Kusakabe T
Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan.
Zoolog Sci. 1997 Oct;14(5):707-18. doi: 10.2108/zsj.14.707.
Actin is a ubiquitous protein in eukaryotic cells and plays an important role in cell structure, cell motility, and the generation of contractile force in both muscle and nonmuscle cells. Multiple genes encoding muscle or nonmuscle actins have been isolated from several species of ascidians and their expression patterns have been investigated. Sequence and expression analyses of muscle actin genes have shown that ascidians have at least two distinct isoforms of muscle actin, the larval muscle and body-wall isoforms. In the ascidian Halocynthia roretzi, two clusters of actin genes are expressed in the larval muscle cells. The HrMA2/4 cluster contains at least five actin genes and the HrMA1 cluster contains a pair of actin genes whose expression is regulated by a single bidirectional promoter. cis-Regulatory elements essential for muscle-specific expression of a larval muscle actin gene HrMA4a have been identified. The adult body-wall muscle actin is clearly distinguished from the larval muscle actin by diagnostic amino acids. The adult muscle actin genes may be useful tools to investigate the mechanisms of muscle development in ascidian adults. The evolution of chordate actin genes has been inferred by comparing the organization and sequences of actin genes and performing molecular phylogenetic analysis. The results suggest a close relationship between ascidian and vertebrate actins. The chordate ancestor seems to have evolved the "chordate-type" cytoplasmic and muscle actins before its divergence into vertebrates and urochordates. The phylogenetic analysis also suggests that the vertebrate muscle actin isoforms evolved after the separation of the vertebrates and urochordates. Muscle actin genes have been used to investigate the mechanism of muscle cell regression during the evolution of anural development. The results suggest that the regression of muscle cell differentiation is mediated by changes in the structure of muscle actin genes rather than in the trans-acting regulatory factors required for their expression. Actin genes have provided a unique system to study developmental and evolutionary mechanisms in chordates.
肌动蛋白是真核细胞中一种普遍存在的蛋白质，在细胞结构、细胞运动以及肌肉和非肌肉细胞收缩力的产生中发挥着重要作用。已从几种海鞘物种中分离出多个编码肌肉或非肌肉肌动蛋白的基因，并对其表达模式进行了研究。肌肉肌动蛋白基因的序列和表达分析表明，海鞘至少有两种不同的肌肉肌动蛋白亚型，即幼虫肌肉亚型和体壁亚型。在海鞘柄海鞘中，两簇肌动蛋白基因在幼虫肌肉细胞中表达。HrMA2/4簇至少包含五个肌动蛋白基因，HrMA1簇包含一对肌动蛋白基因，其表达受单个双向启动子调控。已鉴定出幼虫肌肉肌动蛋白基因HrMA4a肌肉特异性表达所必需的顺式调控元件。成体体壁肌肉肌动蛋白通过诊断性氨基酸与幼虫肌肉肌动蛋白明显区分。成体肌肉肌动蛋白基因可能是研究海鞘成体肌肉发育机制的有用工具。通过比较肌动蛋白基因的组织和序列并进行分子系统发育分析，推断了脊索动物肌动蛋白基因的进化。结果表明海鞘和脊椎动物肌动蛋白之间存在密切关系。脊索动物祖先似乎在分化为脊椎动物和尾索动物之前就进化出了“脊索动物型”细胞质和肌肉肌动蛋白。系统发育分析还表明，脊椎动物肌肉肌动蛋白亚型是在脊椎动物和尾索动物分离后进化而来的。肌肉肌动蛋白基因已被用于研究无耳发育进化过程中肌肉细胞退化的机制。结果表明，肌肉细胞分化的退化是由肌肉肌动蛋白基因结构的变化介导的，而不是由其表达所需的反式作用调控因子的变化介导的。肌动蛋白基因提供了一个独特的系统来研究脊索动物的发育和进化机制。
