Hart P E, Wolniak S M
Department of Cell Biology and Molecular Genetics, University of Maryland, College Park 20742, USA.
Cell Motil Cytoskeleton. 1998;41(1):39-48. doi: 10.1002/(SICI)1097-0169(1998)41:1<39::AID-CM3>3.0.CO;2-A.
The motile male gamete of the water fern Marsilea vestita is a spirally shaped cell that possesses a complex cytoskeletal array of microtubules and approximately 140 cilia. Spermiogenesis in this organism is a rapid process that requires only approximately 11 h at 20 degrees C and involves the de novo synthesis of basal bodies from an organelle known as a blepharoplast. The developmental program that gives rise to the spermatozoids begins with nine mitotic divisions that occur in rapid succession during the first 5.5 h after imbibition of the dry microspore. During the next 5.5 h, the spermatids undergo a complicated differentiation process. We have asked what new proteins must be made for differentiation to proceed. Inhibitor treatments reveal that some translation is a necessary prerequisite for the differentiation and release of spermatozoids, but methionine-labeling studies demonstrate that relatively few types of proteins must be translated for this developmental program to reach completion. We have found that the dry microspores contain alpha-, beta-, and gamma-tubulin, at levels that may be sufficient for the entire developmental process. The abundance of the tubulins remains essentially constant until very late stages of spermiogenesis. In contrast to the tubulins, we show that centrin begins to increase in abundance at approximately 4 h after imbibition and that it reaches a peak at 6 h after imbibition. We also show that centrin mRNA is stored in the dry microspore, and that centrin protein abundance is regulated at the translational level. We believe that the translation of stored centrin transcripts serves as a rate-limiting step in the rapid differentiation process of spermiogenesis M. vestita. We suggest that centrin functions in the microtubule organizing centers that are required for the construction of the cytoskeleton and the motile apparatus in these structurally complicated cells.
水蕨（Marsilea vestita）的可游动雄配子是一种螺旋形细胞，具有由微管和大约140根纤毛组成的复杂细胞骨架阵列。该生物体中的精子发生是一个快速过程，在20摄氏度下仅需约11小时，并且涉及从一种称为生毛体的细胞器从头合成基体。产生精子的发育程序始于九个有丝分裂，这些分裂在干燥小孢子吸水后的前5.5小时内迅速连续发生。在接下来的5.5小时内，精细胞经历复杂的分化过程。我们探究了分化过程中必须合成哪些新蛋白质。抑制剂处理表明，一些翻译是精子分化和释放的必要前提，但甲硫氨酸标记研究表明，对于这个发育程序的完成，只需翻译相对较少类型的蛋白质。我们发现干燥小孢子中含有α-、β-和γ-微管蛋白，其水平可能足以支持整个发育过程。微管蛋白的丰度在精子发生的很晚阶段之前基本保持恒定。与微管蛋白不同，我们发现中心蛋白在吸水后约4小时开始丰度增加，并在吸水后6小时达到峰值。我们还表明中心蛋白mRNA储存在干燥小孢子中，并且中心蛋白的丰度在翻译水平上受到调节。我们认为储存的中心蛋白转录本的翻译是水蕨精子发生快速分化过程中的一个限速步骤。我们推测中心蛋白在这些结构复杂的细胞中构建细胞骨架和运动装置所需的微管组织中心中发挥作用。
