Direct, Ca2+-dependent Interaction between Tubulin and Synaptotagmin I

The synaptic vesicle protein synaptotagmin I probably plays important roles in the synaptic vesicle cycle. However, the mechanisms of its action remain unclear. In this study, we have searched for cytoplasmic proteins that interact with synaptotagmin I. We found that the cytoskeletal protein tubulin directly and stoichiometrically bound to recombinant synaptotagmin I. The binding depended on mm Ca2+, and 1 mol of tubulin dimer bound 2 mol of synaptotagmin I with half-maximal binding at 6.6 μm tubulin. The Ca2+ dependence mainly resulted from Ca2+ binding to the Ca2+ ligands of synaptotagmin I. The C-terminal region of β-tubulin and both C2 domains of synaptotagmin I were involved in the binding. The YVK motif in the C2 domains of synaptotagmin I was essential for tubulin binding. Tubulin and synaptotagmin I were co-precipitated from the synaptosome extract with monoclonal antibodies to tubulin and SNAP-25 (synaptosome-associated protein of 25 kDa), indicating the presence of tubulin/synaptotagmin I complex and tubulin binding to synaptotagmin I in SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complexes. Synaptotagmin I promoted tubulin polymerization and bundled microtubules in the presence of Ca2+. These results suggest that direct interaction between synaptotagmin I and tubulin provides a mechanism for attaching synaptic vesicles to microtubules in high Ca2+ concentrations.