Hsp26 蛋白調控酵母菌孢子形成的機制

Abstract

酵母菌的生長方式會隨著環境中養分的不同而做出適當的改變，當環境中缺乏可發酵的醣類及氮源時，二倍體酵母菌的生長方式會從出芽分裂轉變成減數分裂，並產生孢子以度過逆境。 先前研究酵母菌Hsp26蛋白在孢子產生過程所扮演的角色時，發現hsp26突變株的產孢比例比野生型酵母菌為高，進一步分析減數分裂和孢子產生的過程發現，產孢差異的原因不是發生在減數分裂的啟始或分裂期，而是在孢子形成期。當使用benomyl (一種干擾紡錘絲形成的藥劑) 處理細胞後，此藥劑影響部分野生型酵母菌的產孢比例，但其孢子存活率不受影響；相反的，此藥劑不會影響hsp26突變株的產孢比例，但孢子存活率下降。因此推測存在一個與Hsp26有關的紡錘體位置檢控點 (spindle position checkpoint)，可監控從第二次減數分裂到孢子形成的過程，子細胞核在母細胞中的位置或細胞內形成的紡錘體是否適合形成孢子。而本篇論文確認此藥劑會影響紡錘體的形成，和確認此藥劑會加重野生型酵母菌不能形成孢子的比例，進一步支持此檢控機制的存在。 為了瞭解此檢控點的詳細調控機制，先前找出幾個和Hsp26 蛋白有交互作用且可能參與在此調控機制的蛋白質。其中一個為Ady3蛋白。Ady3 蛋白位於原生孢子膜的前端，形成像甜甜圈般的環狀結構，稱為前緣蛋白質複合體 (leading edge complex)，Ady3 蛋白主要和原生孢子膜形成以及孢子壁合成有關。而本篇研究顯示，以benomyl處理細胞後，許多野生型酵母菌細胞內的Ady3 蛋白無法參與前緣蛋白質複合體，但在hsp26突變細胞仍形成此Ady3複合體之比例不變。進一步分析此前緣蛋白質複合體的另一主要蛋白質Ssp1時，也得到類似的結果。由此結論推論，Hsp26蛋白能調控Ady3蛋白以及Ssp1蛋白形成前緣蛋白質複合體。 綜合所有的結果，我們提出，在酵母菌產生孢子的過程中，有一個Hsp26蛋白參與的紡錘體位置檢控點 (spindle position checkpoint)，可監控從第二次減數分裂時，子細胞核在母細胞中的位置或細胞內形成的紡錘體是否適合形成孢子，若不適合，此檢控機制會透過調控Ady3蛋白及Ssp1蛋白形成前緣蛋白質複合體而阻止孢子形成，以確保形成的孢子都具有能力存活。

Diploid cells of the yeast Saccharomyces cerevisiae switch their growth in response to nutrient starvation. The complete absence of fermentable sugar and nitrogen resources causes cells to exit from the mitotic cycle, enter meiosis, and produce haploid spores. During the study of the roles of heat-shock proteins in yeast sporulation, our laboratory found that the hsp26 mutant increased sporulation ~20% more than that of the wild type. This increase took place not at the beginning of meiosis, neither during meiotic divisions, but at the step of spore formation. Based on the experiments using benomyl to interfere the meiotic spindles, it was proposed that Hsp26 might be involved in a novel spindle position checkpoint that regulates spore formation in response to meiosis II progression. In this study, the existence of the Hsp26-dependent spindle position checkpoint was further confirmed by analyzing the defect of spindle formation after benomyl treatment. To explore the mechanism of Hsp26 in controlling spore formation, previous studies have identified Ady3 as an Hsp26-interacting protein. Ady3 has been reported to form a ring-like structure at the leading edge of each growing prospore membrane that initiates the spore formation process. In this study, we showed that Ady3 localization was affected after benomyl treatment in the wild type, but not in the hsp26 mutant. Similar effect was also observed on the localization of Ssp1, which is the main component of the leading edge complex. Our experiments suggested that Hsp26 regulates Ady3 and Ssp1 localization. Our results confirmed the novel Hsp26-dependent spindle position checkpoint that monitors the distribution or separation of daughter nuclei within the mother cell and controls spore formation. This checkpoint regulates Ady3 and Ssp1 localization and in turn, controls spore formation to ensure spore viability.