Crystal structure of mouse MD-1 with endogenous phospholipid bound in its cavity

Area of Science:

• Immunology
• Structural Biology
• Biochemistry

Background:

• MD-1 is a glycoprotein associated with RP105 on B cells, implicated in lipopolysaccharide (LPS) recognition.
• Its structural and functional characteristics, particularly in LPS sensing, remain largely uncharacterized.
• MD-1 shares low sequence identity with MD-2, a known LPS-binding molecule associated with Toll-like receptor 4.

Purpose of the Study:

• To determine the crystal structure of mouse MD-1.
• To elucidate the structural basis for MD-1's function in lipid recognition.
• To investigate the oligomeric state and ligand-binding properties of MD-1.

Main Methods:

• X-ray crystallography to determine the 3D structure of mouse MD-1 at 1.65 Å resolution.
• Analysis of the MD-1 structure, including cavity dimensions and surface properties.
• Solution-based assays and crystal lattice analysis to assess oligomerization.
• Lipid-binding assays using phosphatidylcholine and tetra-acylated lipid IVa.

Main Results:

• The crystal structure of mouse MD-1 revealed a hydrophobic cavity, structurally analogous to MD-2 but with distinct dimensions (longer, narrower, shallower).
• The MD-1 cavity entrance lacks charged residues, and electron densities consistent with phosphatidylcholine were observed within the cavity.
• MD-1 exists primarily as a monomer in solution but forms dimers in crystal lattices, with cavities facing each other.
• Binding assays confirmed MD-1's ability to bind lipid IVa, supporting its role as a lipid-binding protein.

Conclusions:

• MD-1 possesses a distinct hydrophobic cavity capable of binding lipids like phosphatidylcholine and lipid IVa.
• The structural and binding data identify MD-1 as a coreceptor involved in lipid recognition, likely mediating LPS sensing in conjunction with RP105.
• Further studies are warranted to fully understand MD-1's role in B-cell immune responses.