Mechanistic Insights into Synaptotagmin-1 Mediated Membrane Fusion and Interactions

We present two innovative approaches to investigate the dynamics of membrane fusion and the strength of protein-membrane interactions. The first approach employs pore-spanning membranes (PSMs), which allow for the observation of protein-assisted fusion processes. The second approach utilizes colloidal probe microscopy with membrane-coated probes with reconstituted proteins. PSMs enable one to obtain detailed information about the fusion process with particular emphasis on fusion intermediates and fusion pore formation. We demonstrate the potential of the PSM system using SNARE-mediated fusion. Accompanied by colloidal probe microscopy, molecular information can be gathered on how full-length synaptotagmin-1 (syt-1) contributes to the fusion process. We propose that syt-1 engages with anionic bilayers, significantly modifying the adhesion between membranes. The introduction of Ca²⁺ transforms these interactions, shifting from a state of minimal interaction force between bilayers to one of pronounced strength. This syt-1 interaction facilitates fusion in the presence of Ca²⁺ with a significant reduction in the occurrence of stalled intermediate fusion states. Moreover, the presence of Ca²⁺ significantly accelerates the fusion process, an effect that is further amplified by the addition of multivalent anions such as ATP.