Peterson M D, Novak K D, Reedy M C, Ruman J I, Titus M A
Department of Cell Biology, Duke University Medical Center, Durham, NC 27710, USA.
J Cell Sci. 1995 Mar;108 ( Pt 3):1093-103. doi: 10.1242/jcs.108.3.1093.
The protozoan myosin Is are widely expressed actin-based motors, yet their in vivo roles remain poorly understood. Molecular genetic studies have been carried out to determine their in vivo function in the simple eukaryote Dictyostelium, an organism that contains a family of four myosin Is. Here we report the characterization of myoC, a gene that encodes a fifth member of this family. Analysis of the deduced amino acid sequence reveals that the myoC gene encodes a myosin that is homologous to the well-described Acanthamoeba myosin Is as well as to Dictyostelium myoB and -D. The expression pattern of the myoC mRNA is similar to that of myoB and myoD, with a peak of expression at times of maximal cell migration, around 6 hours development. Deletion of the myoB gene has been previously shown to result in mutant cells that are defective in pseudopod extension and phagocytosis. However, no obvious differences in cell growth, development, phagocytosis or motility were detected in cells in which the myoC gene had been disrupted by homologous recombination. F-actin localization and ultrastructural organization also appeared unperturbed in myoC- cells. This apparent 'lack' of phenotype in a myosin I single knockout cannot be simply explained by redundancy of function. Our results rather suggest that the present means of assessing myosin I function in vivo are insufficient to identify the unique roles of these actin-based motors.
原生动物肌球蛋白I广泛表达，是基于肌动蛋白的分子马达，但它们在体内的作用仍知之甚少。为了确定它们在简单真核生物盘基网柄菌中的体内功能，已经开展了分子遗传学研究，盘基网柄菌含有一个由四个肌球蛋白I组成的家族。在此，我们报告了myoC基因的特征，该基因编码这个家族的第五个成员。对推导的氨基酸序列分析表明，myoC基因编码的肌球蛋白与描述详尽的棘阿米巴肌球蛋白I以及盘基网柄菌的肌球蛋白B和D同源。myoC mRNA的表达模式与肌球蛋白B和D相似，在细胞迁移达到最大值时表达达到峰值，大约在发育6小时时。先前已表明，肌球蛋白B基因的缺失会导致突变细胞在伪足伸展和吞噬作用方面存在缺陷。然而，在通过同源重组破坏了myoC基因的细胞中，未检测到细胞生长、发育、吞噬作用或运动性方面的明显差异。在缺失myoC的细胞中，F-肌动蛋白的定位和超微结构组织也似乎未受干扰。肌球蛋白I单基因敲除中这种明显的“无”表型不能简单地用功能冗余来解释。我们的结果反而表明，目前在体内评估肌球蛋白I功能的方法不足以确定这些基于肌动蛋白的分子马达的独特作用。
