The polyelectrolyte behavior of actin filaments: a 25Mg NMR study.
Under physiological conditions, filamentous actin (F-actin) is a polyanionic protein filament. Key features of the behavior of F-actin are shared with other well-characterized polyelectrolytes, in particular, duplex DNA. For example, the bundle formation of F-actin by polyvalent cations, including divalent metal ions such as Mg2+, has been proposed to be a natural consequence of the polyelectrolyte nature of actin filaments [Tang and Janmey (1996) J. Biol. Chem. 271, 8556-8563]. This recently proposed model also suggests that weak interactions between F-actin and Mg2+ ions reflect a nonspecific trapping of counterions in the electric field surrounding F-actin due to its polyelectrolyte nature. To test this hypothesis, we have performed 25Mg NMR measurements in F-actin solutions. Based on the NMR data, we estimate that the rotational correlation times of Mg2+ are independent of the overall rotational dynamics of the actin filaments. Moreover, competitive binding experiments demonstrate a facile displacement of F-actin-bound Mg2+ by Co(NH3)63+. At higher Co(NH3)63+ concentrations, a fraction of the magnesium ions are trapped as actin filaments aggregate. ATP also competes effectively with actin filaments for binding to Mg2+. These results support the hypothesis that magnesium ions bind loosely and nonspecifically to actin filaments, and thus show a behavior typical of counterions in polyelectrolyte solutions. The observed features mimic to some extent the well-documented behavior of counterions in DNA solutions.
Xian
W
W
Department of Chemistry, The University of Nebraska-Lincoln 68588-0304, USA.
Tang
J X
JX
Janmey
P A
PA
Braunlin
W H
WH
D008841:Actin Cytoskeleton / Q000737:chemistry* / Q000378:metabolism; D000199:Actins / Q000737:chemistry* / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000737:chemistry / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D003035:Cobalt / Q000737:chemistry; D004573:Electrolytes / Q000737:chemistry* / Q000378:metabolism; D018260:Gelsolin / Q000737:chemistry / Q000378:metabolism; D007554:Isotopes; D008274:Magnesium / Q000737:chemistry* / Q000378:metabolism; D019906:Nuclear Magnetic Resonance, Biomolecular / Q000379:methods; D010590:Phalloidine / Q000737:chemistry; D011108:Polymers / Q000737:chemistry* / Q000378:metabolism; D011817:Rabbits; D055672:Static Electricity; D017186:Titrimetry
D000199:Actins; D004573:Electrolytes; D018260:Gelsolin; D007554:Isotopes; D011108:Polymers; C030862:cobalt ammonium complex; D010590:Phalloidine; D003035:Cobalt; D000255:Adenosine Triphosphate; D008274:Magnesium