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Structure of the Month

Structure of the Month: May 2009 [see all]

The RIG-I like Receptor LGP2 Recognizes the Termini of Double-stranded RNA

Xiaojun Li¹, C. T. Ranjith-Kumar², Monica T. Brooks³, S. Dharmaiah², Andrew B. Herr³, Cheng Kao², Pingwei Li^1*

Department of Biochemistry and Biophysics, Texas A&M University, College Station, TX 77843-2128
Department of Biology and the Multidisciplinary Biochemistry Program, Indiana University, Bloomington IN 47405
Department of Molecular Genetics, Biochemistry, and Microbiology, University of Cincinnati College of Medicine, Cincinnati, OH 45267-0524

The innate immune response is the first line of defense against invading pathogens; it is the ubiquitous system of defense against microbial infections (1). Toll-like receptors (TLRs) and RIG-I-like receptors (RLRs) play key roles in innate immune response toward viral infection (2-5). The RIG-I-like receptors (RLRs), RIG-I and MDA5, recognize single-stranded RNA with 5' triphosphates (5' ppp ssRNA) and double-stranded RNA (dsRNA) to initiate innate antiviral immune responses. LGP2, a homolog of RIG-I and MDA5 that lacks signaling capability, regulates the signaling of the RLRs.

To establish the structural basis of dsRNA recognition by the RLRs, we have determined the 2.0 Å resolution crystal structure of human LGP2 C-terminal domain (CTD) bound to an 8 bp dsRNA. Two LGP2 CTD molecules bind to the termini of dsRNA with minimal contacts between the protein molecules. The complete details of this work were recently published in the Journal of Biological Chemistry, Vol. 284(20):13881-13891.

An intriguing possibility presented by the recognition of LGP2 to the two ends of blunt-ended dsRNA is that this recognition could allow for a response to the concentration of viral dsRNAs independent of the lengths of the RNAs. Future structural studies of full-length RIG-I or LGP2 in isolation and in complex with longer RNA molecules should reveal additional mechanisms of RLR activation and regulation.

This work was performed in the laboratory of Pingwei Li, in the Department of Biochemistry and Biophysics at Texas A&M University. The lab focuses on elucidating the structural basis of viral RNA sensing by the RIG-I like receptors in innate antiviral immune responses. A combination of biochemical and structural approaches are used to study the structure and function of this family of receptors.


Figure 1: Crystals of LGP2/dsRNA complex grown with 16-18% (w/v) PEG3350 in a buffer containing 0.2 M (NH₄)₂SO₄, 0.1 M Tris-HCl at pH 8.5.		Figure 2: Diffraction pattern of LGP2 CTD dsRNA complex crystal.

Figure 3: 2|Fo|-|Fc| map of LGP2/dsRNA complex.

Figure 4: Structure of human LGP2 C-terminal domain in complex with an 8 bp dsRNA. Two LGP2 CTD bound to the termini of an 8 bp blunt-ended dsRNA in the crystallographic asymmetric unit. The two protein molecules are shown as cyan (LGP2, A) and green (LGP2, B) ribbons. The dsRNA is shown as stick models. The zinc ions in LGP2 CTD are shown as gray spheres.

Data collection details

PDB ID	3EQT
Space group	C2
Unit cell	a =116.46 Å, b = 54.19 Å, c = 67.20 Å β = 97.26°
Radiation	Cu Kα
Generator	MicroMax-007 HF
Optic	VariMax HR
Detector	R-AXIS IV++
Crystal-to-detector distance	150 mm
Exposure time per frame	10 min
Oscillation width	1°
Number of frames	180
Data Processing	HKL-2000®
Resolution range	50.0 – 2.0 Å

References

Janeway, C. A., Jr., and Medzhitov, R. (2002) Annu Rev Immunol 20, 197-216.
Akira, S., Uematsu, S., and Takeuchi, O. (2006) Cell 124(4), 783-801.
Yoneyama, M., and Fujita, T. (2007) J Biol Chem 282(21), 15315-15318.
Pichlmair, A., and Reis e Sousa, C. (2007) Immunity 27(3), 370-383.
Thompson, A. J., and Locarnini, S. A. (2007) Immunol Cell Biol 85(6), 435-445.

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