CDP-diacylglycerol synthases (Cds) are conserved from bacteria to eukaryotes. Bacterial CdsA is involved not only in phospholipid biosynthesis but also in biosynthesis of glycolipid MPIase, an essential glycolipid that catalyzes membrane protein integration. We found that both Cds4 and Cds5 of Arabidopsis chloroplasts complement cdsA knockout by supporting both phospholipid and MPIase biosyntheses. Comparison of the sequences of CdsA and Cds4/5 suggests a difference in membrane topology at the C-termini, since the region assigned as the last transmembrane region of CdsA, which follows the conserved cytoplasmic domain, is missing in Cds4/5. Deletion of the C-terminal region abolished the function, indicating the importance of the region. Both 6 × His tag attachment to CdsA and substitution of the C-terminal 6 residues with 6 × His did not affect the function. These 6 × His tags were sensitive to protease added from the cytosolic side in vitro, indicating that this region is not a transmembrane one but forms a membrane-embedded reentrant loop. Thus, the C-terminal region of Cds homologues forms a reentrant loop, of which structure is important for the Cds function.

中文翻译：

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拟南芥叶绿体 Cds4/5 在大肠杆菌中的表达揭示了 CDP-二酰基甘油合酶 C 端区域的膜拓扑结构

CDP-二酰基甘油合酶 (Cds) 从细菌到真核生物都是保守的。细菌 CdsA 不仅参与磷脂生物合成，而且参与糖脂 MPIase 的生物合成，糖脂 MPIase 是一种催化膜蛋白整合的必需糖脂。我们发现，Cds4和Cds5都拟南芥叶绿体补充CDSA通过支持磷脂和 MPIase 生物合成来敲除。CdsA 和 Cds4/5 的序列比较表明 C 末端的膜拓扑结构存在差异，因为 Cds4/5 中缺少 Cds4/5 中缺失的区域，该区域位于 CdsA 的最后一个跨膜区域，位于保守的细胞质结构域之后。删除 C 端区域取消了该功能，表明该区域的重要性。将 6 × His 标签连接到 CdsA 和用 6 × His 替换 C 端 6 个残基都不会影响功能。这些 6 × His 标签对体外细胞溶质侧添加的蛋白酶敏感，表明该区域不是跨膜区域，而是形成了膜嵌入的折返环。因此，Cds 同源物的 C 端区域形成一个可重入环，其结构对 Cds 功能很重要。