Copolymerization of two distinct tubulin isotypes during microtubule assembly in vitro.
Copolymerization of two distinct tubulin isotypes during microtubule assembly in vitro.
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DOI：
10.1083/jcb.110.1.97
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发表时间：
1990-01
期刊：
影响因子：
--
通讯作者：
Murphy DB
中科院分区：
文献类型：
--
作者：
Baker HN;Rothwell SW;Grasser WA;Wallis KT;Murphy DB
细胞含有多种微管蛋白同种型，它们是不同基因和翻译后修饰的产物。有人提出，微管蛋白同种型被分为不同类别的微管，每种微管适应特定的活动和功能。为了确定微管蛋白同种型的混合物在体外是否分离成不同类别的聚合物，我们使用免疫电子显微镜检查了由鸡脑和红细胞纯化的微管蛋白亚基混合物组装而成的微管共聚物的组成，每种微管蛋白亚基均已被证明在体外表现出不同的组装特性。我们观察到（a）尽管每种同型表现出其独特的生化和组装特性，但两种同型快速有效地共组装； (b) 在低单体浓度下，微管蛋白同种型的比例沿着延伸的共聚物的长度变化，导致免疫金标记的梯度； (c) 两种不同类别的共聚物，各自含有不同比例的同种型，同时组装在同一亚基混合物中； (d)不同同种型的亚基和聚合物彼此之间的缔合几乎同样好，仅存在轻微的偏向于相同同种型的亚基和聚合物之间的相互作用。这些观察结果与之前关于细胞内多种同种型均匀分布的研究一致，并表明如果体内确实发生同种型分离，则很可能是由细胞特异性微管相关蛋白（MAP）或特殊的细胞内条件引导的。
Cells contain multiple tubulin isotypes that are the products of different genes and posttranslational modifications. It has been proposed that tubulin isotypes become segregated into different classes of microtubules each adapted to specific activities and functions. To determine if mixtures of tubulin isotypes segregate into different classes of polymers in vitro, we used immunoelectron microscopy to examine the composition of microtubule copolymers that assembled from mixtures of purified tubulin subunits from chicken brain and erythrocytes, each of which has been shown to exhibit distinct assembly properties in vitro. We observed that (a) the two isotypes coassemble rapidly and efficiently despite the fact that each isotype exhibits its own unique biochemical and assembly properties; (b) at low monomer concentrations the ratio of tubulin isotypes changes along the lengths of elongating copolymers resulting in gradients in immuno-gold labeling; (c) two distinct classes of copolymers each containing a distinct ratio of isotypes assemble simultaneously in the same subunit mixture; and (d) subunits and polymers of different isotypes associate nearly equally well with each other, there being only a slight bias favoring interactions among subunits and polymers of the same isotype. The observations agree with previous studies on the homogeneous distribution of multiple isotypes within cells and suggest that if segregation of isotypes does occur in vivo, it is most likely directed by cell-specific microtubule-associated proteins (MAPs) or specialized intracellular conditions.