Dutta Angshu, Prasad Kanaujia Shankar
Department of Biosciences and Bioengineering, Indian Institute of Technology Guwahati, Guwahati - 781039, Assam, India.
Department of Biosciences and Bioengineering, Indian Institute of Technology Guwahati, Guwahati - 781039, Assam, India.
J Struct Biol. 2022 Dec;214(4):107896. doi: 10.1016/j.jsb.2022.107896. Epub 2022 Sep 7.
The outer membrane (OM) of Gram-negative bacteria acts as a formidable barrier against a plethora of detrimental compounds owing to its asymmetric nature. This is because the OM possesses lipopolysaccharides (LPSs) in the outer leaflet and phospholipids (PLs) in the inner leaflet. The maintenance of lipid asymmetry (Mla) system is involved in preserving the distribution of PLs in OM. The periplasmic component of the system MlaC serves as the substrate-binding protein (SBP) that shuttles PLs between the inner and outer membranes. However, an in-depth report highlighting its mechanism of ligand binding is still lacking. This study reports the crystal structure of MlaC from Escherichia coli (EcMlaC) at a resolution of 2.5 Å in a quasi-open state, complexed with PL. The structural analysis reveals that EcMlaC and orthologs comprise two major domains, viz. nuclear transport factor 2-like (NTF2-like) and phospholipid-binding protein (PBP). Each domain can be further divided into two subdomains arranged in a discontinuous fashion. This study further reveals that EcMlaC is polyspecific in nature and follows a reverse mechanism of the opening of the substrate-binding site during the ligand binding. Furthermore, MlaC can bind two PLs by forming subsites in the binding pocket. These findings, altogether, have led to the proposition of the unique "segmented domain movement" mechanism of PL binding, not reported for any known SBP to date. Further, unlike typical SBPs, MlaC has originated from a cystatin-like fold. Overall, this study establishes MlaC to be a non-canonical SBP with a unique ligand-binding mechanism.
革兰氏阴性菌的外膜由于其不对称性，对大量有害化合物起着强大的屏障作用。这是因为外膜在外层小叶中含有脂多糖（LPS），在内层小叶中含有磷脂（PL）。脂质不对称性维持（Mla）系统参与维持外膜中PL的分布。该系统的周质成分MlaC作为底物结合蛋白（SBP），在内外膜之间穿梭PL。然而，仍缺乏一份深入报告突出其配体结合机制。本研究报道了来自大肠杆菌（EcMlaC）的MlaC在与PL复合的准开放状态下分辨率为2.5Å的晶体结构。结构分析表明，EcMlaC及其直系同源物包含两个主要结构域，即核转运因子2样（NTF2样）和磷脂结合蛋白（PBP）。每个结构域可进一步分为以不连续方式排列的两个亚结构域。本研究进一步表明，EcMlaC本质上具有多特异性，在配体结合过程中遵循底物结合位点开放的反向机制。此外，MlaC可通过在结合口袋中形成亚位点结合两个PL。总的来说，这些发现提出了独特的PL结合“分段结构域运动”机制，这是迄今为止任何已知SBP都未报道过的。此外，与典型的SBP不同，MlaC起源于胱抑素样折叠。总体而言，本研究确定MlaC是一种具有独特配体结合机制的非典型SBP。
