Interaction of Calmodulin with Striatin, a WD-repeat Protein Present in Neuronal Dendritic Spines

Rat striatin, a quantitatively minor protein belonging to the WD-repeat family of proteins, is a Ca2+/calmodulin-binding protein mostly expressed in the striatum and in the motor and olfactory systems (Castets, F., Bartoli, M., Barnier, J. V., Baillat, G., Salin, P., Moqrich, A., Bourgeois, J. P., Denizot, F., Rougon, G., Calothy, G., and Monneron, A. (1996) J. Cell. Biol. 134, 1051–1062). Generally associated with membranes, striatin is mostly found in dendritic spines where it is likely to play a role in Ca2+-signaling events. In this paper, we characterize its calmodulin-binding properties. By using deletion mapping and site-directed mutagenesis, we identified the sequence located between amino acids 149 and 166 as the main calmodulin-binding site. The predicted corresponding peptide is potentially able to form a basic amphiphilic helix, as is often the case for many known calmodulin-binding sites. Calmodulin binding to striatin is Ca2+-dependent, with half-maximal binding occurring around 0.5 μm free Ca2+. In the presence of Ca2+, the equilibrium dissociation constant of calmodulin/striatin fusion protein complex is 40 ± 5 nm. We also show that brain striatin subcellular localization, as studied by tissue fractionation, is Ca2+-dependent, this effect being probably mediated by calmodulin. Our results are in agreement with the hypothesis that striatin is a transducer involved in Ca2+signaling or an adapter protein involved in regulating macromolecular assemblies within dendritic spines.