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 promoters with a M- (mobile) state and a I- (immobile) state in equilibrium. In the M-state a number of residues in the actin promoter are mobile and give rise to observable NMR signals. This equilibrium is shifted towards the I-state specifically by replacing Mg2+ with Ca(2+)-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.
No takes yet. Share an insight, caveat, or question.
Ślósarek et al. (1994) studied this question.
Synapse has enriched 5 closely related papers on similar clinical questions. Consider them for comparative context: