在植物、哺乳动物以及其它的真核细胞中，肌动蛋白是最丰富和保守的蛋白之一，其主要以单体 (G-actin) 和微丝 (F-actin) 两种形式存在，并且处于聚合和解聚的动态过程中。在很多情况下，微丝是肌动蛋白的主要功能形式，它在囊泡和细胞器运输、胞吞、胞吐以及细胞分裂和生长等许多基本的生理过程中发挥着至关重要的作用。因此，解析微丝的结构对于理解它的功能机制非常重要。近来，伴随着冷冻电子显微镜技术的革新，3.3到4.7 ?分辨率的不同核苷酸状态的兔骨骼肌微丝结构以及3.8 ? 分辨率的Jasplakinolide稳定镰状疟原虫Ⅰ型微丝结构得到解析。除此之外，一些真细菌和古菌类微丝的高分辨率结构在近几年里也得到了解析。虽然植物和动物细胞肌动蛋白的氨基酸序列保守性很高，但是它们之间的生化活性以及在细胞内的功能并不一样。由于植物细胞微丝结构的未知，因此无法从结构水平上去解释这些功能差异。 本研究使用了冷冻电子显微镜技术首次解析出玉米花粉微丝的3.9 ?结构。其是一个右手、平行以及交错的螺旋结构，由链内和链间互作所稳定。虽然整体结构与其它微丝类似，但是DNase I-binding loop (D-loop) 更往外弯曲。此开放型D-loop构象类似于Jasplakinolide或者BeFx稳定的兔骨骼肌微丝。因此，这一结构特征暗示着与兔骨骼肌微丝相比较，玉米花粉微丝可能有一个较高稳定性的结构。 结构模型显示玉米花粉微丝的开放D-loop和链内相邻肌动蛋白亚基的羧基端存在互作，为了验证此构象，本研究对羧基端进行荧光标记，使用全内反射荧光显微镜技术观察了肌动蛋白的聚合，结果显示当玉米花粉肌动蛋白的羧基端被标记上之后，肌动蛋白不能正常聚合成丝，相反，同样位点的标记对兔骨骼肌肌动蛋白聚合没有影响。 为了直接测定微丝的稳定性，本研究在细胞外建立了单分子磁镊技术，其结果显示单根玉米花粉微丝平均被拉断所需要的力比兔骨骼肌微丝大了11.3 pN。因此单分子磁镊分析揭示了玉米花粉微丝比兔骨骼肌微丝要更抗拉力。 总之，目前的结构和生化数据显示与兔骨骼肌微丝相比，玉米花粉微丝有更强的稳定性，这为植物细胞微丝作为长距离囊泡和细胞器运输的轨道而在动物细胞内微管作为长距离运输轨道这一现象提供了一种可能的解释。

Actins are among the most abundant and conserved proteins in plant, mammalian and other eukaryotic cells. Actin exists as two states in vivo, actin monomers (G-actin) and actin filaments (F-actin), which are subject to a dynamic equilibrium of polymerization and depolymerization. In most instances, F-actin is the functional form of actin proteins, which performs vital roles in many fundamental physiological processes ranging from vesicle and organelle transportation, endo- and exocytosis to cell division and growth. Thus, studying the structure of F-actin is of particular importance for understanding its functional mechanism. Recently, the evolution of cryo-electron microscopy (cryo-EM) technology has enabled the determination of filamentous structures of RSMA in different nucleotide states with resolution ranging from 3.3 to 4.7 ? and the structure of jasplakinolide-stabilized malaria parasite Plasmodium falciparum actin 1 (JASP-PfAct1) at 3.8 ? resolution. In addition to those of eukaryotic F-actins, high-resolution cryo-EM structures of bacterial and archaeal actin-like filaments have also been revealed in the last few years. Despite the high protein sequence identity between plant and animal actins, their biochemical activities and cellular functions are different. However, the structural basis accounting for these differences remains poorly understood, largely because none of the plant F-actin structures have been resolved. Here, we report a 3.9 ? structure of Zea mays pollen actin (ZMPA) filaments using electron cryo-microscopy (cryo-EM). The structure shows a right-handed, parallel and staggered architecture that is stabilized by the intra- and interstrand interactions. While the overall structure resembles that of other actin filaments, the DNase I-binding loop (D-loop) bends further outward, adopting an open-D-loop conformation similar to that of the Jasplakinolide- or BeFx- stabilized rabbit skeleton muscle actin (RSMA) filament. Therefore, the structural characteristic implies that ZMPA filaments may have enhanced stability in comparison with RSMA filaments. It shows that there is a connection between open-D-loop and carboxyl terminus of adjoining intrastrand actin subunit. To verify it, we labeled the carboxyl terminus of actin with the fluorescent molecule. Then, the actin polymerization was observed by using total internal reflection fluorescent microscope (TIRFM). The results showed that when the carboxyl terminus of actin was labeled, ZMPA was not able to polymerize normally but RSMA did. To test filament stability directly, the single-molecule magnetic tweezers analysis was set up in our study. Its result showed that the averaged disruption force of the ZMPA filament was 11.3 pN greater than that of the RSMA filament. Therefore, single-molecule magnetic tweezer analysis revealed that the ZMPA filament can resist greater stretching force than the RSMA filament. In summary, the present structural and biochemical data suggest that ZMPA filament is more stable than RSMA filament, which provides a possible explanation for the phenomenon that plant actin filaments serve as tracks for long-distance vesicle and organelle transportations that generate extensive tensions, which is different from the microtubule-centric cargo transportation systems in mammalian cells.