Diatom plastids depend on nucleotide import from the cytosol
Diatom plastids depend on nucleotide import from the cytosol
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DOI：
10.1073/pnas.0808862106
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发表时间：
2009-03-03
影响因子：
11.1
通讯作者：
Haferkamp, Ilka
中科院分区：
文献类型：
--
作者：
Ast, Michelle;Gruber, Ansgar;Haferkamp, Ilka
硅藻是生态上重要的藻类，通过次生内共生获得质体，与具有初级质体的生物体相比，导致更复杂的细胞结构和代谢途径分布的改变。硅藻质体被 4 层膜包围；最外层的膜与内质网连续。基因组分析表明，与高等植物相比，核苷酸生物合成不是位于质体中，而是位于细胞质中。因此，核苷酸必须被输入到细胞器中。然而，核苷酸进入复杂质体的机制尚不清楚。我们在硅藻假海链藻和三角褐指藻中鉴定出了大量假定的核苷酸转运蛋白 (NTT)，并鉴定了前 2 个亚型（NTT1 和 NTT2）。基于 GFP 的定位研究表明，所研究的两个 NTT 均靶向质体膜，并且 NTT1 最有可能通过基质进入最内质体包膜。异源表达的 NTT1 作为质子依赖性腺嘌呤核苷酸输入蛋白，而 NTT2 促进（脱氧）三磷酸核苷的反向交换。因此，这些转运蛋白在功能上类似于来自核苷酸代谢受损的专性细胞内细菌的 NTT，而不是来自初级质体的 ATP/ADP 交换 NTT。我们认为硅藻具有专门适应的核苷酸运输系统，并且 NTT 是向复杂质体供应核苷酸的关键角色。
Diatoms are ecologically important algae that acquired their plastids by secondary endosymbiosis, resulting in a more complex cell structure and an altered distribution of metabolic pathways when compared with organisms with primary plastids. Diatom plastids are surrounded by 4 membranes; the outermost membrane is continuous with the endoplasmic reticulum. Genome analyses suggest that nucleotide biosynthesis is, in contrast to higher plants, not located in the plastid, but in the cytosol. As a consequence, nucleotides have to be imported into the organelle. However, the mechanism of nucleotide entry into the complex plastid is unknown. We identified a high number of putative nucleotide transporters (NTTs) in the diatoms Thalassiosira pseudonana and Phaeodactylum tricornutum and characterized the first 2 isoforms (NTT1 and NTT2). GFP-based localization studies revealed that both investigated NTTs are targeted to the plastid membranes, and that NTT1 most likely enters the innermost plastid envelope via the stroma. Heterologously expressed NTT1 acts as a proton-dependent adenine nucleotide importer, whereas NTT2 facilitates the counter exchange of (deoxy-) nucleoside triphosphates. Therefore, these transporters functionally resemble NTTs from obligate intracellular bacteria with an impaired nucleotide metabolism rather than ATP/ADP exchanging NTTs from primary plastids. We suggest that diatoms harbor a specifically-adapted nucleotide transport system and that NTTs are the key players in nucleotide supply to the complex plastid.