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微丝 .
小家鼠 胚胎 成纤维细胞 的肌动蛋白细胞骨架,荧光素异硫氰酸酯-鬼笔环肽 染色微丝 (microfilament)是由肌动蛋白 (Actin)组成的直径 约为7nm 的纤维结构。肌动蛋白单体(全称为“球状肌动蛋白”,简称“G肌动蛋白”)表面上有一个ATP 结合位点。肌动蛋白单体可一个接一个连成一串肌动蛋白链,而微丝则由两串这样的肌动蛋白链互相缠绕扭曲成而成。微丝这种肌动蛋白多聚体又被称为“纤维形肌动蛋白”。
微絲也普遍存在于所有真核細胞中,是一個實心狀的纖維,一般細胞中含量約占細胞內總蛋白質的1%-2%,但在活動較強的細胞中可占20%-30%。微絲的主要化學成分是肌動蛋白(actin)和肌球蛋白 (myosin),如同微管蛋白,肌動蛋白的基因組成一個超家族并有多種結構極為相似的組成。在肌細胞中至少存在4種不同的肌動蛋白。
微丝是由双股单链扭曲而成的。
组装与去组装
微丝能被组装和去组装。当单体上结合的是ATP时,就会有较高的相互亲和力,单体趋向于聚合成多聚体,此过程即为微丝的组装。而当ATP水解成ADP后,单体亲和力就会下降,多聚体趋向解聚,则为微丝的去组装。高ATP浓度 有利于微丝的组装。所以当将细胞质 放入富含ATP的溶液时,细胞质会因为微丝的大量组装迅速凝固成胶。而微丝的两端组装速度并不一样。较快的一端(+极)比较慢的一端(-极)快5-10倍。当ATP浓度达一定临界值时,可以观察到+极组装而-极同时去组装的现象。
微丝的组装和去组装受到细胞质基质 内多种蛋白质 的调节,这些蛋白能结合到微丝上,影响其组装去组装速度,被称之为微丝结合蛋白(association protein)。
调节
微丝的组装先需要“核化”(nucleation),即几个单体首先聚合,其它单体再与之结合成更大的多聚体。Arp复合体 (Actin related-protein)是一种能与肌动蛋白结合的蛋白,它起到模板的作用,促进肌动蛋白的多聚化。Arp复合体由Arp2、Arp3和其它5种蛋白构成。封闭蛋白(end-blocking protein)则是微丝两端的“帽子”。当这种蛋白结合到微丝上时,微丝的组装和去组装就会停止。这对一些长度固定的蛋白来说很重要,如细肌丝。而前纤维蛋白(Profilin,或译G肌动蛋白结合蛋白 )则是促进多聚的,相应地促解聚的蛋白则有丝切蛋白(Cofilin)。纤丝切割蛋白(filament severing protein),如溶胶蛋白(Gelsolin),能将微丝从中间切断。粘着斑蛋白(Vinculin)则能固定微丝到细胞膜上,形成粘着斑。交联蛋白(cross-linking protein)有两个以上肌动蛋白结合位点,起到连接微丝的作用,其中,丝束蛋白(fimbrin)帮助微丝结成束状,而细丝蛋白(filamin)则将微丝交联成网状。
參考書目
Zhang, Dong (EDT)Advances in Filament Yarn Spinning of Textiles and Polymers (2014) Elsevier Science Ltd public
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