Effects of formamide on the polymerization and depolymerization of muscle actin

The interaction of 6-propionyl-2-(NN-dimethyl)aminonaphthalene (PRODAN)-labelled actin with actin-binding proteins and drugs

The influence of various actin-binding proteins and drugs on the fluorescence emission of rabbit muscle actin labelled with the fluorescent probe acrylodan (6-acryloyl-2-dimethylaminonaphthalene) at Cys-374, the penultimate amino acid residue of the actin amino acid sequence, was studied. Addition of myosin, tropomyosin or phalloidin, agents known to bind only to filamentous F-actin, did not change the emission energy or the integrated intensity of the fluorescence spectrum. The presence of heavy meromyosin or of the glycolytic enzyme aldolase led to a small (approx. 2%) increase in the integrated intensity, and in the energy of the emitted fluorescence. The interaction of 6-propionyl-2-(NN-dimethyl)aminonaphthalene (PRODAN)-F-actin with pancreatic DNAase I and with a filament-severing 19 kDa protein from pig brain resulted in the gradual reduction of the integrated intensity of the emission and a red shift of the emission energy, suggestive of a disintegration of the actin filament structure. Profilin caused a < 10% change in the emission energy. Cytochalasin D reduced the integrated intensity of PRODAN-F-actin and red-shifted the emission energy, while cytochalasin B was without influence. Pancreatic DNAase I did not change the fluorescence emission of PRODAN-G-actin, suggesting that binding of this enzyme does not alter the environment of the probe. When the 19 kDa protein bound to PRODAN-G-actin, however, the integrated intensity was reduced and the emission energy was lowered. This effect was exploited to estimate the binding constant for the interaction between the 19 kDa protein and PRODAN-G-actin. The Kd was found to be about 0.25 microM.

Spectroscopic and functional characterization of an environmentally sensitive fluorescent actin conjugate

Rabbit skeletal muscle F-actin has been selectively labeled at a cysteine residue with the environmentally sensitive fluorophore 6-acryloyl-2-(dimethylamino)naphthalene. The fluorescent actin conjugate behaves similarly to native actin with respect to the polymerization kinetics, critical monomer concentration, and ability to form F-actin paracrystals. Upon polymerization to F-actin, the absorption of the actin conjugate is red-shifted, whereas the fluorescence emission is blue-shifted 740 wavenumbers and is accompanied by a decrease in the fluorescence bandwidth of 470 wavenumbers. These large shifts in the spectral properties of 6-propionyl-2-(dimethylamino)naphthalene (Prodan) in actin provide a simple method for obtaining a spectral discrimination between the G- and F-actin populations during the polymerization reaction. Steady-state fluorescence techniques were used to study the environment of the fluorophore in the monomeric and polymeric forms of actin. Fluorescence emission spectral analysis and quenching and polarization studies of G-actin-Prodan indicated that the fluorophore lies immobile on the protein surface but with one of its faces in full contact with the solvent. In F-actin, the fluorophore has a limited exposure to the solvent and is located in a dielectric environment similar to those seen for Prodan in polar, aprotic solvents or buried within a protein matrix [Macgregor, R. B., Jr., & Weber, G. (1986) Nature (London) 318, 70-73]. Additionally, our results demonstrate that the Prodan molecule conjugated to F-actin is completely immobile during its fluorescence lifetime, exhibits an increase in the resonance energy transfer (RET) from tryptophan residues compared to that observed in G-actin, and shows evidence of homologous RET within the polymer.

Immunocytochemical studies using a monoclonal antibody to bovine cardiac titin on intact and extracted myofibrils

A monoclonal antibody specific to bovine cardiac titin has been identified. The antibody recognizes a common antigenic site in striated muscles of several species. In relaxed myofibrils, specific staining at the A-I junction resulted in a doublet of fluorescent bands within a sarcomere. The distance between the doublets in successive sarcomeres varied according to the degree of myofibrillar contraction. Staining on formamide-extracted myofibrils has confirmed that this epitope is located near the outer edges of isolated A bands. Selective extraction of myofibrillar proteins resulted in different staining patterns. Disrupting the structural integrity of the M-line or the A-band centre caused a significant amount of titin to translocate toward the Z-line region. In contrast, shortening of the A-band by removal of myosin from the ends of the thick filaments resulted in anti-titin staining moving closer to the M-line region. Several conclusions can be drawn from this study: (a) two aligned groups of titin molecules are placed symmetrically to the M-line in a sarcomere; (b) titin may attach directly or via intermediary protein(s) to sites near the M-line and Z-line such that the protein is under tension and (c) removal of proteins from either region results in titin staining in the opposite region. However, the edges of the A-band give some hindrance to collapse of the titin toward the M-line.

Stability differences of muscle F-actin in formamide in the presence of Mg2+ and Ca2+

Muscle G-actin was polymerized by addition of 2 mM Mg2+ or 2 mM Ca2+. Subsequent addition of formamide reduced the specific viscosity of the polymer solution. However, kinetic analysis of this reduction in the presence or absence of 0.1 M KCl revealed differences between F-actin formed in the presence of Mg2+ and F-actin formed in the presence of Ca2+. In the presence of Mg2+ the viscosity dropped instantaneously, reaching within minutes a steady-state level that was constant for many hours. In contrast, in the presence of Ca2+ the high-shear viscosity continued to decrease slowly after an initial drop, and it could take hours until a quasi-equilibrium was obtained. The time was dependent on both formamide and protein concentration. Addition of formamide increased the critical actin concentration in the presence of Ca2+, but not in the presence of Mg2+. This is taken as evidence that in the presence of Ca2+, but not in the presence of Mg2+, formamide causes partial depolymerization of F-actin.