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Immunobiology. 2006;211(6-8):437-47. Epub 2006 Jul 18.


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  • 1Division of Infectious Diseases and Immunology, University of Massachusetts Medical School, Worcester, MA 01605, USA.


Toll-like receptors (TLRs) are a small family of type-I glycoproteins that bind to and are activated by conserved non-self molecular signatures carried by microorganisms. Toll-like receptor 4 is triggered by most lipopolysaccharides (LPS). LPS is a complex amphipathic saccharolipidic glycan derived from Gram-negative bacteria. Unique among TLRs, TLR4 activity and interaction with its natural ligand(s) strictly depends on the presence of the extracellular adaptor MD-2. MD-2 is a small secreted glycoprotein that binds with cytokine-like affinities to both the hydrophobic portion of LPS and to the extracellular domain of TLR4. The interaction between MD-2 and LPS induces a triggering event on TLR4, which involves the molecular rearrangement of the receptor complex and its homotypic aggregation. In silico analysis suggests that MD-2 and MD-1 are paralogs derived from a common predecessor at the level of early vertebrates. In this review, we summarize the current state of knowledge concerning MD-2.

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