Sun H, Zusman D R, Shi W
Molecular Biology Institute and School of Dentistry, University of California, Los Angeles, California 90095, USA.
Curr Biol. 2000 Sep 21;10(18):1143-6. doi: 10.1016/s0960-9822(00)00705-3.
Although flagella are the best-understood means of locomotion in bacteria [1], other bacterial motility mechanisms must exist as many diverse groups of bacteria move without the aid of flagella [2-4]. One unusual structure that may contribute to motility is the type IV pilus [5,6]. Genetic evidence indicates that type IV pili are required for social gliding motility (S-motility) in Myxococcus, and twitching motility in Pseudomonas and Neisseria [6,7]. It is thought that type IV pili may retract or rotate to bring about cellular motility [6,8], but there is no direct evidence for the role of pili in cell movements. Here, using a tethering assay, we obtained evidence that the type IV pilus of Myxococcus xanthus functions as a motility apparatus. Pili were required for M. xanthus cells to adhere to solid surfaces and to generate cellular movement using S-motility. Tethered cells were released from the surface at intervals corresponding to the reversal frequency of wild-type cells when gliding on a solid surface. Mutants defective in the control of directional movements and cellular reversals (frz mutants) showed altered patterns of adherence that correlate reversal frequencies with tethering. The behavior of the tethered cells was consistent with a model in which the pili are extruded from one cell pole, adhere to a surface, and then retract, pulling the cell in the direction of the adhering pili. Cellular reversals would result from the sites of pili extrusion switching from one cell pole to another and are controlled by the frz chemosensory system.
尽管鞭毛是细菌中最被了解的运动方式[1]，但由于许多不同种类的细菌在没有鞭毛帮助的情况下也能移动[2 - 4]，所以必然存在其他细菌运动机制。一种可能有助于运动的不寻常结构是IV型菌毛[5,6]。遗传学证据表明，IV型菌毛是黏球菌社会滑行运动（S - 运动）、假单胞菌和奈瑟氏菌的颤动运动所必需的[6,7]。人们认为IV型菌毛可能会收缩或旋转以实现细胞运动[6,8]，但菌毛在细胞运动中的作用尚无直接证据。在这里，我们通过系留试验获得证据，表明黄色黏球菌的IV型菌毛起到运动装置的作用。菌毛是黄色黏球菌细胞附着于固体表面并利用S - 运动产生细胞运动所必需的。当在固体表面滑行时，系留细胞以与野生型细胞反转频率相对应的间隔从表面释放。在定向运动和细胞反转控制方面有缺陷的突变体（frz突变体）表现出改变的附着模式，其反转频率与系留相关。系留细胞的行为与一个模型一致，在该模型中，菌毛从一个细胞极伸出，附着于表面，然后缩回，将细胞拉向附着菌毛的方向。细胞反转将由菌毛伸出位点从一个细胞极切换到另一个细胞极引起，并由frz化学感应系统控制。
