Characterization of the Aluminum and Beryllium Fluoride Species Bound to F-actin and Microtubules at the Site of the γ-Phosphate of the Nucleotide

Aluminum fluoride and beryllium fluoride complexes have previously been shown to bind tightly to F-ADP-actin and GDP-microtubules in competition with Pi and to mimic the XDP-Pi transient state of the polymerization. The structure of the bound complexes is investigated here in further detail. Using a fluoride ion-specific electrode, the number of fluoride atoms per aluminum or beryllium atom in the bound complex could be determined. The results indicate that AIF-4 and either BeF2(OH)-.H2O or BeF3-.H2O are the tightly bound species in both F-actin and microtubules. The dependences of the binding on pF and pH are consistent with this conclusion. The possible geometries of aluminum and beryllium fluorides in the gamma-phosphate subsite of the nucleotide are discussed in correlation with the catalytic mechanism of nucleotide hydrolysis.