a posteriori analysis. This difficulty can be overcome with the use of multiresolution models, in which different regions of the same system are concurrently described at different levels of detail. In enzymes, computationally expensive AT detail is crucial in the modeling of the active site in order to capture, for example, the chemically subtle process of ligand binding. In contrast, important yet more collective properties of the remainder of the protein can be reproduced with a coarser description. In the present work, we demonstrate the effectiveness of this approach through the calculation of the binding free energy of hen egg white lysozyme with the inhibitor di-N-acetylchitotriose. Particular attention is payed to the impact of the mapping, that is, the selection of AT and coarse-grained residues, on the binding free energy. It is shown that, in spite of small variations of the binding free energy with respect to the active site resolution, the separate contributions coming from different energetic terms (such as electrostatic and van der Waals interactions) manifest a stronger dependence on the mapping, thus pointing to the existence of an optimal level of intermediate resolution.

Expression, Purification, and Characterization of Anti-Zika virus Envelope Protein: Polyclonal and Chicken-Derived Single Chain Variable Fragment Antibodies.

Zika virus (ZIKV) is a new and emerging virus that has caused outbreaks worldwide. The virus has been linked to congenital neurological malformations in neonates and Guillain-Barré syndrome in adults. Currently there are no effective vaccines available. As a result, there is a great need for ZIKV treatment. In this study, we developed single chain variable fragment (scFv) antibodies that target the ZIKV envelope protein using phage display technology. We first induced an immune response in white leghorn laying hens against the ZIKV envelope (E) protein. Chickens were immunized and polyclonal immunoglobulin yolk (IgY) antibodies were extracted from egg yolks. A high-level titer of anti-ZIKV_E IgY antibodies was detected using enzyme-linked immunosorbent assay (ELISA) after the third immunization. The titer persisted for at least 9 weeks. We constructed two antibody libraries that contained 5.3 × 106 and 4.5 × 106 transformants. After biopanning, an ELISA phage assay confirmed the enrichment of specific clones. We randomly selected 26 clones that expressed ZIKV scFv antibodies and classified them into two groups, short-linker and long-linker. Of these, four showed specific binding activities toward ZIKV_E proteins. These data suggest that the polyclonal and monoclonal scFv antibodies have the diagnostic or therapeutic potential for ZIKV.

Soluble expression, rapid purification, biological identification of chicken interferon-alpha using a thioredoxin fusion system in E. coli and its antiviral effects to H9N2 avian influenza virus.

In this paper, we report a soluble expression based on Escherichia coli and two-step purification of a novel thioredoxin-tagged chicken interferon-α fusion protein (Trx-rChIFN-α) by using pET32a(+) expression system. The mature ChIFN-α gene was amplified by Reverse transcriptase-polymerase chain reaction (RT-PCR) and subcloned into pET-32a (+) vector prior to transformation into Rosetta (DE3) competent cells. After IPTG induction, the recombinant fusion protein was expressed efficiently in the soluble fraction. The protein purification was performed by nickel affinity chromatography and DEAE anion exchange chromatography. The purified product has a purity of 95% with a yield of 47.3 mg/L of culture. The specific activity of the fusion protein reaches to 2.0 × 107 IU/mg as determined in the CEF/VSV titration system. After excision of the Trx tag by enterokinase, the remaining solo protein was confirmed as rChIFN-α protein by SDS-PAGE, N-terminal sequencing and mass spectrometry. The effects of this Trx-rChIFN-α fusion protein against H9N2 influenza virus infection were also evaluated in ovo. The results showed that the Trx-rChIFN-α protein could significantly reduce the hemagglutination titer of H9N2 virus, and the H9N2 viruses HA gene copy numbers. These findings will enable us to produce large amount and bio-active rChIFN-α protein for future applications.

Isolation, purification, and structure identification of antioxidant peptides from embryonated eggs.

