Capsule and lipopolysaccharide - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 530

in Munford, 2008 ; Bryant et al., 2010 ). TLR4 was the first member of the
Toll-like receptor family of pattern recognition receptors to be described
( Bryant et al., 2010 ). It is a type 1 transmembrane protein found on the sur-
face of cells of the immune system, including monocytes and macrophages,
as well as non-immune cells, such as epithelial and endothelial cells. The
extracellular domain of TLR4 contains multiple leucine-rich repeats and these
motifs confer the characteristic curved shape found in TLR4 and other related
proteins ( Figure 17.5 ). The cytoplasmic Toll-interleukin-1 receptor (TIR)
domain of TLR4 activates downstream signaling pathways in the context of
LPS binding. In order to perform this function, TLR4 forms a complex with
MD2. MD2 is a member of a family of lipid-binding proteins and provides a
soluble co-receptor for LPS ( Inohara and Nunez, 2002 ). In mice, both TLR4
and MD2 are required to raise an immune response to LPS and mice lacking
either protein are resistant to endotoxic shock.
The crystal structure of TLR4-MD2 in complex with an LPS molecule
was invaluable in elucidating the details of LPS binding and TLR4 activa-
tion ( Figure 17.5 ) ( Park et al., 2009 ). Previous crystal structures of MD2 and
TLR4-MD2 in complex with lipid IV A and eritoran, respectively, revealed
an LPS binding pocket in the β-sandwich fold of MD2 ( Kim et al., 2007 ;
Ohto et al., 2007 ). However, these antagonist structures only have four acyl
chains and do not result in an activating conformational change in the recep-
tor. In contrast, hexaacyl LPS promotes dimerization of TLR4-MD2 in an
activated complex. The MD2 LPS-binding pocket cannot accommodate six
acyl chains, leaving one (at C2) exposed, where it contributes to a hydropho-
bic region that forms a dimerization interface with TLR4. This repositions
the diglucosamine backbone, allowing the phosphates to interact with each
of the TLR4 molecules in the complex and the inner core Kdo and heptose
FIGURE 17.5 Crystal structure of the TLR4-MD2-LPS complex from PDB 3FXI ( Park et al.,
2009 ). The two TLR4 monomers are in green and magenta, the MD2 proteins are in blue and the
LPS molecule is shown as spheres with phosphates colored orange. Five of the six acyl chains are
bound in the MD2 pocket while the sixth forms an interaction with both MD2 and TLR4. Only the
extracellular domain of TLR4 was crystallized; in the full-length protein the C-terminal transmem-
brane and TIR effector domains would be located at the base of the structure.