Inhibition of muscle tropomyosin polymerization by DNAse I

Comparisons of endothelial cell G- and F-actin distribution in situ and in vitro

Numerous studies have described the F-actin cytoskeleton; however, little information relevant to G-actin is available. The actin pools of bovine aortic endothelial cells were examined using in situ and in vitro conditions and fluorescent probes for G-(deoxyribonuclease I, 0.3 microM) or F-actin (phalloidin, 0.2 microM). Cells in situ displayed a diffuse G-actin distribution, while F-actin was concentrated in the cell periphery and in fine stress fibers that traversed some cells. Cells of subconfluent or just confluent cultures demonstrated intense fluorescence, with many F-actin stress fibers. Postconfluent cultures resembled the condition in situ; peripheral F-actin was prominent, traversing actin stress fibers were greatly reduced and fluorescent intensity was diminished. Postconfluency had little influence on G-actin, with only an enhancement in the intensity of G-actin punctate fluorescence. When post-confluent cultures were incubated with cytochalasin D (15 min; 10(-4) M), F-actin networks were disrupted and actin punctate and diffuse fluorescence increased. G-actin fluorescence was not altered by the incubation. Although its unstructured nature may account for the minor changes observed, the stability of the G-actin pool in the presence of notable F-actin modulations suggested that filamentous actin was the key constituent involved in these actin cytoskeletal alterations. A separate finding illustrated that the concomitant use of actin probes with image enhancement and fluorescent microscopy could reveal simultaneously the G- and F-actin pools within the same cell.

Interaction of muscle and non-muscle tropomyosins with deoxyribonuclease I

The sulfhydryl-selective fluorescent reagent acrylodan (6-acryloyl-2-dimethylaminonaphthalene) was used to label tropomyosins from rabbit cardiac muscle and from equine platelets. Addition of bovine pancreatic deoxyribonuclease I to solutions of acrylodan-modified tropomyosins significantly altered the emission properties of the samples. Muscle and non-muscle tropomyosin fluorescence were affected in qualitatively similar manners; emission maxima were red-shifted by about 8 nm to 522-525 nm and maximal intensities were reduced by approximately 15%. Addition of KI to each of the fluorescent samples caused a greater degree of fluorescence quenching in the presence of DNase I than in its absence. The slopes of Stern-Volmer plots were 15-25% steeper in the presence of DNase I. Fluorescence polarization values for acrylodan-labelled tropomyosin samples were 25-35% lower in the presence of DNase I. Each of these effects could be saturated by addition of about a two-fold molar excess of DNase I to tropomyosin. Together they suggest that interaction with DNase I causes localized unfolding of tropomyosin, thereby allowing the fluorescent label to become more exposed to the solvent and less restricted in its local motions. Circular dichroism measurements support this idea. Addition of DNase I to solutions of either labelled or unlabelled tropomyosin results in a net 14-18% loss in ellipticity near 220 nm, indicative of unfolding of alpha-helix.