Xian W, Tang J X, Janmey P A, Braunlin W H
Department of Chemistry, The University of Nebraska-Lincoln 68588-0304, USA.
Biochemistry. 1999 Jun 1;38(22):7219-26. doi: 10.1021/bi982301f.
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.
在生理条件下，丝状肌动蛋白（F-肌动蛋白）是一种多阴离子蛋白质细丝。F-肌动蛋白行为的关键特征与其他特征明确的聚电解质，特别是双链DNA相同。例如，包括二价金属离子如Mg2+在内的多价阳离子可使F-肌动蛋白形成束状结构，这被认为是肌动蛋白丝聚电解质性质的自然结果[Tang和Janmey（1996年）《生物化学杂志》271卷，8556 - 8563页]。这个最近提出的模型还表明，F-肌动蛋白与Mg2+离子之间的弱相互作用反映了由于其聚电解质性质，抗衡离子在F-肌动蛋白周围电场中的非特异性捕获。为了验证这一假设，我们对F-肌动蛋白溶液进行了25Mg核磁共振测量。基于核磁共振数据，我们估计Mg2+的旋转相关时间与肌动蛋白丝的整体旋转动力学无关。此外，竞争性结合实验表明，Co(NH3)63+能轻易取代与F-肌动蛋白结合的Mg2+。在较高的Co(NH3)63+浓度下，一部分镁离子会随着肌动蛋白丝聚集而被捕获。ATP也能有效地与肌动蛋白丝竞争结合Mg2+。这些结果支持了镁离子与肌动蛋白丝松散且非特异性结合的假设，因此显示出聚电解质溶液中抗衡离子的典型行为。观察到的特征在一定程度上模仿了DNA溶液中抗衡离子的充分记录的行为。
