Janmey P A, Hvidt S, Lamb J, Stossel T P
Massachusetts General Hospital, Department of Medicine, Harvard Medical School, Boston 02114.
Nature. 1990 May 3;345(6270):89-92. doi: 10.1038/345089a0.
The maintainance of the shape of cells is often due to their surface elasticity, which arises mainly from an actin-rich cytoplasmic cortex. On locomotion, phagocytosis or fission, however, these cells become partially fluid-like. The finding of proteins that can bind to actin and control the assembly of, or crosslink, actin filaments, and of intracellular messages that regulate the activities of some of these actin-binding proteins, indicates that such 'gel-sol' transformations result from the rearrangement of cortical actin-rich networks. Alternatively, on the basis of a study of the mechanical properties of mixtures of actin filaments and an Acanthamoeba actin-binding protein, alpha-actinin, it has been proposed that these transformations can be accounted for by rapid exchange of crosslinks between actin filaments: the cortical network would be solid when the deformation rate is greater than the rate of crosslink exchange, but would deform or 'creep' when deformation is slow enough to permit crosslinker molecules to rearrange. Here we report, however, that mixtures of actin filaments and actin-binding protein (ABP), an actin crosslinking protein of many higher eukaryotes, form gels rheologically equivalent to covalently crosslinked networks. These gels do not creep in response to applied stress on a time scale compatible with most cell-surface movements. These findings support a more complex and controlled mechanism underlying the dynamic mechanical properties of cortical cytoplasm, and can explain why cells do not collapse under the constant shear forces that often exist in tissues.
细胞形状的维持通常归因于其表面弹性，这种弹性主要源于富含肌动蛋白的细胞质皮层。然而，在移动、吞噬作用或裂变过程中，这些细胞会部分呈现出类似液体的状态。发现能够与肌动蛋白结合并控制肌动蛋白丝组装或交联的蛋白质，以及调节其中一些肌动蛋白结合蛋白活性的细胞内信息，表明这种“凝胶-溶胶”转变是由富含肌动蛋白的皮层网络重排导致的。另外，基于对肌动蛋白丝与一种棘阿米巴肌动蛋白结合蛋白α-辅肌动蛋白混合物力学性质的研究，有人提出这些转变可以通过肌动蛋白丝之间交联的快速交换来解释：当变形速率大于交联交换速率时，皮层网络是固态的，但当变形足够缓慢以至于允许交联分子重新排列时，网络会变形或“蠕变”。然而，我们在此报告，肌动蛋白丝与肌动蛋白结合蛋白（ABP，许多高等真核生物的一种肌动蛋白交联蛋白）的混合物形成的凝胶在流变学上等同于共价交联网络。在与大多数细胞表面运动相适应的时间尺度上，这些凝胶不会因施加的应力而蠕变。这些发现支持一种更复杂且受控制的机制，该机制是皮层细胞质动态力学性质的基础，并且可以解释为什么细胞在组织中经常存在的恒定剪切力作用下不会塌陷。