Antioxidant peptides are increasingly attracting researchers in medicine and foods. In the present study, egg white hydrolysates of embryonated eggs hatched on the sixth day (EWHD) were segmented consecutively by ultrafiltration membranes with small tangential flow ultrafiltration system. Four segments with more than 30, 30 to 10 kDa, 10 to 5 kDa, and less than 5 kDa of molecular weight cut-off (MWCO) values were separated and were labeled as MWCOI, MWCOII, MWCOIII, and MWCOIV, respectively. The antioxidant activities of segments were investigated by performing DPPH•, •OH radical scavenging, ultra oxygen anion (O2-•), total antioxidant capacity, and reducing power experiments. The results indicated that MWCOI has the strongest scavenging activities on DPPH• radical. However, MWCOIV has the strongest scavenging activities on •OH, O2-•, total antioxidant capacity, and reducing power, which revealed that MWCOIV has strong antioxidant activity. MWCOIV was further separated into 14 fractions via semipreparative reverse-phased high-performance liquid chromatography (RP-HPLC), and their antioxidant activity was evaluated by different antioxidant assays in vitro. The fractions 10 and 7 had strong antioxidant activities. The purities of these 2 fractions were determined by analytical RP-HPLC. Moreover, both fractions 10 and 7 displayed high purity levels, and they were identified by quadrupole time-of-flight tandem mass spectrometry. Only fraction 10, with a molecular weight of 204 Da, can be identified to be Ser-Val. EWHD can be considered as a promising source of natural food antioxidants for the development of functional food.

New reagents for poultry research: preparation, purification, and in vitro evaluation of non-PEGylated and mono-PEGylated chicken prolactin.

Recombinant chicken prolactin (chPRL), expressed in Escherichia coli and purified as a monomer, was successfully PEGylated and purified to homogeneity as a mono-PEGylated protein (PEG-chPRL). Its biological activity was estimated by its ability to interact with human prolactin receptor extracellular domain (hPRLR-ECD) and stimulate PRLR-mediated proliferation in Nb2-11C cells. PEG-chPRL activity in a cell bioassay was 10-fold lower than that of non-PEGylated chPRL, but only 2-fold lower in a binding assay to hPRLR-ECD. The CD spectra of non-PEGylated and PEGylated chPRL were almost identical and similar to that of hPRL, indicating proper refolding. Although the PEGylation of chPRL resulted in lower activity in vitro, PEG-chPRL was absorbed more slowly than chPRL, remained in the circulation 16 h longer. Furthermore the effects of PEG-chPRL injections in chickens on subsequent corticosteroid levels in blood were significantly profound compared to chPRL. These favorable PEGylation-induced pharmacokinetic alterations should improve efficacy of PEG-chPRL in in vivo experiments, as dosing frequency can be reduced due to its prolonged persistence in the circulation, and thus reduce the frequency of dosing. Furthermore, hydrophobic interaction chromatography was successfully adopted to isolate PEG-chPRL as a better alternative for separation of PEGylated PRL, and is likely to be successfully applicable to other proteins.

Purification of glutathione S-transferase enzyme from quail liver tissue and inhibition effects of (3aR,4S,7R,7aS)-2-(4-((E)-3-(aryl)acryloyl)phenyl)-3a,4,7,7a-tetrahydro-1H-4,7-methanoisoindole-1,3(2H)-dione derivatives on the enzyme activity.

The use of quail meat and eggs has made this animal important in recent years, with its low cost and high yields. Glutathione S-transferases (GST, E.C.2.5.1.18) are an important enzyme family, which play a critical role in detoxification system. In our study, GST was purified from quail liver tissue with 47.88-fold purification and 12.33% recovery by glutathione agarose affinity chromatography. The purity of enzyme was checked by SDS-PAGE method and showed a single band. In addition, inhibition effects of (3aR,4S,7R,7aS)-2-(4-((E)-3-(aryl)acryloyl)phenyl)-3a,4,7,7a-tetrahydro-1H-4,7methanoisoindole-1,3(2H)-dion derivatives (1a-g) were investigated on the enzyme activity. The inhibition parameters (IC50 and Ki values) were calculated for these compounds. IC50 values of these derivatives (1a-e) were found as 23.00, 15.75, 115.50, 10.00, and 28.75 µM, respectively. Ki values of these derivatives (1a-e) were calculated in the range of 3.04 ± 0.50 to 131.50 ± 32.50 µM. However, for f and g compounds, the inhibition effects on the enzyme were not found.

Expression and activity analysis of ß Gallinacin-3 in Arabidopsis.

