据报道,维持脂质不对称(Mla)系统的膜相关溶质结合蛋白(SBP)MlaD有助于在革兰氏阴性细菌的外膜和内膜之间运输磷脂(PL)。尽管有结构信息,PLs转运的分子机制和MlaD蛋白的起源尚不清楚.在这项研究中,我们报告了来自大肠杆菌(EcMlaD)的MlaD周质区域的晶体结构,分辨率范围为2.3-3.2。EcMlaD原型由两个不同的区域组成,viz.N-末端β-桶折叠由七条链(称为MlaD结构域)和C-末端α-螺旋结构域(HD)组成。蛋白质EcMlaD寡聚化产生具有中心通道的同源六聚体环,所述中心通道是疏水的并且具有可变直径的连续通道。有趣的是,结构分析表明,HD,而不是MlaD域,在确定蛋白质的寡聚状态中起着至关重要的作用。基于对可用结构信息的分析,我们提出了一种PL传输的工作机制,viz.\"非对称探针移动(APM)\"。其中EcMlaD六聚体的一半会随着孔环的向外运动而在周质侧上升,导致中央通道几何形状的变化。此外,这项研究强调,与典型的SBP不同,EcMlaD具有类似于EF/AMT型β(6)-桶的褶皱和独特的祖先。总之,这些发现牢固地证明EcMlaD是一种具有独特配体转运机制的非规范SBP。
The membrane-associated solute-binding protein (SBP) MlaD of the maintenance of lipid asymmetry (Mla) system has been reported to help the transport of phospholipids (PLs) between the outer and inner membranes of Gram-negative bacteria. Despite the availability of structural information, the molecular mechanism underlying the transport of PLs and the ancestry of the protein MlaD remain unclear. In this study, we report the crystal structures of the periplasmic region of MlaD from Escherichia coli (EcMlaD) at a resolution range of 2.3-3.2 Å. The EcMlaD protomer consists of two distinct regions, viz. N-terminal β-barrel fold consisting of seven strands (referred to as MlaD domain) and C-terminal α-helical domain (HD). The protein EcMlaD oligomerizes to give rise to a homo-hexameric ring with a central channel that is hydrophobic and continuous with a variable diameter. Interestingly, the structural analysis revealed that the HD, instead of the MlaD domain, plays a critical role in determining the oligomeric state of the protein. Based on the analysis of available structural information, we propose a working mechanism of PL transport, viz. \"asymmetric protomer movement (APM)\". Wherein half of the EcMlaD hexamer would rise in the periplasmic side along with an outward movement of pore loops, resulting in the change of the central channel geometry. Furthermore, this study highlights that, unlike typical SBPs, EcMlaD possesses a fold similar to EF/AMT-type beta(6)-barrel and a unique ancestry. Altogether, the findings firmly establish EcMlaD to be a non-canonical SBP with a unique ligand-transport mechanism.