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The structure of a tunicate C-type lectin from polyandrocarpa misakiensis complexed with d-galactose11Edited by R. Huber

Authors :
Poget, Se´bastien F.
Legge, Glen B.
Proctor, Mark R.
Butler, P. Jonathan G.
Bycroft, Mark
Williams, Roger L.
Source :
JMB Online (Journal of Molecular Biology); July 23, 1999, Vol. 290 Issue: 4 p867-879, 13p
Publication Year :
1999

Abstract

C-type lectins are calcium-dependent carbohydrate-recognising proteins. Isothermal titration calorimetry of the C-type Polyandrocarpa lectin (TC14) from the tunicate Polyandrocarpa misakiensis revealed the presence of a single calcium atom per monomer with a dissociation constant of 2.6 μM, and confirmed the specificity of TC14 for d-galactose and related monosaccharides. We have determined the 2.2 A˚ X-ray crystal structure of Polyandrocarpa lectin complexed with d-galactose. Analytical ultracentrifugation revealed that TC14 behaves as a dimer in solution. This is reflected by the presence of two molecules in the asymmetric unit with the dimeric interface formed by antiparallel pairing of the two N-terminal β-strands and hydrophobic interactions. TC14 adopts a typical C-type lectin fold with differences in structure from other C-type lectins mainly in the diverse loop regions and in the second α-helix, which is involved in the formation of the dimeric interface. The d-galactose is bound through coordination of the 3 and 4-hydroxyl oxygen atoms with a bound calcium atom. Additional hydrogen bonds are formed directly between serine, aspartate and glutamate side-chains of the protein and the sugar 3 and 4-hydroxyl groups. Comparison of the galactose binding by TC14 with the mannose binding by rat mannose-binding protein reveals how monosaccharide specificity is achieved in this lectin. A tryptophan side-chain close to the binding site and the distribution of hydrogen-bond acceptors and donors around the 3 and 4-hydroxyl groups of the sugar are essential determinants of specificity. These elements are, however, arranged in a very different way than in an engineered galactose-specific mutant of MBPA. Possible biological functions can more easily be understood from the fact that TC14 is a dimer under physiological conditions.

Details

Language :
English
ISSN :
00222836 and 10898638
Volume :
290
Issue :
4
Database :
Supplemental Index
Journal :
JMB Online (Journal of Molecular Biology)
Publication Type :
Periodical
Accession number :
ejs746878
Full Text :
https://doi.org/10.1006/jmbi.1999.2910