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Published on: April 23, 2017
The Structural Features of MlaD Illuminate its Unique Ligand-Transporting Mechanism and Ancestry
Angshu Dutta1, Shankar Prasad Kanaujia2
1Department of Biosciences and Bioengineering, Indian Institute of Technology Guwahati, Guwahati, Assam, 781039, India.
Abstract:
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.
Insights
The study reveals the crystal structure of Escherichia coli MlaD, a membrane protein crucial for phospholipid transport in bacteria. It proposes a novel "asymmetric protomer movement" mechanism for lipid transport and highlights MlaD
Area of Science:
- Structural biology
- Bacterial outer membrane transport
- Molecular mechanisms
Background:
- The MlaD protein is a membrane-associated solute-binding protein (SBP) involved in maintaining lipid asymmetry in Gram-negative bacteria.
- Previous studies provided structural information but lacked clarity on the molecular mechanism of phospholipid transport and MlaD's ancestry.
- Understanding these aspects is crucial for deciphering bacterial envelope biogenesis and potential therapeutic targets.
Purpose of the Study:
- To elucidate the crystal structures of the periplasmic region of MlaD from Escherichia coli (EcMlaD).
- To investigate the molecular mechanism of phospholipid transport mediated by EcMlaD.
- To determine the evolutionary ancestry and oligomeric behavior of EcMlaD.
Main Methods:
- X-ray crystallography was employed to determine the structures of EcMlaD at resolutions between 2.3-3.2 Å.
- Bioinformatic and structural analyses were performed to understand protein oligomerization and functional domains.
- A novel mechanism, 'asymmetric protomer movement' (APM), was proposed based on structural observations.
Main Results:
- The crystal structures revealed EcMlaD as a homo-hexameric ring with a hydrophobic central channel.
- The C-terminal helical domain (HD) was identified as critical for the oligomeric state, unlike the N-terminal MlaD domain.
- A unique 'asymmetric protomer movement' (APM) mechanism was proposed for phospholipid transport, involving dynamic changes in the central channel.
- EcMlaD exhibits a fold similar to EF/AMT-type beta(6)-barrel proteins, suggesting a unique ancestry distinct from typical SBPs.
Conclusions:
- EcMlaD functions as a non-canonical SBP with a distinctive mechanism for phospholipid transport.
- The study provides significant insights into the structural basis and molecular dynamics of the Mla system.
- Findings contribute to understanding bacterial lipid homeostasis and open avenues for future research on MlaD function and evolution.
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