ß Gallinacin-3 (ß Gal-3) is an antimicrobial peptide with strong antibacterial activity against Escherichia coli, Staphylococcus aureus and Salmonella typhimurium. In this study, the ß Gal-3 gene was transferred into a plant genome by genetic engineering techniques. These transgenic plants can be used as feed additives to prevent poultry diseases and them might replace the antibiotics used in poultry industry. To ensure the ß Gal-3 expresses effectively in Arabidopsis seeds, the expression was driven by promoter Ppha cloned from the ß-phaseolin storage protein gene. A total of 294 transgenic lines were obtained by Agrobacterium-mediated transformation into Arabidopsis, and five transgenic lines were selected in which the expression levels of ß Gal-3 were more than 0.10% of the total soluble proteins. The transgenic lines with single locus were identified by Southern blotting. The expression of ß Gal-3 and the highest protein accumulation level (about 4.76 mg/g fresh weight with a maximum of 0.27% of total soluble proteins) was measured by Western blotting and ELISA, respectively. After ultrafiltration by centrifugation, the purity of recombinant ß Gal-3 was up to 73%. Taken together, our data showed that expression of ß Gal-3 with antimicrobial activity is possible and effective in Arabidopsis seeds.

Extraction and physicochemical characterization of broiler (Gallus gallus domesticus) skin gelatin compared to commercial bovine gelatin.

Gelatin was extracted from broiler (Gallus gallus domesticus) skins and analyzed to compare its physicochemical properties with those of commercial bovine gelatin. The average yield of broiler skin gelatin was 6.5% on a wet weight basis. Broiler skin gelatin had more α1-and α2-chains than ß-chain and contained high molecular weight (γ-chain) polymers. Glycine was the dominant amino acid in broiler skin gelatin (20.26%), followed by proline (Pro) (15.12%) then hydroxyproline (Hyp) (11.36%). Compared to commercial bovine gelatin, broiler skin gelatin had less total imino acids (Pro and Hyp) but a higher (33.65 vs. 31.38°C) melting temperature (P < 0.01). The differences in physical properties between the broiler and commercial bovine gelatins appeared to be associated with differences in their amino acid composition and molecular weight distribution. The sensory evaluation results revealed that broiler skin gelatin could be a potential alternative to commercial bovine gelatin, useful in various food products.

Simultaneous separation of taxon-specific crystallins from Mule duck and characterization of their enzymatic activities and structures.

Methods to obtain pure proteins in large amounts are indispensible in protein research. We report here a large-scale/simultaneous isolation of taxon-specific crystallins (ɛ- and δ-crystallin) from the eye lenses of Mule duck. We also investigate the compositions, enzymatic activities, and structures of these purified taxon-specific proteins. A relatively mild method of ion-exchange chromatography was developed to fractionate ɛ-crystallin and δ-crystallin in large amount, ca. ∼6.60mg/g-lens and ∼41.0mg/g-lens, respectively. Both crystallins were identified by electrophoresis, HPLC, and MALDI-TOF-MS. ɛ-Crystallin, with native composition of Mr 142kDa, consisted of two subunits of 35kDa and 36kDa, while δ-Crystallin, with native molecular mass of 200kDa, comprised single subunit of Mr ∼50kDa. Both ɛ- and δ-crystallin were tetramers. The former showed lactate dehydrogenase (LDH) activity, while the latter appeared slightly active in an argininosuccinate lyase (ASL) assay. Raman spectroscopic results indicated that the secondary structures of ɛ- and δ-crystallin were predominantly α-helix as evidenced by the vibrational stretching of amide III over 1260cm-1 and amide I at 1255cm-1, in greatly contrast to the anti-parallel ß-sheet of α- and ß-crystallin as demonstrated by amide III at 1238cm-1 and amide I at 1672cm-1. The microenvironments of aromatic amino acids and the status of thiol groups also vary in different crystallins. The compositions, enzyme activities, and structures of the ɛ- and δ-crystalline of Mule duck are different from those of Muscovy duck (Cairina moschata) or Kaiya duck (Anas Platyrhynchos var. domestica), which reflect faithfully species specificity.

Recombinant expression, purification, and crystallization of the sterile α-motif/histidine-aspartate domain-containing protein from chicken.

The sterile α-motif and HD domain containing protein 1 (SAMHD1) family is a newly identified protein family, involved in innate immunity restriction. This family possesses a broad-spectrum of antiviral activity. The SAMHD1 family in chicken has not been clearly documented. Here, we expressed chicken SAMHD1 (101-614) fused with a SUMO tag in an Escherichia coli (E. coli) system. For the first time, chicken SAMHD1 (101-614) was found to possess dNTPase cleavage activities in vitro. This suggests that chicken SAMHD1 may be a potential antiviral factor against avian viruses. Through a unique purification method, the purity of the protein as estimated by SDS-PAGE was >95% after a double Ni affinity chromatography and gel filtration purification. Using a sitting-drop vapor-diffusion method, protein crystals were obtained. This study provides some essential method and information for further structure and function determinations of chicken SAMHD1.

