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Abstract

Synaptic vesicles (SVs) are small, membrane-bound organelles that are found in the synaptic terminal of neurons. Although tremendous progress has been made in understanding the protein machinery that drives fusion of SVs with the presyn- aptic membrane, little progress has been made in understanding changes in the membrane structure that accompany this process. We used lipid monolayers of defined composition to mimic biological membranes, which were probed by x-ray reflec- tivity and grazing incidence x-ray diffraction. These techniques allowed us to successfully monitor structural changes in the membranes at molecular level, both in response to injection of SVs in the subphase below the monolayer, as well as to phys- iological cues involved in neurotransmitter release, such as increases in the concentration of the membrane lipid PIP 2 , or addi- tion of physiological levels of Ca 2 þ . Such structural changes may well modulate vesicle fusion in vivo.