Abstract
摘要:細胞自噬 (autophagy) 泛指真核細胞面臨壓力 (stress) 時—例如在缺乏養分的環境中,由雙層膜形成的細胞自噬體將細胞質成分送入濾泡(vacuole)或溶小體 (lysosome) 中,分解巨分子物質以便循環利用其基本成分,用以合成維持生命所需新分子的細胞生理現象。近年來研究發現,在腫瘤與組織退化疾病的細胞中,常有異常的細胞自噬活性,一般推測這與細胞自噬可選擇性去除受損的巨分子及胞器,甚至促成細胞死亡 (type II programmed cell death),以避免這些物質干擾正常生理功能有關。因此,研究細胞自噬的貨物篩選分子機制,不但有助於認識細胞自噬在疾病發生中所扮演的角色,更可能在將來用以發展新的療法。本計畫預定以三年時間,研究出芽酵母菌及哺乳動物細胞的選擇性細胞自噬的分子機制。以往的實驗發現,細胞自噬體的膜蛋白Atg8與貨物受體 (cargo receptor) Atg19會直接接合,提供貨物篩選過程中一部分的辨識能力。我們最新的實驗則觀察到一個新的貨物辨識機制,經由Atg11與Atg9間的作用來達成。Atg9的功能在調控自噬體的形成,而Atg11則已知與選擇性細胞自噬的貨物篩選可關,但其詳細機制則未有報導。我們計畫進一步分析Atg9及Atg8這二個貨物篩選途徑之間的關係,並由新建立的選擇性過氧化體代謝來印證其一般性,並將由酵母菌系統所得知識,應用在研究哺乳動物細胞選擇性代謝蛋白質沈積過程之分子機制。
Abstract: Autophagy is a membrane trafficking mechanism that delivers cytosolic cargo to the vacuole/lysosome for degradation and recycling. Since its discovery, autophagy had been considered simply as a non-specific catabolic pathway for accommodating cells to starvation challenges. Recent studies, however, have uncovered the intimate relation between autophagy activities and human diseases, such as pathogen infections and tissue-degenerative diseases. Based on animal model studies, autophagy activities are also proposed as a mechanism to prevent aging by eliminating dysfunctional mitochondria. All these newly characterized autophagy functions depend on efficient sorting mechanisms to recognize and segregate specific cargoes for degradation, which make the sorting machinery a potential target for medical manipulations. Working on a suitable model will accelerate our characterization on the molecular mechanisms of cargo-sorting processes. We have found a selective autophagic cargo-sorting model in the budding yeast Saccharomyces cerevisiae. The precursors of aminopeptidase I enzyme (prApe1) are delivered directly from cytosol to the vacuole through the actions of autophagy proteins. Atg19 is the sorting receptor for prApe1. Atg11 regulates vacuolar delivery of not only prApe1 but also another specific autophagic cargo—peroxisome, revealing its adaptor function. Atg11 is proposed to perform this function by sending cargo to the site of vesicle formation, the pre-autophagosomal structure (PAS). Subsequent events at the PAS load cargo into forming vesicles. We have previously reported our observations that Atg19 interacts with the vesicle membrane protein Atg8 to media prApe1 loading during vesicle formation but how prApe1 is targeted to the PAS with the help of Atg11 is not clear. Our recent studies identified another interaction involving Atg11 and Atg9, which may be responsible for the cargo-targeting function. Atg9 is an integral membrane protein that resides at the PAS to recruit other autophagy proteins for vesicle formation. We propose that interaction between Atg9 and Atg11 helps prApe1-targeting to the PAS, whereas interaction between Atg8 and Atg19 loads prApe1 into transport vesicles. We will work on this hypothesis to dissect the sorting mechanisms for prApe1. In addition, we will screen new mutants to study the molecular mechanism of pexophagy pathway. Based on the collected information, the potentially conserved cargo-sorting machineries between yeast and mammalian cells will be examined.
Keyword(s)
細胞自噬
貨物篩選
蛋白質傳遞
濾泡
腫瘤
細胞死亡
出芽酵母菌
autophagy
cargo-sorting mechanisms
protein targeting
membrane trafficking
vacuole
programmed cell death
Saccharomyces cerevisiae