Distinct structural changes detected by X-ray fiber diffraction in stabilization of F-actin by lowering pH and increasing ionic strength.
Lowering pH or raising salt concentration stabilizes the F-actin structure by increasing the free energy change associated with its polymerization. To understand the F-actin stabilization mechanism, we studied the effect of pH, salt concentration, and cation species on the F-actin structure. X-ray fiber diffraction patterns recorded from highly ordered F-actin sols at high density enabled us to detect minute changes of diffraction intensities and to precisely determine the helical parameters. F-actin in a solution containing 30 mM NaCl at pH 8 was taken as the control. F-actin at pH 8, 30 to 90 mM NaCl or 30 mM KCl showed a helical symmetry of 2.161 subunits per turn of the 1-start helix (12.968 subunits/6 turns). Lowering pH from 8 to 6 or replacing NaCl by LiCl altered the helical symmetry to 2.159 subunits per turn (12.952/6). The diffraction intensity associated with the 27-A meridional layer-line increased as the pH decreased but decreased as the NaCl concentration increased. None of the solvent conditions tested gave rise to significant changes in the pitch of the left-handed 1-start helix (approximately 59.8 A). The present results indicate that the two factors that stabilize F-actin, relatively low pH and high salt concentration, have distinct effects on the F-actin structure. Possible mechanisms will be discussed to understand how F-actin is stabilized under these conditions.
Oda
T
T
International Institute for Advanced Research, Matsushita Electric Industrial Co. Ltd., Seika, Kyoto 619-0237, Japan. oda@mpimf-heidelberg.mpg.de
Makino
K
K
Yamashita
I
I
Namba
K
K
Maéda
Y
Y
D000199:Actins / Q000737:chemistry*; D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D003545:Cysteine / Q000737:chemistry; D004355:Drug Stability; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D009994:Osmolar Concentration; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D011817:Rabbits; D013816:Thermodynamics; D014961:X-Ray Diffraction
D000199:Actins; D003545:Cysteine