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  • 學位論文

探討Atg24在出芽酵母菌細胞自噬調控上所扮演的角色

Characterization of the role of Atg24 in autophagy regulation in Saccharomyces cerevisiae

指導教授 : 黃偉邦

摘要


細胞自噬(autophagy)是普遍存在所有真核生物的一種分解機制。當細胞處於壓力環境,特別是營養缺乏狀況,細胞自噬的活性會上升,誘導雙層膜構造形成,將周遭的物質非選擇性地包裹起來,此種雙層膜狀囊胞稱為自噬體(autophagosome)。自噬體最後會與液胞融合而分解內容物質,所釋出的小分子單體可供重新利用,維持細胞在惡劣環境中的生理恆定。細胞自噬也可以選擇性地移除過量或受損胞器,像是過氧化物酶體自噬(pexophagy)和粒線體自噬(mitophagy)。在出芽酵母菌(Saccharomyces cerevisiae)中,細胞質至液胞傳遞途徑(cytoplasm-to-vacuole targeting pathway, Cvt pathway)是一種特殊的選擇性細胞自噬,當細胞處於養分充足的生理環境時,細胞質至液胞傳遞途徑仍維持低度活性,持續運送液胞水解酶aminopeptidase I前驅物(prApeI)及α-mannosidase I (AmsI)。細胞質至液胞傳遞途徑和非選擇性細胞自噬共享許多核心調節分子,然而,細胞質至液胞傳遞途徑也需要特殊的調節蛋白參與,例如Atg24蛋白質。 Atg24是一個具有PI3P分子結合能力的蛋白質;它參與選擇性細胞自噬的調控,但並非養分缺乏引發之細胞自噬所必需。此外,Atg24也參與在內膜系統的蛋白質運輸途徑當中。本篇研究發現,Atg24調控在細胞質至液胞傳遞途徑之囊胞形成步驟。Atg24運送至囊胞形成位置(pre-autophagosomal structure, PAS)受到PI3K複合體I、Atg20和Trs85的調節。進一步分析Atg24蛋白質結構發現,Atg24的羰基端具有兩個螺旋結構(coiled-coil domain):其中第一螺旋結構是選擇性細胞自噬所必需,而兩個螺旋結構都參與Snc1蛋白質在內膜系統的回收機制當中。然而,第一螺旋結構並不負責Atg24與已知的結合蛋白Atg17和Atg20之交互作用,卻藉由未知的機制調控Atg24本身以及Atg20在細胞質內的分布。由本文的研究結果顯示:在細胞質至液胞傳遞途徑的過程中,Atg24-Atg20複合體移動到囊胞形成位置,隨後Atg24和Atg20再共同調控雙層膜囊胞的形成。然而,Atg24如何與細胞自噬的核心調控分子合作完成選擇性細胞自噬的過程,有待更進一步的研究了解。

並列摘要


Autophagy is a highly conserved degradation pathway among all eukaryotes that can be induced under stress conditions, such as starvation. During the process, portions of cytoplasm are non-selectively engulfed into double-membrane vesicles called autophagosomes and subsequently delivered to the vacuole for degradation. The small molecules released from the process can be used to maintain cellular homeostasis. However, the degradation processes can occur selectively to remove excess or damaged organelles, for example, pexophagy and mitophagy. In the budding yeast Saccharomyces cerevisiae, the cytoplasm-to-vacuole targeting (Cvt) pathway is a unique type of selective autophagy that is constitutively active in growth condition for biosynthetic transport of vacuolar enzymes, such as precursors of aminopeptidase I and α-mannosidase. Although the Cvt pathway shares the same core machinery with bulk autophagy, it also requires specific regulatory proteins. In this study, I used the Cvt pathway as a model for analyzing the regulatory mechanisms underlying selective autophagy and investigated the role of selective autophagy-specific regulatory protein Atg24. Atg24 is a PI3P-binding protein essential for selective autophagy but not bulk autophagy. Moreover, Atg24 is also involved in the endosome-to-Golgi protein traffic. I found that Atg24 participates in the vesicle formation step of the Cvt pathway and its recruitment to the PAS (pre-autophagosomal structure) is regulated by the PI3K complex I, Atg20 and Trs85. Besides, Atg24 contains two coiled-coil domains at the C terminus. Only the coiled-coil domain 1 is necessary for selective autophagy, whereas both two coiled-coil domains are required for retrieval of Snc1 in endomembrane trafficking. Although the coiled-coil domain 1 of Atg24 does not mediate its association with known interaction partners Atg17 and Atg20, it determines the subcellular localization of itself as well as Atg20. Our data suggested that during the Cvt pathway, the Atg24-Atg20 complex appears at the PAS in a mutually dependent manner and together they directly assist the Cvt vesicle formation. Future studies are needed to characterize how Atg24 cooperates with the core autophagic machinery in details.

並列關鍵字

autophagy the Cvt pathway pexophagy Atg24 Snc1 retrieval

參考文獻


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