Stege J T, Shaulsky G, Loomis W F
Department of Biology, University of California San Diego, La Jolla 92093, USA.
Dev Biol. 1997 May 1;185(1):34-41. doi: 10.1006/dbio.1997.8538.
About 8 hr after the initiation of development in Dictyostelium discoideum, a few randomly scattered cells express prestalk specific genes and subsequently sort out to the top of the aggregate where they form a tip. The tip elongates and forms the anterior of the migrating slug before differentiating into a stalk which supports the ball of spores in a mature fruiting body. Using REMI mutagenesis we isolated a mutant strain, AK244, in which the initial aggregate subdivides to give a highly papillated surface. This mutant fails to form slugs and appears to have a defect in sorting of prestalk cells. The disrupted gene, tipA, encodes a novel 83-kDa protein and is preferentially expressed in PST-O cells after the cell types have sorted out. Mutant strains that lack TipA express the prestalk-specific gene ecmA at reduced levels and form very few spores. These defects cannot be overcome by developing the mutant cells in the presence of wild-type cells. Thus, TipA acts in a cell-autonomous manner at an early stage in development. Using strains carrying reporter constructs, we found that mutant cells expressing a prestalk marker remain dispersed in the aggregates. Prespore cells appear to sort such that the base is free of cells expressing cell-type-specific markers. Even after 20 hr of development, when wild-type cells are undergoing terminal differentiation, prestalk cells in tipA mutants form very small clumps, most of which fail to sort to the periphery or the tops of aggregates. The tipA gene appears to play an essential role in the sorting of the initial cell types.
在盘基网柄菌发育开始约8小时后，少数随机散布的细胞表达前柄特异性基因，随后分选到聚集体顶部形成一个尖端。该尖端伸长并在迁移的蛞蝓体前端形成，然后分化成支撑成熟子实体中孢子球的柄。利用REMI诱变，我们分离出一个突变株AK244，其初始聚集体会细分形成高度乳头状的表面。该突变体无法形成蛞蝓体，似乎在前柄细胞分选方面存在缺陷。被破坏的基因tipA编码一种新的83 kDa蛋白，在细胞类型分选后优先在PST-O细胞中表达。缺乏TipA的突变株以较低水平表达前柄特异性基因ecmA，且形成的孢子很少。在野生型细胞存在的情况下培养突变细胞并不能克服这些缺陷。因此，TipA在发育早期以细胞自主方式发挥作用。利用携带报告构建体的菌株，我们发现表达前柄标记的突变细胞在聚集体中仍分散存在。前孢子细胞似乎进行了分选，使得基部没有表达细胞类型特异性标记的细胞。即使在发育20小时后，当野生型细胞正在进行终末分化时，tipA突变体中的前柄细胞形成非常小的团块，其中大多数无法分选到聚集体的周边或顶部。tipA基因似乎在初始细胞类型的分选中起着至关重要的作用。
