Versatility of a Glycosylphosphatidylinositol Fragment in Forming Highly Ordered Polymorphs

Glycosylphosphatidylinositols (GPIs) are often attributed with the ability to associate with the organized membrane microdomains. GPI fragment <b>1</b> forms a highly ordered subgel-phase structure characterized by ordering of both headgroups and alkyl chains in thin layers. While investigating the driving forces behind the formation of these ordered monolayers, we have studied polymorphism of <b>1</b> under different conditions employing surface-sensitive X-ray diffraction methods. Three distinct polymorphs of <b>1</b> (<b>I</b>, <b>II</b>, and <b>III</b>) were identified and characterized by grazing incidence X-ray diffraction. Polymorphs <b>II</b> (a condensed monolayer structure) and <b>III</b> (highly ordered subgel phase) coexist on an 8 M urea solution subphase allowing for a detailed thermodynamic and kinetic analysis of the processes leading to the formation of these polymorphs. They are enantiotropic and can be directly interconverted by changes in temperature or lateral surface pressure. As a consequence, polymorph <b>III</b> nuclei of critical size (or larger) could be formed by density fluctuations in a multicomponent system, and they could continue to exist for a period of time even under conditions that would normally not allow for the nucleation of polymorph <b>III</b>. The processes described here could also lead to the formation of patches of highly ordered structures in a disordered environment of a cell membrane suggesting that GPIs may play a role in the formation of such domains

Molecular Recognition of Complex-Type Biantennary <i>N</i>‑Glycans by Protein Receptors: a Three-Dimensional View on Epitope Selection by NMR

The current surge in defining glycobiomarkers by applying lectins rekindles interest in definition of the sugar-binding sites of lectins at high resolution. Natural complex-type <i>N</i>-glycans can present more than one potential binding motif, posing the question of the actual mode of interaction when interpreting, for example, lectin array data. By strategically combining <i>N</i>-glycan preparation with saturation-transfer difference NMR and modeling, we illustrate that epitope recognition depends on the structural context of both the sugar and the lectin (here, wheat germ agglutinin and a single hevein domain) and cannot always be predicted from simplified model systems studied in the solid state. We also monitor branch-end substitutions by this strategy and describe a three-dimensional structure that accounts for the accommodation of the α2,6-sialyl­ated terminus of a biantennary <i>N</i>-glycan by viscumin. In addition, we provide a structural explanation for the role of terminal α2,6-sialyl­ation in precluding the interaction of natural <i>N</i>-glycans with lectin from Maackia amurensis. The approach described is thus capable of pinpointing lectin-binding motifs in natural <i>N</i>-glycans and providing detailed structural explanations for lectin selectivity