Mobile segments in rabbit skeletal muscle F-actin detected by ¹H nuclear magnetic resonance spectroscopy

Authors:

• Genowefa Ślósarek,
• Daniela Heintz,
• Hans Robert Kalbitzer

Abstract

Polymerization of actin by increasing the ionic strength leads to a quenching of almost all ¹H NMR signals. Surprisingly, distinct signals with relatively small line widths can still be observed in actin filaments (F-actin) indicating the existence of mobile, NMR visible residues in the macromolecular structure. The intensity of the F-actin spectrum is much reduced if one replaces Mg²⁺ with Ca²⁺, and a moderate reduction of the signal intensity can also be obtained by increasing the ionic strength. These results can be explained in a two-state model of the actin protomers with a M- (mobile) state and a I- (immobile) state in equilibrium. In the M-state a number of residues in the actin protomer are mobile and give rise to observable NMR signals. This equilibrium is shifted towards the I-state specifically by replacing Mg²⁺ with Ca²⁺-ions and unspecifically by addition of monovalent ions such as K₊. The binding of phalloidin to its high-affinity site in the filaments does not influence the equilibrium between M- and I-state. Phalloidin itself is completely immobilized in F-actin, its exchange with the solvent being slow on the NMR time scale. © 1994.