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Complement Factor H and Simian Virus 40 bind the GM1 ganglioside in distinct conformations.
Mammalian cell surfaces are decorated with a variety of glycan chains that orchestrate development and defense and are exploited by pathogens for cellular attachment and entry. While glycosidic linkages are, in principle, flexible, the conformational space that a given glycan can sample is subject to spatial and electrostatic restrictions imposed by its overall chemical structure. Here, we show how the glycan moiety of the GM1 ganglioside, a branched, monosialylated pentasaccharide that serves as a ligand for various proteins, undergoes differential conformational selection in its interactions with different lectins. Using STD NMR and X-ray crystallography, we found that the innate immune regulator Complement Factor H (FH) binds a previously not reported GM1 conformation that is not compatible with the GM1 binding sites of other structurally characterized GM1-binding lectins such as the Simian Virus 40 (SV40) capsid. Molecular dynamics simulations of the free glycan in explicit solvent on the ten microsecond timescale reveal that the FH-bound conformation nevertheless corresponds to a minimum in the Gibbs free energy plot. In contrast to the GM1 conformation recognized by SV40 the FH-bound GM1 conformation is associated with poor NOE restraints, explaining how it escaped (1)H-(1)H NOE restrained modeling in the past and highlighting the necessity for ensemble representations of glycan structures.
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