Insights into the structural basis of the pH-dependent dimer-tetramer equilibrium through crystallographic analysis of recombinant Diocleinae lectins
Résumé
The structural ground underlying the pH-dependency of the dimer-tetramer transition of Diocleinae lectins was investigated by equilibrium sedimentation and X-ray crystal structure determination of wild-type and site-directed mutants of recombinant lectins. Synthetic genes coding for the full-length α-chains of the seed lectins of Dioclea guianensis (r-αDguia) and Dioclea grandiflora (r-αDGL) were designed and expressed in E. coli. This pioneer approach, which will be described in detail, yielded recombinant lectins displaying carbohydrate-binding activity, dimer-tetramer equilibria, and crystal structures indistinguisable from their natural homologues. Conversion of the pH-stable tetrameric r-αDGL into a structure exhibiting pH-dependent dimer-tetramer transition was accomplished through mutations that abolished the interdimeric interactions at the central cavity of the tetrameric lectins. Both, the central and the peripheral interacting regions bear structural information for formation of the canonical legume lectin tetramer. We hypothesize that the strength of the ionic contacts at these sites may be modulated by the pH, leading to dissociation of those lectin structures that are not locked into a pH-stable tetramer through interdimeric contacs networking the central cavity loops.
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