Tuberculosis (Tb), caused by Mycobacterium tuberculosis (Mtb), is responsible for more than a million deaths annually. In the latent phase of infection, Mtb uses lipids as the source of carbon and energy for its survival. The lipid molecules are transported across the cell wall via multiple transport systems. One such set of widely present and less-studied transporters is the Mammalian-cell-entry (Mce) complexes. Here, we report the properties of the substrate-binding proteins (SBPs; MceA-F) of the Mce1 and Mce4 complexes from Mtb which are responsible for the import of mycolic acid/fatty acids, and cholesterol respectively. MceA-F are composed of four domains namely, transmembrane, MCE, helical and tail domains. Our studies show that MceA-F are predominantly monomeric when purified individually and do not form homohexamers unlike the reported homologs (MlaD, PqiB and LetB) from other prokaryotes. The crystal structure of MCE domain of Mtb Mce4A (MtMce4A39-140) determined at 2.9 Å shows the formation of an unexpected domain-swapped dimer in the crystals. Further, the purification and small-angle X-ray scattering (SAXS) analysis on MtMce1A, MtMce4A and their domains suggest that the helical domain requires hydrophobic interactions with the detergent molecules for its stability. Combining all the experimental data, we propose a heterohexameric arrangement of MtMceA-F SBPs, where the soluble MCE domain of the SBPs would remain in the periplasm with the helical domain extending to the lipid layer forming a hollow channel for the transport of lipids across the membranes. The tail domain would reach the cell surface assisting in lipid recognition and binding.

中文翻译：

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对结核分枝杆菌哺乳动物细胞进入（Mce）1和4复合物的底物结合蛋白的结构见解

由结核分枝杆菌（Mtb）引起的结核病（Tb）每年造成超过一百万的死亡。在潜在的感染阶段，Mtb使用脂质作为其生存的碳和能量来源。脂质分子通过多种转运系统转运穿过细胞壁。这类广泛存在且研究较少的转运蛋白是哺乳动物细胞进入（Mce）复合体。在这里，我们报告了Mtb的Mce1和Mce4复合物的底物结合蛋白（SBPs; MceA-F）的特性，它们分别负责霉菌酸/脂肪酸和胆固醇的进口。MceA-F由四个结构域组成，即跨膜，MCE，螺旋和尾部结构域。我们的研究表明，单独纯化时，MceA-F主要为单体，与其他原核生物报道的同系物（MlaD，PqiB和LetB）不同，它们不会形成同六聚体。Mtb的MCE结构域的晶体结构在2.9处测定的Mce4A（MtMce4A39-140）显示了晶体中意外的域交换二聚体的形成。此外，对MtMce1A，MtMce4A及其结构域的纯化和小角度X射线散射（SAXS）分析表明，螺旋结构域需要与去污剂分子发生疏水性相互作用才能保持稳定性。结合所有实验数据，我们提出了MtMceA-F SBP的异六聚体排列，其中SBP的可溶性MCE结构域将保留在周质中，而螺旋结构域则延伸到脂质层，形成了一个中空通道，用于脂质跨膜运输。膜。尾部结构域将到达细胞表面，帮助脂质识别和结合。