Human M-ficolin is a pathogen-associated molecular recognition molecule in the innate immune system, and it binds to some sugars, such as GlcNAc (N-acetylglucosamine), on pathogen surfaces. From previous structural and functional studies of the FD1 (M-ficolin fibrinogen-like domain), we proposed that the ligand-binding region of FD1 exists in a conformational equilibrium between active and non-active states depending on three groups with a pKa of 6.2, which are probably histidine residues, and suggested that the 2-state conformational equilibrium as well as the trimer formation contributes to the discrimination mechanism between self and non-self of FD1 [Tanio, M., Kondo, S., Sugio, S. and Kohno, T. (2007) J. Biol. Chem. 282, 3889–3895]. To investigate the origins of the pH dependency, mutational analyses were performed on FD1 expressed by Brevibacillus choshinensis. The GlcNAc binding study of a series of single histidine mutants of FD1 demonstrated that His251, His284 and His297 are required for the activity, and thus we concluded that the three histidines are the origins of the pH dependency of FD1. Monomeric mutants of FD1 show weaker affinity for the ligand than the trimeric wild-type, indicating that trimer formation confers high avidity for the ligand. In addition, analyses of the GlcNAc association and dissociation of FD1 provided evidence that FD1 always exchanges between the active and non-active states with the pH-dependent populations in solution. The biological roles of the histidine-regulated conformational equilibrium of M-ficolin are discussed in terms of the self and non-self discrimination mechanism.
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Research Article|
December 23 2008
Histidine-regulated activity of M-ficolin
Michikazu Tanio;
Michikazu Tanio
1Mitsubishi Kagaku Institute of Life Sciences (MITILS), 11 Minamiooya, Machida, Tokyo 194-8511, Japan
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Toshiyuki Kohno
Toshiyuki Kohno
1
1Mitsubishi Kagaku Institute of Life Sciences (MITILS), 11 Minamiooya, Machida, Tokyo 194-8511, Japan
1To whom correspondence should be addressed (email tkohno@mitils.jp).
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Publisher: Portland Press Ltd
Received:
August 11 2008
Revision Received:
September 26 2008
Accepted:
October 01 2008
Accepted Manuscript online:
October 01 2008
Online ISSN: 1470-8728
Print ISSN: 0264-6021
© The Authors Journal compilation © 2009 Biochemical Society
2009
Biochem J (2009) 417 (2): 485–491.
Article history
Received:
August 11 2008
Revision Received:
September 26 2008
Accepted:
October 01 2008
Accepted Manuscript online:
October 01 2008
Citation
Michikazu Tanio, Toshiyuki Kohno; Histidine-regulated activity of M-ficolin. Biochem J 15 January 2009; 417 (2): 485–491. doi: https://doi.org/10.1042/BJ20081640
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