Evidence for an F-actin like conformation in the actin:DNase I complex.

We demonstrate that a ribose modified analogue of ATP, TNP-ATP, can exchange with a resident nucleotide in F-actin, but fails to bind to G-actin. TNP-ATP is also able to bind to actin in the actin:DNase I complex, suggesting that the nucleotide binding site in the actin:DNase I complex adopts a conformation similar to that found in F-actin. This result is consistent with the hypothesis that the two major domains of actin on either side of the cleft are able to "flex" or move relative to each other in G-actin, but that this flexing motion is limited as a consequence of either polymerisation or DNase I binding. F-actin, in which approximately 80% of the bound nucleotide is TNP-ADP, appears to be functionally similar to native ADP-F-actin. It can superprecipitate with myosin and, following regulation with troponin-tropomyosin, exhibits a Ca(2+)-sensitivity during superprecipitation. Sonication induced nucleotide exchange in regulated F-actin was not sensitive to the presence of Ca2+ which argues against a significant conformational change in the vicinity of the nucleotide binding site during Ca(2+)-sensitive thin filament regulation.