Kishi Y, Clements C, Mahadeo D C, Cotter D A, Sameshima M
Department of Physics, Rikkyo (St Paul's) University, Toshima-ku, Tokyo, Japan.
J Cell Sci. 1998 Oct;111 ( Pt 19):2923-32. doi: 10.1242/jcs.111.19.2923.
Upon removal of nutrients, the amoebae of the cellular slime mold Dictyostelium discoideum differentiate into dormant spores which survive starvation stress. In this study, we demonstrate that half of the actin molecules in the spores are tyrosine-phosphorylated. The phosphorylated actin is distributed around immobile crenate mitochondria and vesicles, as well as in the cytoplasm of the spores. The actin isolated from spore lysates contains phosphorylated and unphosphorylated forms at the same molar ratio as that of the original whole spore lysate. Under actin polymerizing conditions they form actin filaments and then they are completely depolymerized under actin depolymerizing conditions, indicating that tyrosine phosphorylation of actin may not prohibit actin polymerization nor stimulate depolymerization. The phosphorylation levels increase at the end of the culmination stage when spores have matured morphologically and physiologically, and reach maximum levels after an additional 12 hours of development. The levels are stable for 20 days following spore maturation, and decline to undetectable levels within the next 10 days. Spores having high levels of phosphorylation show high viability, and vice versa. Following activation of spores with nutrient medium containing spore germination promoters, the phosphorylation levels quickly decrease with a half-life of about 5 minutes. After 20 minutes spores begin to swell. At this later time, most of the phosphorylated actin already has been dephosphorylated. Also, in heat-activated spores actin dephosphorylation occurs prior to spore swelling. However, addition of phosphatase inhibitors following heat-activation, prevented spore swelling and dephosphorylation of actin. Our data indicate that the high levels of actin tyrosine phosphorylation, specific to the spore stage, may be required for maintaining dormancy to withstand starvation stress. The rapid dephosphorylation of actin leads to a reactivated dynamic actin system which participates in spore swelling, vesicle movement, and mitochondrial shape changes during the spore germination process.
去除营养物质后，细胞黏菌盘基网柄菌的变形虫会分化成休眠孢子，这些孢子能够在饥饿胁迫下存活。在本研究中，我们证明孢子中一半的肌动蛋白分子发生了酪氨酸磷酸化。磷酸化的肌动蛋白分布在静止的锯齿状线粒体和囊泡周围，以及孢子的细胞质中。从孢子裂解物中分离出的肌动蛋白含有磷酸化和未磷酸化形式，其摩尔比与原始全孢子裂解物相同。在肌动蛋白聚合条件下，它们形成肌动蛋白丝，然后在肌动蛋白解聚条件下完全解聚，这表明肌动蛋白的酪氨酸磷酸化可能既不阻止肌动蛋白聚合，也不刺激解聚。在成熟阶段末期，当孢子在形态和生理上成熟时，磷酸化水平会升高，并在额外发育12小时后达到最高水平。孢子成熟后，这些水平在20天内保持稳定，并在接下来的10天内降至无法检测的水平。磷酸化水平高的孢子具有高活力，反之亦然。用含有孢子萌发促进剂的营养培养基激活孢子后，磷酸化水平迅速下降，半衰期约为5分钟。20分钟后，孢子开始膨胀。在这个较晚的时间，大多数磷酸化的肌动蛋白已经去磷酸化。此外，在热激活的孢子中，肌动蛋白去磷酸化发生在孢子膨胀之前。然而，热激活后添加磷酸酶抑制剂可阻止孢子膨胀和肌动蛋白去磷酸化。我们的数据表明，孢子阶段特有的高水平肌动蛋白酪氨酸磷酸化可能是维持休眠以承受饥饿胁迫所必需的。肌动蛋白的快速去磷酸化导致重新激活的动态肌动蛋白系统，该系统参与孢子萌发过程中的孢子膨胀、囊泡运动和线粒体形态变化。
