Acanthamoeba profilin binding to fluorescein-labeled actins

The real-time monitoring of the thermal unfolding of tetramethylrhodamine-labeled actin

Modification of actin at Cys (374) with tetramethylrhodamine maleimide (TMR-actin) has been used for visualization of actin filaments and to produce high-resolution crystal structures of actin. We show that TMR-actin exhibits a 21% decrease in absorbance at 557 nm upon thermal unfolding, likely due to the movement of TMR to a more hydrophobic environment upon rapid unfolding and protein aggregation. We took advantage of this property to test models of actin protein unfolding. A transition temperature ( T m) of 60.2 +/- 0.2 degrees C for Ca (2+).ATP.TMR-actin was determined using A 557 and agreed with our own determinations employing different techniques and previous work with unlabeled actin. Our data show that the dependence of TMR-actin thermal stability on the bound nucleotide and cations follows a trend of Ca (2+).ATP > Mg (2+).ATP > Ca (2+).ADP > Mg (2+).ADP. The activation energies and frequency factors for the thermal unfolding of TMR-actin determined with two methods were in good agreement with those previously determined for unlabeled actin. We observed a biphasic trend in the T m of TMR-actin with increasing nucleotide concentrations, supporting a two-pathway model for actin protein unfolding where one pathway dominates at different concentrations of nucleotide. Additionally, TMR-actin bound by DNase I or gelsolin segment-1 exhibited elevated transition temperatures.

Elongation factor 1beta is an actin-binding protein

A 17 kDa polypeptide found in association with actin in cellular extracts of Dictyostelium discoideum was identified as a proteolytic fragment of eEF1beta. Antibody elicited against the 17 kDa protein reacted with a single 29 kDa polypeptide in Dictyostelium, indicating that the 17 kDa peptide arises from degradation of a larger precursor. The cDNA isolated from a Dictyostelium library using this antibody as a probe encodes Dictyostelium elongation factor 1beta. Amino acid degradation of the 17 kDa protein fragment confirmed the identity of the protein as eEF1beta. Direct interaction of eEF1beta with actin in vitro was further demonstrated in mixtures of actin with the 17 kDa protein fragment of Dictyostelium eEF1beta, recombinant preparations of Dictyostelium eEF1beta expressed in Escherichia coli, and the intact eEF1betagamma complex purified from wheat germ. Localization of eEF1beta in Dictyostelium by immunofluorescence microscopy reveals both diffuse cytoplasmic staining, and some concentration in the cortical and hyaline cytoplasm. The results support the existence of physical and functional interactions of the translation apparatus with the cytoskeleton, and suggest that eEF1beta may function in a dual role both to promote the elongation phase of protein synthesis, and to interact with cytoplasmic actin.

Analysis of rhodamine and fluorescein-labeled F-actin diffusion in vitro by fluorescence photobleaching recovery

Properties of filamentous acetamidofluorescein-labeled actin and acetamidotetramethylrhodamine-labeled actin (AF and ATR-actin, respectively) were examined to resolve discrepancies in the reported translational diffusion coefficients of F-actin measured in vitro by FPR and other techniques. Using falling-ball viscometry and two independent versions of fluorescence photobleaching recovery (FPR), the present data indicate that several factors are responsible for these discrepancies. Gel filtration chromatography profoundly affects the viscosity of actin solutions and filament diffusion coefficients. ATR-actin and, to a lesser degree, AF-actin show a reduction in viscosity in proportion to the fraction labeled, presumably due to filament shortening. Actin filaments containing AF-actin or ATR-actin are susceptible to photoinduced damage, including a covalent cross-linking of actin protomers within filaments and an apparent cleavage of filaments detected by a decrease of the measured viscosity and an increase in the measured filament diffusion coefficients. Quantum yields of the two photoinduced effects are quite different. Multiple cross-links are produced relative to each photobleaching event, whereas less than 1% filament cleavage occurs. Substantial differences in the filament diffusion coefficients measured by FPR are also the result of differences in illumination geometry and sampling time. However, under controlled conditions, FPR can be used as a quantitative tool for measuring the hydrodynamic properties of actin filaments. Incremented filament shortening caused by photoinduced cleavage or incremental addition of filament capping proteins produces a continuous and approximately linear increase of filament diffusion coefficients, indicating that filaments are not associated in solution. Our results indicate that actin filaments exhibit low mobilities and it is inferred that actin filaments formed in vitro by column-purified actin, under standard conditions, are much longer than has conventionally been presumed.