Solution structure of the silkworm βGRP/GNBP3 N-terminal domain reveals the mechanism for β-1,3-glucan-specific recognition

doi:10.1073/pnas.0901671106

Solution structure of the silkworm βGRP/GNBP3 N-terminal domain reveals the mechanism for β-1,3-glucan-specific recognition

^aDepartment of Structural Biology, Graduate School of Pharmaceutical Sciences, Hokkaido University, N-21, W-11, Kita-ku, Sapporo 001-0021, Japan; and
^bInstitute of Low Temperature Science, Hokkaido University, N-19, W-8, kita-ku, Sapporo 060-0819, Japan

Edited by John H. Law, University of Georgia, Athens, GA, and approved May 15, 2009
↵¹K.T. and M.O. contributed equally to this work. (received for review February 12, 2009)

Abstract

The β-1,3-glucan recognition protein (βGRP)/Gram-negative bacteria-binding protein 3 (GNBP3) is a crucial pattern-recognition receptor that specifically binds β-1,3-glucan, a component of fungal cell walls. It evokes innate immunity against fungi through activation of the prophenoloxidase (proPO) cascade and Toll pathway in invertebrates. The βGRP consists of an N-terminal β-1,3-glucan-recognition domain and a C-terminal glucanase-like domain, with the former reported to be responsible for the proPO cascade activation. This report shows the solution structure of the N-terminal β-1,3-glucan recognition domain of silkworm βGRP. Although the N-terminal domain of βGRP has a β-sandwich fold, often seen in carbohydrate-binding modules, both NMR titration experiments and mutational analysis showed that βGRP has a binding mechanism which is distinct from those observed in previously reported carbohydarate-binding domains. Our results suggest that βGRP is a β-1,3-glucan-recognition protein that specifically recognizes a triple-helical structure of β-1,3-glucan.

Footnotes

²To whom correspondence should be addressed. E-mail: finagaki{at}pharm.hokudai.ac.jp
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: The atomic coordinates have been deposited in the Protein Data Bank, www.pdb.org (PDB ID code 2RQE).
This article contains supporting information online at www.pnas.org/cgi/content/full/0901671106/DCSupplemental.