An ATP binding site regulates synapsin IIa function

The synapsins are neuron-specific proteins mostly known fortheir capability to cluster synaptic vesicles and to control vesiclemovement between adjacent en-passant synapses. Early studiesrevealed various sites that regulate the activity of synapsin I, the firstsynapsin to be described. Although synapsin IIa was recently shownto be a dominant member of this family in hippocampal glutama-tergic synapses, its regulation has not been studied to date. Allmembers of the synapsin family bind ATP at a well-conserved site in their central “C” domain. Intriguingly, ATP binding is differen-tially regulated by calcium ions, suggesting a physiologicalfunction, but its significance has not been investigated in thecontext of neurotransmission. To study this subject, we measuredthe rescue of synaptic depression in autaptic neurons lacking allsynapsins when overexpressing either exogenous wild type synapsinIIa or synapsin IIa carrying a mutation (K270A) which renders itincapable of binding ATP. We found that while K270A-SynIIa fullyrescued the clustering of vesicles at the presynaptic terminal, it onlypartially rescued synaptic depression. We infer that K270A-SynIIacan maintain vesicles in proximity to the release site, but its abilityto couple synaptic activity to vesicle mobilization is compromised.This result is consistent with ATP-binding playing a key role inregulating the mobilization of vesicles for release under conditionsof intense neuronal activity.