植物與動物細(xì)胞的主要區(qū)別就是光能轉(zhuǎn)換成化學(xué)能的過程不同。當(dāng)沒有光可用時,，能量從碳水化合物和糖的分解中而來,，就象動物和細(xì)菌細(xì)胞一樣。兩種細(xì)胞器負(fù)責(zé)這兩個過程,，即葉綠體負(fù)責(zé)光合作用,，線粒體負(fù)責(zé)糖分解,。一項新研究已打開了一扇通往葉綠體進化早期階段的窗口。這項研究在線發(fā)表在2月27日到3月2日的PNAS上,。
大家普遍認(rèn)為,，葉綠體源自單細(xì)胞藍細(xì)菌（cyanobacteria，藍綠藻類原核生物）,，15億年前,，藍細(xì)菌被更復(fù)雜的非光合細(xì)胞所吞噬。這兩種生物之間原本是共生的,，但是藍細(xì)菌將其大部分遺傳物質(zhì)轉(zhuǎn)移至宿主生物體細(xì)胞核內(nèi),，原來的藍細(xì)菌便轉(zhuǎn)變成葉綠體，這樣藍細(xì)菌沒有宿主就不能存活了,。
導(dǎo)致線粒體產(chǎn)生的類似過程
為了維持細(xì)胞器功能,，于是在細(xì)胞質(zhì)或細(xì)胞內(nèi)部合成轉(zhuǎn)化基因編碼的蛋白質(zhì)，然后再將合成的蛋白質(zhì)輸回細(xì)胞器中,。在所研究的許多系統(tǒng)中,，它們通過一種多蛋白質(zhì)輸入復(fù)合物將蛋白質(zhì)運至葉綠體中，這種輸入復(fù)合物能使蛋白質(zhì)穿過葉綠體周圍的囊膜,。
葉綠體和線粒體之間的事件永遠(yuǎn)地改變了世界,，但很難再研究這個事件發(fā)生的過程，因為它發(fā)生在很久以前,。若要闡明這一過程的進化方式,，就先要確定一種生物體，在此生物體中,，存在有細(xì)菌到宿主依賴性細(xì)胞器的轉(zhuǎn)換事件發(fā)生,。
Nowack和 Grossman重點研究了一種阿米巴原蟲（Paulinella chromatophora），它包含兩個起源于一個內(nèi)共生藍細(xì)菌的室,，但這是完全進化細(xì)胞器形成的早期階段,。
這些室稱為色素體，能轉(zhuǎn)移30多種原始藍細(xì)菌基因到宿主生物細(xì)胞核中,。雖然,，在其他內(nèi)共生菌中也觀察到基因轉(zhuǎn)移，但是還不清楚被轉(zhuǎn)移基因的功能,，因為不會出現(xiàn)內(nèi)共生體（與細(xì)胞器相反,，可奪回被轉(zhuǎn)移基因所編碼的蛋白質(zhì)），它們沒有適當(dāng)?shù)牡鞍踪|(zhì)輸入機制,。
利用一種先進的芯片技術(shù),，他們確定了三種在細(xì)胞質(zhì)合成然后轉(zhuǎn)入葉綠體內(nèi)的蛋白質(zhì)，這三種蛋白質(zhì)在細(xì)胞質(zhì)中與內(nèi)在編碼的其他蛋白組裝成光合過程一部分的工作蛋白復(fù)合物,。
有趣的是,，這個將蛋白質(zhì)輸入色素體的過程可能是新的,，包括通過高爾基體運輸，再成為本地化色素體,。這表明存在蛋白質(zhì)跨膜產(chǎn)生葉綠體囊膜的原始過程與退化過程,。最終這個過程進化成一個更加精細(xì)、用特定蛋白復(fù)合物有效運輸?shù)倪^程,。
這項工作表明,，阿米巴原蟲是一個潛在的、研究進化過程的模型,，通過進化,，細(xì)胞器發(fā)育，并獲得輸入色素體的蛋白質(zhì)詳細(xì)清單,，包括蛋白質(zhì)的功能與來源,、被輸入的途徑，這能使我們更好地了解真核細(xì)胞奴役細(xì)菌并將它們轉(zhuǎn)化為葉綠體與線粒體之類的細(xì)胞器的機制,。（生物谷bioon.com）
doi:10.1073/pnas.1118800109
PMC:
PMID:
Trafficking of protein into the recently established photosynthetic organelles of Paulinella chromatophora
Eva C. M. Nowack ,，Arthur R. Grossman
Endosymbiotic acquisition of bacteria by a protist, with subsequent evolution of the bacteria into mitochondria and plastids, had a transformative impact on eukaryotic biology. Reconstructing events that created a stable association between endosymbiont and host during the process of organellogenesis-including establishment of regulated protein import into nascent organelles-is difficult because they date back more than 1 billion years. The amoebaPaulinella chromatophora contains nascent photosynthetic organelles of more recent evolutionary origin (?60 Mya) termed chromatophores (CRs). After the initial endosymbiotic event, the CR genome was reduced to approximately 30% of its presumed original size and more than 30 expressed genes were transferred from the CR to the amoebal nuclear genome. Three transferred genes-psaE,psaK1, and psaK2-encode subunits of photosystem I. Here we report biochemical evidence that PsaE, PsaK1, and PsaK2 are synthesized in the amoeba cytoplasm and traffic into CRs, where they assemble with CR-encoded subunits into photosystem I complexes. Additionally, our data suggest that proteins routed to CRs pass through the Golgi apparatus. Whereas genome reduction and transfer of genes from bacterial to host genome have been reported to occur in other obligate bacterial endosymbioses, this report outlines the import of proteins encoded by such transferred genes into the compartment derived from the bacterial endosymbiont. Our study showcases P. chromatophora as an exceptional model in which to study early events in organellogenesis, and suggests that protein import into bacterial endosymbionts might be a phenomenon much more widespread than currently assumed
