Mobile segments in rabbit skeletal muscle F-actin detected by 1H nuclear magnetic resonance spectroscopy.
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.
Slósarek
G
G
Max-Planck-Institute for Medical Research, Dept. Biophysics, Heidelberg, Germany.
Heintz
D
D
Kalbitzer
H R
HR
D000199:Actins / Q000737:chemistry*; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000737:chemistry; D008274:Magnesium / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D009132:Muscles / Q000737:chemistry*; D009994:Osmolar Concentration; D010590:Phalloidine / Q000737:chemistry; D011108:Polymers; D011817:Rabbits; D014783:Viscosity
D000199:Actins; D011108:Polymers; D010590:Phalloidine; D008274:Magnesium; D002118:Calcium