The Structural Basis for Lipid a Recognition in the CD14 Innate Immune Co-Receptor

Abstract

The CD14 co-receptor is specialized for recognition of bacterial lipopolysaccharide (LPS). On the surface of macrophages and other immune cells, it transfers LPS and its bioactive component lipid A to the MD-2 protein in complex with Toll-Like Receptor 4 (TLR4), and is hence crucial in activating the innate immune system via the TLR4 signalling pathway. In the case of severe infections, lipid A can cause sepsis through over-activation of the immune response, leading to multiple organ failure and death, and has become a major target for anti-septic drugs. Unfortunately, the mechanism by which lipid A is transferred to CD14, and the detailed mode(s) of associated binding, are unknown. In this study we have used atomically detailed molecular dynamics simulation approaches to uncover the mechanism by which lipid A is transferred to, interacts with and binds to a hypothesized amino-terminal pocket in CD14. We modelled the interactions and dynamics of CD14 in the presence of a range of lipid ligands, including control fatty acid systems, and lipid A in monomeric and aggregate/micelle forms. These simulations were run in order to observe the spontaneous ligand binding process, and have subsequently been extended to establish the thermodynamics of ligand recognition. Our results emphasise the dynamic nature of the amino-terminal pocket which allows it to adapt its volume to widely varying ligand size, consistent with the broad specificity of CD14. We have also identified a possible ligand gating mechanism consistent with available NMR data, and key sites that may be essential for LPS/lipid A binding which may ultimately be targeted by novel anti-septic drugs.