Studies of the gelsolin(150--169)-phosphatidylinositol 4,5-bisphosphate interaction and the filamentous actin-magnesium ion interaction

A 20-amino acid segment, G(150-169), of the actin-binding protein gelsolin is the binding site for both filamentous actin (F-actin) and the phospholipid phosphatidyl-inositol 4,5-bisphosphate (PIP$\sb2$). The goal of this work is to study the interaction between G(150-169) and PIP$\sb2$. Circular dichroism (CD) experiments show that G(150-169) undergoes a coil-helix transition in the presence of PIP$\sb2$ and in TFE-H$\sb2$O co-solvent. The TFE-induced helical structure of the peptide is studied as a model for the PIP$\sb2$-induced structure using two-dimensional NMR spectroscopy. The region from Pro-154 to Lys-166 is found to be helical, and the N-terminus forms a turn. SDS micelle solution is then used as a model lipid environment. The structure of G(150-169) in SDS micelle solution studied by NMR is somewhat different from that in TFE co-solvent, and the peptide is involved in hydrophobic interactions with SDS micelles. Finally, the direct interaction between G(150-169) and PIP$\sb2$ is studied using mixed SDS/PIP$\sb2$ micelle solution. The peptide structure is found to be similar to that observed in SDS micelle solution. The effect of the binding of G(150-169) on the esterphosphate groups of PIP$\sb2$ is measured by the $\sp{31}$P longitudinal relaxation time. The monoesterphosphates on the inositol ring are immobilized by the binding of the peptide, whereas the diesterphosphate is not at all affected. These results explain the specificity of the binding of PIP$\sb2$ to G(150-169). The interaction between Mg$\sp{2+}$ and actin filaments is studied by $\sp{25}$Mg NMR. The dynamics of the Mg$\sp{2+}$ ions in actin filament solution demonstrate that F-actin possesses polyelectrolyte properties. The displacement of Mg$\sp{2+}$ by Co(NH$\sb3)\sb6\sp{3+}$ suggests that the binding of Mg$\sp{2+}$ to F-actin is non-specific