MOBILE SEGMENTS IN RABBIT SKELETAL-MUSCLE F-ACTIN DETECTED BY H-1 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY

Polymerization of actin by increasing the ionic strength leads to a qu enching of almost all H-1 NMR signals. Surprisingly, distinct signals with relatively small line widths can still be observed in actin filam ents (F-actin) indicating the existence of mobile, NMR visible residue s in the macromolecular structure. The intensity of the F-actin spectr um is much reduced if one replaces Mg2+ with Ca2+, and a moderate redu ction of the signal intensity can also be obtained by increasing the i onic 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 i n equilibrium. In the M-state a number of residues in the actin protom er are mobile and give rise to observable NMR signals. This equilibriu m is shifted towards the I-state specifically by replacing Mg2+ with C a2+-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 sol vent being slow on the NMR time scale.