Design of peptidase-resistant peptide inhibitors of myosin light chain kinase.

Myosin light chain kinase (MLCK) is a key regulator of various forms of cell motility including smooth muscle contraction, cell migration, cytokinesis, receptor capping, secretion, etc. Inhibition of MLCK activity in endothelial and epithelial monolayers using cell-permeant peptide Arg-Lys-Lys-Tyr-Lys-Tyr-Arg-Arg-Lys (PIK, Peptide Inhibitor of Kinase) allows protecting the barrier capacity, suggesting a potential medical use of PIK. However, low stability of L-PIK in a biological milieu prompts for development of more stable L-PIK analogues for use as experimental tools in basic and drug-oriented biomedical research. Previously, we designed PIK1, H-(Nα Me)Arg-Lys-Lys-Tyr-Lys-Tyr-Arg-Arg-Lys-NH2 , that was 2.5-fold more resistant to peptidases in human plasma in vitro than L-PIK and equal to it as MLCK inhibitor. In order to further enhance proteolytic stability of PIK inhibitor, we designed the set of six site-protected peptides based on L-PIK and PIK1 degradation patterns in human plasma as revealed by 1 H-NMR analysis. Implemented modifications increased half-live of the PIK-related peptides in plasma about 10-fold, and these compounds retained 25-100% of L-PIK inhibitory activity toward MLCK in vitro. Based on stability and functional activity ranking, PIK2, H-(Nα Me)Arg-Lys-Lys-Tyr-Lys-Tyr-Arg-D-Arg-Lys-NH2 , was identified as the most stable and effective L-PIK analogue. PIK2 was able to decrease myosin light chain phosphorylation in endothelial cells stimulated with thrombin, and this effect correlated with the inhibition by PIK2 of thrombin-induced endothelial hyperpermeability in vitro. Therefore, PIK2 could be used as novel alternative to other cell-permeant inhibitors of MLCK in cell culture-based and in vivo studies where MLCK catalytic activity inhibition is required. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.

Interactomic landscape of PA-X-chicken protein complexes of H5N1 influenza A virus.

UNLABELLED: As a newly identified isoform generated from the PA segment and an essential factor for viral virulence, little is known about PA-X-host interactions. Here we present the interactomic landscape of PA-X protein of H5N1 influenza A virus (IAV), described using data generated from affinity purification and mass spectrometry (AP-MS). PA-X was exogenously expressed in chicken fibroblast cells and PA-X associated protein complexes were identified by AP-MS. Using a high confidence threshold for interaction 56 unique proteins were found to have physical interactions with PA-X. PA-X associated host factors showed strong enrichment for specific protein domains, including annexin and WD40 domains. Many proteins that have been described as pro or antiviral host proteins interact with PA-X, indicating the possible effect of these proteins on influenza A viral infections facilitated by interactions with PA-X. This study has uncovered the comprehensive interactomic landscape of PA-X and laid the foundation for further understanding of PA-X function in terms of viral-host protein interactions. BIOLOGICAL SIGNIFICANCE: Identification of viral-host interacting proteins is vital for the comprehensive understanding of how virus recruits the host cellular machinery and how host antagonizes virus infection. Our study reveals the viral-host interactome of PA-X and uncovers interactions between host proteins and PA-X which might have crucial roles in viral infection.

Efficient purification of lysozyme from egg white by 2-mercapto-5-benzimidazolesulfonic acid modified Fe3O4/Au nanoparticles.

2-Mercapto-5-benzimidazolesulfonic acid (MBISA) modified Fe3O4/Au nanoparticles were synthesized in aqueous solution and characterized by photo correlation spectroscopy (PCS) and vibrating sample magnetometer (VSM). The so-obtained Fe3O4/Au-MBISA nanoparticles were capable of specific adsorbing lysozyme. The maximum amount of lysozyme adsorbed on 1.0mg Fe3O4/Au-MBISA nanoparticles was 346µg. The lysozyme desorption behavior was studied and the lysozyme recovery from Fe3O4/Au-MBISA nanoparticles approached 100% under optimal conditions, and the reusability studies showed that the nanoparticles could maintain about 91% of the initial lysozyme adsorption capacity after 7 repeated adsorption-elution cycles. The Fe3O4/Au-MBISA nanoparticles were used in the purification of lysozyme from chicken egg white, which was verified by a single SDS-PAGE band. Therefore, the obtained Fe3O4/Au-MBISA nanoparticles exhibited excellent performance in the direct purification of lysozyme from egg white.

