Crystalline actin tubes. II. The effect of various lanthanide ions on actin tube formation.

The ability of skeletal muscle actin to aggregate in the form of crystalline tubular structures was examined using all 14 of the available trivalent lanthanide cations. Under conditions which normally cause F-actin formation, (0.1 M KCl, pH 6.9) only those lanthanide ions which interact with actin with a molar ratio of 5:1 (Ce3+, Pr3+, Nd3+, Sm3+ and Eu3+) or 6:1 (Gd3+, Tb3+, Dy3+ and Ho3+) can promote the formation of ordered tubular structures. Under the same conditions, the remaining ions (Er3+, Tm3+, Yb3+, Lu3+ as well as La3+) interact with a 7:1 molar ratio of lanthanide to actin, but these were unable to form actin tubes. Actin tube dimensions undergo systematic changes depending on which lanthanide ion binds. The dimensions of the actin monomers and their packing arrangement (i.e. number of rows of subunits per helical repeat and the pitch angle) determines the actin tube diameter. It is suggested that the failure of actin tube formation when 7 lanthanide ions bind is due to additional charge on the monomer, possibly conferring on it a net positive charge.