Welnhofer E A, Travis J L
Department of Biological Studies, University of Albany, SUNY, New York 12222, USA.
Cell Motil Cytoskeleton. 1998;41(2):107-16. doi: 10.1002/(SICI)1097-0169(1998)41:2<107::AID-CM2>3.0.CO;2-B.
In Allogromia, tubulin lattices transform between microtubule and helical filament states. Helical filaments are composed of approximately 10-nm-thick tubulin filaments wound into approximately 30-nm diameter coils. The transition pathway between these two lattice states was examined in vitro in detergent-lysed pseudopods. Microtubules represented the majority of the assembled tubulin polymers in the detergent extracted pseudopodia. However, microtubules transformed into helical filaments upon exposure to 10 mM Ca2+ or 50 mM Mg2+. The conversion of microtubules into helical filaments involved fragmentation of the tubulin lattice and reduction in total polymer length. Divalent cations were required for the maintenance of the helical filament state; their removal resulted in the loss of helical filaments and the re-formation of microtubules. The data support a direct transition model in which the tubulin lattice interconverts between the helical filament and microtubule states, independently of tubulin subunit concentration. We propose a structural model for the direct pathway whereby disruption of longitudinal bonds between tandem tubulin dimers in protofilaments causes the microtubule lattice to unwind into helical filaments--continuous ribbons of laterally connected tubulin dimers--from the microtubule end. Helical filaments may rewind into microtubules as longitudinal intersubunit bonds reform.
在异网足虫属中，微管蛋白晶格在微管和螺旋丝状态之间转变。螺旋丝由大约10纳米厚的微管蛋白丝组成，这些丝缠绕成直径约30纳米的螺旋。在体外，于经去污剂裂解的伪足中研究了这两种晶格状态之间的转变途径。在经去污剂提取的伪足中，微管是组装好的微管蛋白聚合物的主要形式。然而，当暴露于10 mM Ca2+ 或50 mM Mg2+ 时，微管会转变为螺旋丝。微管向螺旋丝的转变涉及微管蛋白晶格的碎片化和聚合物总长度的缩短。二价阳离子是维持螺旋丝状态所必需的；去除它们会导致螺旋丝消失并重新形成微管。这些数据支持一种直接转变模型，即微管蛋白晶格在螺旋丝和微管状态之间相互转换，与微管蛋白亚基浓度无关。我们提出了一种直接途径的结构模型，即原丝中串联微管蛋白二聚体之间的纵向键断裂会导致微管晶格从微管末端展开成螺旋丝——横向连接的微管蛋白二聚体的连续带。随着纵向亚基间键的重新形成，螺旋丝可能会重新缠绕成微管。
