BACKGROUND All cells rely on lipids for key functions. Lipid transfer proteins allow lipids to exit the hydrophobic environment of bilayers, and cross aqueous spaces. One lipid transfer domain fold present in almost all eukaryotes is the TUbular LIPid binding (TULIP) domain. Three TULIP families have been identified in bacteria (P47, OrfX2 and YceB), but their homology to eukaryotic proteins is too low to specify a common origin. Another recently described eukaryotic lipid transfer domain in VPS13 and ATG2 is Chorein-N, which has no known bacterial homologues. There has been no systematic search for bacterial TULIPs or Chorein-N domains. RESULTS Remote homology predictions for bacterial TULIP domains using HHsearch identified four new TULIP domains in three bacterial families. DUF4403 is a full length pseudo-dimeric TULIP with a 6 strand β-meander dimer interface like eukaryotic TULIPs. A similar sheet is also present in YceB, suggesting it homo-dimerizes. TULIP domains were also found in DUF2140 and in the C-terminus DUF2993. Remote homology predictions for bacterial Chorein-N domains identified strong hits in the N-termini of AsmA and TamB in diderm bacteria, which are related to Mdm31p in eukaryotic mitochondria. The N-terminus of DUF2993 has a Chorein-N domain adjacent to its TULIP domain. CONCLUSIONS TULIP lipid transfer domains are widespread in bacteria. Chorein-N domains are also found in bacteria, at the N-terminus of multiple proteins in the intermembrane space of diderms (AsmA, TamB and their relatives) and in Mdm31p, a protein that is likely to have evolved from an AsmA/TamB-like protein in the endosymbiotic mitochondrial ancestor. This indicates that both TULIP and Chorein-N lipid transfer domains may have originated in bacteria.

中文翻译：

---

远程同源性搜索可识别真核脂质转移蛋白的细菌同源物，包括 TamB 和 AsmA 和 Mdm31p 中的 Chorein-N 结构域。

背景所有细胞的关键功能都依赖于脂质。脂质转移蛋白允许脂质离开双层的疏水环境，并穿过水空间。几乎所有真核生物中存在的一种脂质转移结构域折叠是 TUbular LIPid 结合 (TULIP) 结构域。已在细菌中鉴定出三个 TULIP 家族（P47、OrfX2 和 YceB），但它们与真核蛋白质的同源性太低，无法确定共同的起源。VPS13 和 ATG2 中另一个最近描述的真核脂质转移结构域是 Chorein-N，它没有已知的细菌同源物。没有系统地搜索细菌 TULIP 或 Chorein-N 结构域。结果 使用 HHsearch 对细菌 TULIP 域的远程同源性预测在三个细菌家族中确定了四个新的 TULIP 域。DUF4403 是一种全长假二聚体 TULIP，具有 6 链 β-曲折二聚体界面，如真核 TULIP。YceB 中也存在类似的薄片，表明它是同型二聚体。在 DUF2140 和 C 端 DUF2993 中也发现了 TULIP 结构域。对细菌 Chorein-N 结构域的远程同源性预测确定了 diderm 细菌中 AsmA 和 TamB 的 N 末端的强命中，这与真核线粒体中的 Mdm31p 相关。DUF2993 的 N 末端有一个与其 TULIP 结构域相邻的 Chorein-N 结构域。结论 TULIP 脂质转移结构域广泛存在于细菌中。Chorein-N 结构域也存在于细菌中，位于双皮层（AsmA、TamB 及其亲属）和 Mdm31p 膜间隙中多种蛋白质的 N 端，一种很可能是从线粒体内共生祖先中的 AsmA/TamB 样蛋白质进化而来的蛋白质。这表明 TULIP 和 Chorein-N 脂质转移结构域都可能起源于细菌。