A polar SxxS motif drives assembly of the transmembrane domains of Toll-like receptor 4.

Like all members of the Toll-like receptor (TLR) family, TLR4 comprises of a large ectodomain (ECD) involved in ligand recognition at the cell-surface, and a cytosolic Toll/interleukin-1 receptor (TIR) signalling domain, linked by a lipid membrane-anchored transmembrane (TM) domain (TMD). Binding of immunostimulatory pathogen-associated molecular patterns (PAMPs) such as bacterial lipopolysaccharide (LPS) to myeloid differentiation factor 2 (MD-2) coreceptor-complexed TLR4 leads to its dimerization, resulting in productive juxtaposition of TIR domains and the initiation of pro-inflammatory innate immune responses. Whilst the process of PAMP recognition is relatively well understood, thanks to numerous high-resolution crystallographic structures of ECDs, the mechanism by which such recognition is translated into TMD dimerization and activating conformational changes is less clear. Based on available biophysical and biochemical experimental data, ab initio modelling, and multiscale molecular dynamics (MD) simulations entailing a total of >13μs and >200μs of atomistic and coarse-grained sampling, respectively, we investigate the structural basis for TLR4 TMD dimerization within a biologically relevant lipid membrane environment. A key polar-xx-polar (SxxS) motif is shown to drive association of the TLR4 TMDs, and to maintain a flexible interface, which may be disrupted by selected point mutations. Furthermore, MD simulations of various TMD+ECD constructs have been used to investigate the coupling between domains, revealing that flexible linkers abrogate dimerization via aggregation of ECDs at the membrane surface, explaining previous biochemical observations. These results improve our understanding of the assembly and signalling mechanisms of TLR4, and pave the way for rational structure-based development of membrane-associated immunomodulatory molecules.