Molecule of the Month: Toll-like Receptors
Toll-like receptors warn us about bacterial and viral infection
Sending the Signal
Friend or Foe
Exploring the Structure
Toll-like Receptor (PDB entry 2z7x)
The distinctive horseshoe shape of Toll-like receptors is formed by a repeating amino acid sequence, termed a "leucine-rich repeat." This motif is used in many other proteins, and particularly in proteins that bind to other proteins. In some cases, such as ribonuclease inhibitor (PDB entry 1dfj , not shown here), the protein binds where you might expect: inside the horseshoe. In the Toll-like receptors, however, the target molecules bind in a pocket on the side of the horseshoe. This is particularly apparent in the Toll-like receptors that recognize bacterial lipoproteins, shown here from PDB entry 2z7x , where a deep pocket in the receptor encloses the lipid chains. To take a closer look at this structure, click on the image for an interactive Jmol.
Topics for Further Discussion
- Several structures are available for the monomeric form of Toll-like receptors, before they have encountered their target ligands. For instance, compare the monomeric form of TLR2 in PDB entry 3a7c with the active dimer in PDB entry 2z7x.
- Ribonuclease inhibitor also contains leucine-rich repeats that form a horseshoe-shaped protein. However, the curve is much tighter in ribonuclease inhibitor. Use a cartoon or backbone representation to compare the secondary structure of TLR2 in 3a7c and ribonuclease inhibitor in 1dfj, and notice how the alpha helices on the outer edge force the protein into a tighter curve.
November 2011, David Goodsellhttp://doi.org/10.2210/rcsb_pdb/mom_2011_11