Sedimentation Velocity Analysis of Large Oligomeric Chromatin Complexes Using Interference Detection.

Sedimentation velocity experiments measure the transport of molecules in solution under centrifugal force. Here, we describe a method for monitoring the sedimentation of very large biological molecular assemblies using the interference optical systems of the analytical ultracentrifuge. The mass, partial-specific volume, and shape of macromolecules in solution affect their sedimentation rates as reflected in the sedimentation coefficient. The sedimentation coefficient is obtained by measuring the solute concentration as a function of radial distance during centrifugation. Monitoring the concentration can be accomplished using interference optics, absorbance optics, or the fluorescence detection system, each with inherent advantages. The interference optical system captures data much faster than these other optical systems, allowing for sedimentation velocity analysis of extremely large macromolecular complexes that sediment rapidly at very low rotor speeds. Supramolecular oligomeric complexes produced by self-association of 12-mer chromatin fibers are used to illustrate the advantages of the interference optics. Using interference optics, we show that chromatin fibers self-associate at physiological divalent salt concentrations to form structures that sediment between 10,000 and 350,000S. The method for characterizing chromatin oligomers described in this chapter will be generally useful for characterization of any biological structures that are too large to be studied by the absorbance optical system.

Preparation of asparagine-linked monoglucosylated high-mannose-type oligosaccharide from egg yolk.

Monoglucosylated high-mannose-type glycan (Glc1Man9GlcNAc2: G1M9) is well-known as a key glycoform in the glycoprotein folding process, which is specifically recognized by lectin chaperones calnexin (CNX) and calreticulin (CRT) in the endoplasmic reticulum (ER). In this work, we developed an efficient method for the preparation of G1M9-Asn. The G1M9-Asn was obtained from the IgY-rich fraction derived from hen egg yolk by the digestion with pronase. The α-amino group of asparagine in G1M9-Asn was protected with the 9-fluorenylmethyloxycarbonyl (Fmoc) group and the labeled glycans were subsequently purified using high performance liquid chromatography (HPLC). This method will provide useful substrates for analysis of the glycoprotein folding cycle in the ER.

A simple method for isolating chicken egg yolk immunoglobulin using effective delipidation solution and ammonium sulfate.

Chicken egg yolk immunoglobulin (IgY) is a superior alternative to mammalian immunoglobulin. However, the practical application of IgY in research, diagnostics, and functional food is limited due to complex or time-consuming purification procedures. The objective of this study was to develop a simple, safe, large-scale separation method for IgY from egg yolk. Egg yolk was diluted with 6-fold delipidation solutions made of different types (pectin, λ-carrageenan, carboxymethylcellulose, methylcellulose, and dextran sulfate) and concentrations (0.01, 0.05, 0.1, 0.15, and 0.2%) of polysaccharides, respectively. The yolk solution was adjusted to pH 5.0, and then kept overnight at 4°C before being centrifuged at 4°C. The resulting supernatant was added to 35% (w/v) (NH4)2SO4 and then centrifuged. The precipitant, which contained IgY, was dissolved in distilled water and then dialyzed. SDS-PAGE and Western blotting were utilized to conduct qualitative analysis of IgY; high-performance liquid chromatography (HPLC) was used for quantitative analysis. The immunoreactivity of IgY was measured by ELISA. The results showed that yield, purity, and immunoreactivity varied with types and concentrations of polysaccharides. The optimal isolation of IgY for pectin, λ-carrageenan, dextran sulfate, and carboxymethylcellulose was at the concentration of 0.1%; for methylcellulose, optimal isolation was at 0.15%. The best results were obtained in the presence of 0.1% pectin. In this condition, yield and purity can reach 8.36 mg/mL egg yolk and 83.3%, respectively, and the negative effect of IgY on immunoreactivity can be minimized. The procedure of isolation was simplified to 2 steps with a higher yield of IgY, avoiding energy- and time-consuming methods. Therefore, the isolation condition under study has a great potential for food industry production of IgY on a large scale.

