X-ray structural investigations of fusion intermediates: Lipid model systems and beyond.

We review recent X-ray diffraction studies of fusion intermediate structures in lipid model membranes. We illustrate what kind of information can be derived on the stalk structure as well as on the energetics of stalk formation. The role of different lipids is elucidated by a structural fusion assay, highlighting the significant modulation of fusion energetics by the lipid stoichiometry. Extending beyond the pure lipid models, we also include recent work probing the interaction of lipid bilayers and synaptic vesicles, from a structural point of view. The review closes with an outlook on X-ray imaging of lipid bilayers as a possible new experimental method to study membrane fusion.