Kulp D T, Herzfeld J
Department of Chemistry Brandeis University Waltham, MA 02254-9110, USA.
Biophys Chem. 1995 Dec;57(1):93-102. doi: 10.1016/0301-4622(95)00050-8.
The typical cell contains ca. 25 vol.-% protein, of which ca. 10% forms cytoskeletal filaments and ca. 90% is non-aggregating globular protein. It has previously been theoretically predicted that, under such highly crowded conditions, rigid filaments will coalesce into tight bundles coexisting with an isotropic solution of globular proteins. In the present work we show that such spontaneous bundling will occur even when filament flexibility is taken into account because the persistence length of the filaments is much longer than the diameter of the globular proteins. The theoretical results are consistent with experimentally observed bundling of F-actin (the most flexible of the three most common types of cytoskeletal filaments) in the presence of globular macromolecules.The main effect of increased filament flexibility on bundling is to cause somewhat looser packing. In mixtures of filaments, differences in flexibilities can lead to segregation. This segregation is accentuated when the stiffer filament is also wider. The results suggest that actin filaments and microtubules will spontaneously form segregated bundles in the presence of cellular concentrations of globular proteins. While cross-linking proteins may serve to stabilize these bundles, their more important function in bundling may be to fine tune the structure (e.g., polarity and registration of filaments).
典型的细胞含有约25%（体积分数）的蛋白质，其中约10%形成细胞骨架细丝，约90%是不聚集的球状蛋白质。此前从理论上预测，在如此高度拥挤的条件下，刚性细丝会聚集成紧密的束状结构，并与球状蛋白质的各向同性溶液共存。在本研究中我们表明，即使考虑细丝的柔韧性，这种自发成束现象也会发生，因为细丝的持久长度远大于球状蛋白质的直径。理论结果与在球状大分子存在下实验观察到的F - 肌动蛋白（三种最常见的细胞骨架细丝类型中最柔韧的一种）成束现象一致。细丝柔韧性增加对成束的主要影响是导致堆积略显松散。在细丝混合物中，柔韧性的差异会导致分离。当较硬的细丝也更粗时，这种分离会加剧。结果表明，在细胞内球状蛋白质浓度下，肌动蛋白细丝和微管会自发形成分离的束状结构。虽然交联蛋白可能有助于稳定这些束状结构，但它们在成束过程中更重要的功能可能是微调结构（例如细丝的极性和排列）。
