An unusual dimeric structure and assembly for RP105–MD-1 a regulator for the TLR4 response to LPS

摘要

RP105–MD-1 modulates the TLR4–MD-2-mediated, innate immune response against bacterial lipopolysaccharide (LPS). The crystal structure of the bovine 1:1 RP105–MD-1 complex bound to a putative endogenous lipid at 2.9 Å resolution shares a similar overall architecture to its homologue TLR4–MD-2, but assembles into an unusual 2:2 homodimer which differs from any other known TLR-ligand assembly. The homodimer is assembled in a head-to-head orientation that juxtaposes the N-terminal leucine-rich repeats (LRRs) of the two RP105 chains, rather than the usual tail-to-tail configuration of C-terminal LRRs in ligand-activated TLR dimers, such as TLR1–2, 3 and 4. Another novel interaction is mediated by an RP105-specific Asn-linked glycan, which wedges MD-1 into the co-receptor binding concavity on RP105. This unique mode of assembly in RP105–MD-1 represents a new paradigm for TLR complexes and suggests a potential molecular mechanism for regulating LPS responses.