Fractional Binding: A Molecular Analog-To-Digital Converter in Ca++ Regulated Vesicle Differentiation

A critical feature of neurotransmission is that only a subset of vesicles react at a given Ca++ pulse. To achieve such a fractional response the synapse utilises a number of complex spatio-temporal and biochemical mechanisms that are still under debate but collectively appear to create distinct vesicle pools. We currently ignore whether differentiation of seemingly identical vesicles exists also for membrane trafficking vesicles.We recently established that vesicles within an ensemble display large compositional inhomogeneities(1). Annexins, as well as other Ca2+-dependent membrane binding proteins, are known to bind specific phospholipid components in cellular membranes(2). Here we investigated how the unique capability of Ca2+-regulated membrane-binding proteins to bind certain lipids enables them to discriminate between single vesicles based on their lipid compositional heterogeneity. We employed human Annexin A5 (AnxA5), which is widely used as a model system for the membrane binding properties of the Annexin superfamily(2), and our previously described single vesicle assay(3, 4). We demonstrate that heterogeneities in the lipid composition of individual vesicles within a population, in conjunction with Ca2+ concentration, regulate the fraction of vesicles recruiting Annexins with digital precision. This is a recurrent phenomenon observed in multiple vesicle samples and for different Ca2+ sensing proteins. Thus, we anticipate that the cell utilize this feature to translate analog Ca2+ influxes into a selective digital targeting of vesicle sub-populations within the ensemble for specific functions.1. J. Am. Chem. Soc. 133: 10685-10687.2. Nat Rev Mol Cell Biol. 6: 449-461.3. The EMBO Journal. 28: 3303-3314.4. Nat. Chem. Biol. 5: 835-841.