Molecular mechanisms, for cell cycle restart from a quiescent phase in fission yeast

1) 孢子萌发所需的信号转导：裂殖酵母裂殖酵母中子囊孢子的萌发是由葡萄糖诱导的。通过添加葡萄糖，细胞内 cAMP 水平短暂增加。此外，我们发现分别编码腺苷酸环化酶和A-激酶的cyrl和pkal基因的突变阻碍了孢子萌发的启动。打破孢子休眠需要激活cAMP/PKA信号通路。2)孢子生长过程中的细胞周期进程：研究了孢子萌发后的细胞周期。萌发的孢子经过漫长（〜10小时）的G1期后进入S期。在 DNA 复制开始前不久，Cd2 激酶活性增加。巧合的是，Cig2细胞周期蛋白积累，Ruml Cdk抑制剂消失。相反，Cdc13 细胞周期蛋白对于 S 期不是必需的。在 G1 阶段，发生扩张生长，随后发生极性生长。我们推测，细胞需要较长的 G1 期才能达到 S 期开始所需的临界细胞大小。3）主要孢子壁蛋白 p23 的鉴定：萌发被认为伴随着孢子包膜的化学和形态变化。我们发现p23（一种主要的孢子壁蛋白）在萌发过程中逐渐减少。纯化P23并确定其部分氨基酸序列。该序列与 isp3/meu4 基因产物的氨基末端相同。 isp3 基因在减数分裂期间转录，其破坏导致孢子形成能力降低。Isp3-GFP 融合蛋白定位于孢子包膜。有趣的是，P23/Isp3 与花粉过敏原蛋白共享一个富含丙氨酸的区域。

1) Signal transduction required for spore germination.: Germination of ascospores in the fission yeast Schizosaccharomyces pombe are induced by glucose. The intracellular cAMP level increased transiently by the addition of glucose. Furthermore, we found that mutations in the cyrl and pkal genes, encoding adenylyl cyclase and A-kinase, respectively, blocked the initiation of spore germination. Breaking of dormancy of spores requires activation of the cAMP/PKA signal pathway.2) Cell cycle progression during spore outgrowth : A cell cycle following spore germination was studied. Germinated spores enter the S phase after a long (〜10 hours) G1 phase. Cd2 kinase activity increased shortly before the onset of DNA replication. Coincidently, Cig2 cyclin was accumulated and Ruml Cdk inhibitor was disappeared. In contrast, Cdc13 cyclin is not necessary for S phase. During the G1 phase, expansion growth followed by polar growth occurs. We speculate that the long G1 phase is required for cells to reach a critical cell size necessary for the onset of S phase.3) Identification of a major spore wall protein p23 : Germination is thought to accompany chemical and morphological alteration of spore envelopes. We found that p23, a major spore wall protein, decreased gradually during germination. P23 was purified and its partial amino acid sequence was determined. This sequence was identical with the amino terminus of the isp3/meu4 gene product, The isp3 gene is transcribed during meiosis and its disruption results in reduction of sporulation ability.Isp3-GFP fusion protein localized to the spore envelope. Intriguingly, P23/Isp3 shares a characteristic alanine-rich region with pollen allergen proteins.