Co-polymers of Actin and Tropomyosin Account for a Major Fraction of the Human Actin Cytoskeleton
Co-polymers of Actin and Tropomyosin Account for a Major Fraction of the Human Actin Cytoskeleton
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DOI：
10.1016/j.cub.2018.05.053
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发表时间：
2018-07-23
期刊：
影响因子：
9.2
通讯作者：
Gunning, Peter W.
中科院分区：
文献类型：
--
作者：
Meiring, Joyce C. M.;Bryce, Nicole S.;Gunning, Peter W.
原肌球蛋白形成稳定的卷曲螺旋二聚体，沿着肌动蛋白丝的a-螺旋沟聚合[1]。肌动蛋白细胞骨架由肌动蛋白和原肌球蛋白的共聚物和无原肌球蛋白的肌动蛋白的聚合物组成[2]。这两种类型的丝之间的根本区别是原肌球蛋白以亚型依赖性方式决定肌动蛋白丝的功能能力[3-9]。然而，肌动蛋白丝的哪一部分与原肌球蛋白相关尚不清楚。为了解决这一缺陷，我们通过定量各种人类细胞类型（包括骨（U2 OS）、乳腺上皮（MCF-10A）、转化的乳腺上皮（MCF-7）以及原代（BJpar）、永生化（BJeH）和Ras转化（BJeLR）BJ成纤维细胞）中的原肌球蛋白和肌动蛋白水平，测量了这两种细丝群体之间的相对分布[10]。我们的原肌球蛋白和肌动蛋白的测量预测饱和的肌动蛋白细胞骨架，这意味着原肌球蛋白结合必须被抑制，以产生原肌球蛋白自由肌动蛋白丝。我们发现大多数的肌动蛋白丝与原肌球蛋白在四个测试的6个细胞系和部分肌动蛋白丝与原肌球蛋白减少转化。我们还发现，高分子量（HMW），不像低分子量（LMW），原肌球蛋白异构体主要与肌动蛋白在未转化的细胞共聚。原肌球蛋白的这种差异分配不是由于缺乏N-末端乙酰化的LMW原肌球蛋白，但它是，在一定程度上，解释了可溶性高分子量原肌球蛋白的蛋白酶体降解的敏感性。我们得出结论，肌动蛋白原肌球蛋白共聚物构成了人类肌动蛋白细胞骨架的主要部分。
Tropomyosin proteins form stable coiled-coil dimers that polymerize along the a-helical groove of actin filaments [1]. The actin cytoskeleton consists of both co-polymers of actin and tropomyosin and polymers of tropomyosin-free actin [2]. The fundamental distinction between these two types of filaments is that tropomyosin determines the functional capability of actin filaments in an isoform-dependent manner [3-9]. However, it is unknown what portion of actin filaments are associated with tropomyosin. To address this deficit, we have measured the relative distribution between these two filament populations by quantifying tropomyosin and actin levels in a variety of human cell types, including bone (U2OS); breast epithelial (MCF-10A); transformed breast epithelial (MCF-7); and primary (BJpar), immortalized (BJeH), and Ras-transformed (BJeLR) BJ fibroblasts [10]. Our measurements of tropomyosin and actin predict the saturation of the actin cytoskeleton, implying that tropomyosin binding must be inhibited in order to generate tropomyosin-free actin filaments. We find the majority of actin filaments to be associated with tropomyosin in four of the six cell lines tested and the portion of actin filaments associated with tropomyosin to decrease with transformation. We also discover that high-molecular-weight (HMW), unlike low-molecular-weight (LMW), tropomyosin isoforms are primarily co-polymerized with actin in untransformed cells. This differential partitioning of tropomyosins is not due to a lack of N-terminal acetylation of LMW tropomyosins, but it is, in part, explained by the susceptibility of soluble HMW tropomyosins to proteasomal degradation. We conclude that actin-tropomyosin co-polymers make up a major fraction of the human actin cytoskeleton.