A novel monoclonal antibody against the constant region of goose immunoglobulin light chain.

A monoclonal antibody (MAb) against the antigenic determinant of the constant region of goose immunoglobulin light chain (GoIgCL) was produced and characterized for the first time here. Goose immunoglobulin (Ig) in serum was purified by immunoaffinity chromatography and the resulting protein was used as immunogen to immunize BALB/c mice. At the same time, the GoIgCL gene was expressed and purified as the screening antigen for selecting MAb against GoIgCL. One hybridoma that produces antibodies against GoIgCL was selected by indirect ELISA. Then the characterization of the MAb was analyzed by ELISA, Western blot, and flow cytometry. It was found to be IgG1 with κ light chain; the MAB has high specificity to Ig in goose serum, bile, and B lymphocytes from peripheral blood, reacts only with the light chain of goose Ig, and can distinguish Ig from other birds. Therefore, the MAb generated in this study can be used as a specific reagent for detection of goose disease-specific antibodies and as a powerful tool for basic immunology research on geese.

In vitro antioxidant properties of chicken skin enzymatic protein hydrolysates and membrane fractions.

Chicken thigh and breast skin proteins were hydrolysed using alcalase or a combination of pepsin and pancreatin (PP), each at concentrations of 1-4%. The chicken skin protein hydrolysates (CSPHs) were then fractionated by membrane ultrafiltration into different molecular weight peptides (<1, 1-3, 3-5 and 5-10kDa) and analysed for antioxidant properties. Results showed that the CSPHs had a significantly (p<0.05) lower scavenging activity against DPPH radicals when compared to reduced glutathione. The chicken breast skin hydrolysates had significantly higher DPPH scavenging activity than the chicken thigh skin hydrolysates. DPPH scavenging and metal ion chelation increased significantly (p<0.05) from 29-40% to 86-89%, respectively with increasing proteolytic enzyme concentration. In contrast, the antioxidant properties decreased as peptide size increased. We conclude that CSPHs and their peptide fractions may be used as ingredients in the formulation of functional foods and nutraceuticals for the control and management of oxidative stress-related diseases.

Egg white proteins and their potential use in food processing or as nutraceutical and pharmaceutical agents--a review.

Egg white contains many functionally important proteins. Ovalbumin (54%), ovotransferrin (12%), ovomucoid (11%), ovomucin (3.5%), and lysozyme (3.5%) are among the major proteins that have high potentials for industrial applications if separated. The separation methods for these proteins from egg white have been developed since early 1900, but preparation methods of these proteins for commercial applications are still under development. Simplicity and scalability of the methods, use of nontoxic chemicals for the separation, and sequential separation for multiple proteins are very important criteria for the commercial production and application of these proteins. The separated proteins can be used in food and pharmaceutical industry as is or after modifications with enzymes. Ovotransferrin is used as a metal transporter, antimicrobial, or anticancer agent, whereas lysozyme is mainly used as a food preservative. Ovalbumin is widely used as a nutrient supplement and ovomucin as a tumor suppression agent. Ovomucoid is the major egg allergen but can inhibit the growth of tumors, and thus can be used as an anticancer agent. Hydrolyzed peptides from these proteins showed very good angiotensin I converting enzyme inhibitory, anticancer, metal binding, and antioxidant activities. Therefore, separation of egg white proteins and the productions of bioactive peptides from egg white proteins are emerging areas with many new applications.

Avian synaptopodin 2 (fesselin) stabilizes myosin filaments and actomyosin in the presence of ATP.

Smooth muscle cells maintain filaments of actin and myosin in the presence of ATP, although dephosphorylated myosin filaments and actin-myosin interactions are unstable under those conditions in vitro. Several proteins that stabilize myosin filaments and that stabilize actin-myosin interactions have been identified. Fesselin or synaptopodin 2 appears to be another such protein. Rapid kinetic measurements and electron microscopy demonstrated that fesselin, isolated from turkey gizzard muscle, reduced the rate of dissociation of myosin filaments. Addition of fesselin increased both the length and thickness of myosin filaments. The rate of detachment of myosin, but not heavy meromyosin, from actin was also greatly reduced by fesselin. Data from this study suggest that fesselin stabilizes myosin filaments and tethers myosin to actin. These results support the view that one role of fesselin is to organize contractile units of myosin and actin.