GDGT cyclization proteins identify the dominant archaeal sources of tetraether lipids in the ocean
GDGT cyclization proteins identify the dominant archaeal sources of tetraether lipids in the ocean
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DOI：
10.1073/pnas.1909306116
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发表时间：
2019-10
期刊：
影响因子：
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通讯作者：
Z. Zeng;Xiao-Lei Liu;Kristen R Farley;Jeremy H. Wei;W. Metcalf;R. Summons;P. Welander
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文献类型：
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作者：
Z. Zeng;Xiao-Lei Liu;Kristen R Farley;Jeremy H. Wei;W. Metcalf;R. Summons;P. Welander
古细菌合成独特的膜跨越脂质（GDGTs），这些脂质很容易保存在古代沉积物中，并被用作古温度指标来重建地球过去深处的海洋表面温度。然而，正确解释基于GDGT的生物标志物代理需要准确评估现代环境中对GDGT池有贡献的古菌和合成GDGT所需的蛋白质。在这项研究中，我们鉴定了酸藻中产生这些分子所需的2个自由基SAM蛋白。这些GDGT环合成蛋白的生物信息学分析表明，在开放海洋中，thumarchaeota是环化GDGT的主要来源，这使我们能够限制基于GDGT古温度代理的应用中的一个不确定因素。甘油二phytanyl甘油四醚（GDGTs）是独特的古细菌跨膜脂质，具有多达八个环戊烷环和/或一个环己烷环。添加到GDGT核心结构上的环的数量可以随着环境条件的变化而变化，例如生长温度的变化。这种生理反应使得保存在沉积物中的循环GDGTs可以作为重建过去全球和区域温度的代理，并为古代气候变化提供基本的见解。然而，基于GDGT的古细菌群落的不确定性以及影响现存古细菌GDGT循环的环境和生理因素的不确定性阻碍了基于GDGT古温度代用指标的可信度。为了适当地限制这些不确定性，需要对GDGT的生物合成有一个全面的了解。在这里，我们鉴定了2种GDGT环合成酶，GrsA和GrsB，它们是酸藻中GDGT环形成所必需的。这两种蛋白都是自由基s -腺苷蛋氨酸蛋白，表明GDGT环化是通过自由基机制发生的。此外，我们证明GrsA在核心GDGT脂质的C-7位置特异性引入环，而GrsB在C-3位置环化，这表明环化模式受两种不同酶的不同控制，并可能受到不同环境因素的影响。最后，对Grs蛋白的系统发育分析表明，海洋Thaumarchaeota而不是Euryarchaeota是开放海洋环境中环化GDGTs的主要来源，解决了基于gdgt的古温度代理应用的主要不确定性来源。
Significance Archaea synthesize distinctive membrane-spanning lipids (GDGTs) that are readily preserved in ancient sediments and utilized as paleotemperature proxies to reconstruct sea surface temperatures deep in Earth’s past. However, properly interpreting GDGT-based biomarker proxies requires an accurate assessment of the archaea that contribute to GDGT pools in modern environments and of the proteins necessary for synthesizing GDGTs. In this study, we identify 2 radical SAM proteins in Sulfolobus acidocaldarius that are required to produce these molecules. Bioinformatics analyses of these GDGT ring synthesis proteins reveal that Thaumarchaeota are the dominant source of cyclized GDGTs in the open ocean, allowing us to constrain one factor of uncertainty in the application of GDGT-based paleotemperature proxies. Glycerol dibiphytanyl glycerol tetraethers (GDGTs) are distinctive archaeal membrane-spanning lipids with up to eight cyclopentane rings and/or one cyclohexane ring. The number of rings added to the GDGT core structure can vary as a function of environmental conditions, such as changes in growth temperature. This physiological response enables cyclic GDGTs preserved in sediments to be employed as proxies for reconstructing past global and regional temperatures and to provide fundamental insights into ancient climate variability. Yet, confidence in GDGT-based paleotemperature proxies is hindered by uncertainty concerning the archaeal communities contributing to GDGT pools in modern environments and ambiguity in the environmental and physiological factors that affect GDGT cyclization in extant archaea. To properly constrain these uncertainties, a comprehensive understanding of GDGT biosynthesis is required. Here, we identify 2 GDGT ring synthases, GrsA and GrsB, essential for GDGT ring formation in Sulfolobus acidocaldarius. Both proteins are radical S-adenosylmethionine proteins, indicating that GDGT cyclization occurs through a free radical mechanism. In addition, we demonstrate that GrsA introduces rings specifically at the C-7 position of the core GDGT lipid, while GrsB cyclizes at the C-3 position, suggesting that cyclization patterns are differentially controlled by 2 separate enzymes and potentially influenced by distinct environmental factors. Finally, phylogenetic analyses of the Grs proteins reveal that marine Thaumarchaeota, and not Euryarchaeota, are the dominant source of cyclized GDGTs in open ocean settings, addressing a major source of uncertainty in GDGT-based paleotemperature proxy applications.