Kaneko T, Itoh T J, Hotani H
Department of Molecular Biology, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602, Japan.
J Mol Biol. 1998 Dec 18;284(5):1671-81. doi: 10.1006/jmbi.1998.2251.
To examine the role of cytoskeletons in cellular morphogenesis, we generated liposomes encapsulating tubulin, with or without microtubule-associated proteins (MAPs), and observed their transformation using dark-field microscopy. When tubulin was polymerized with MAPs in liposomes, liposomes were transformed into a "bipolar" shape with a central sphere and two tubular membrane protrusions that aligned in a straight line. On the other hand, when pure tubulin was polymerized in liposomes without MAPs, they initially transformed into a bipolar shape but subsequently re-transformed into a "monopolar" shape, i.e. a sphere with only one straight tubular portion. This re-transformation occurred in two ways: first, by shortening of one of the tubular portions due to microtubule disassembly; or second, by fluctuation of the central sphere toward one of the ends without shortening of the tube portion. MAPs prevented this re-transformation, and their role in stabilizing the shape of transformed liposomes was studied by the co-sedimentation method. The results show that MAPs, particularly MAP1 and MAP2, mediate binding between microtubules and the liposomal membrane. However, MAP2 by itself did not bind to liposomes, but was able to stabilize bipolar liposomes. This stabilization is caused not only by direct links between microtubules and liposomes, but also by prevention of Brownian motion of microtubules through an increase in friction.
为了研究细胞骨架在细胞形态发生中的作用，我们制备了包裹微管蛋白的脂质体，其中有的含有微管相关蛋白（MAPs），有的不含，并用暗视野显微镜观察它们的转变。当微管蛋白在脂质体中与MAPs聚合时，脂质体转变为“双极”形状，有一个中心球体和两个呈直线排列的管状膜突起。另一方面，当纯微管蛋白在不含MAPs的脂质体中聚合时，它们最初转变为双极形状，但随后又重新转变为“单极”形状，即只有一个直管状部分的球体。这种重新转变以两种方式发生：第一，由于微管解聚导致其中一个管状部分缩短；第二，中心球体向一端波动而管部分不缩短。MAPs阻止了这种重新转变，并通过共沉降法研究了它们在稳定转变后脂质体形状中的作用。结果表明，MAPs，特别是MAP1和MAP2，介导微管与脂质体膜之间的结合。然而，MAP2本身不与脂质体结合，但能够稳定双极脂质体。这种稳定不仅是由微管与脂质体之间的直接连接引起的，还通过增加摩擦力来防止微管的布朗运动。
