Tetramerization and ATP binding by a protein comprising the A, B, and C domains of rat synapsin I

Synapsins are multidomain proteins that are critical for regulating neurotransmitter release in vertebrates. In the present study, two crystal structures of the C domain of rat synapsin I (rSynI-C) in complex with Ca(2+) and ATP reveal that this protein can form a tetramer and that a flexible loop (the "multifunctional loop") contacts bound ATP. Further experiments were carried out on a protein comprising the A, B, and C domains of rat synapsin I (rSynI-ABC). An ATP-stabilized tetramer of rSynI-ABC is observed during velocity sedimentation and size-exclusion chromatographic experiments. These hydrodynamic results also indicate that the A and B domains exist in an extended conformation. Calorimetric measurements of ATP binding to wild-type and mutant rSynI-ABC demonstrate that the multifunctional loop and a cross-tetramer contact are important for ATP binding. The evidence supports a view of synapsin I as an ATP-utilizing, tetrameric protein made up of monomers that have a flexible, extended N terminus.