Biochem. J. (2004) 378
(399–407) (Printed in Great Britain)
Crystal structure of a platelet-agglutinating factor isolated from the venom of Taiwan habu (Trimeresurus mucrosquamatus)
Kai-Fa HUANG*†, Tzu-Ping KO*, Chin-Chun HUNG*, John CHU*, Andrew H.-J. WANG*†1 and Shyh-Horng CHIOU*†1
*Institute of Biological Chemistry, Academia Sinica, Nankang, Taipei 11529, Taiwan, and †Institute of Biochemical Sciences, National Taiwan University, Taipei 106, Taiwan
Platelet glycoprotein Ib (GPIb)-binding proteins (GPIb-BPs) from snake venoms are usually C-type lectins, which target specific sites of GPIba and elicit distinct effects on platelets. In the present paper, we report a tetrameric platelet-agglutinating factor (molecular mass 121.1 kDa), termed mucrocetin, purified from the venom of Taiwan habu (Trimeresurus mucrosquamatus). Mucrocetin is a GPIba agonist with a binding site distinct from that of flavocetin-A (a snake venom GPIba antagonist) on GPIba, in spite of the high sequence identity (94.6%) between the two venom lectins. The crystal structure of mucrocetin was solved and refined to 2.8 Å (1 Å=0.1 nm) resolution, which shows an interesting crystal packing of six-layer cylinders of doughnut-shaped molecules. The four ab heterodimers are arranged in an unusual square-shaped ring stabilized by four interdimer ‘head-to-tail’ disulphide bridges. Detailed structural comparison between mucrocetin and flavocetin-A suggests that their disparate platelet effects are probably attributable to different charge distributions on the putative concave binding surface. A unique positively charged patch on the binding surface of mucrocetin, formed by Lys102, Lys108, Lys109 and Arg123 in the a-subunit coupled with Lys22, Lys102, Lys116 and Arg117 in the b-subunit, appears to be the primary determinant of its platelet-agglutinating activity. Conceivably, this interesting venom factor may provide a useful tool to study platelet agglutination by binding to the GPIb–IX–V complex.
Key words: flavocetin-A, lectin-like protein, mucrocetin, platelet glycoprotein Ib, snake venom, Trimeresurus mucrosquamatus.
Abbreviations used: CRD, carbohydrate-recognition domain; FL-A, flavocetin-A; GPIb, glycoprotein Ib; GPIb-BP, GPIb-binding protein; mAb, monoclonal antibody; NCS, non-crystallographic symmetry; PRP, platelet-rich plasma; r.m.s.d., root mean square deviation; vWf, von Willebrand factor.
1To whom correspondence should be addressed (e-mail ahjwang@gate.sinica.edu.tw or shchiou@gate.sinica.edu.tw).
2The nucleotide sequences for the mucrocetin a- and b-subunits have been deposited in the DDBJ, EMBL, GenBank® and GSDB Nucleotide Sequence Databases under the accession numbers AY390533 and AY390534 respectively. The atomic co-ordinates of the mucrocetin structure have been deposited at Research Collaboratory for Structural Bioinformatics (RCSB) Protein Data Bank under the accession number 1V4L.
Received 2 October 2003/10 November 2003; accepted 13 November 2003
Published as BJ Immediate Publication 13 November 2003, DOI 10.1042/BJ20031507
The Biochemical Society, London ©2004
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