Spectral studies of the Ca2+-dependent interaction of trifluoperazine with S100b

Phenothiazines inhibit S100A4 function by inducing protein oligomerization

S100A4, a member of the S100 family of Ca(2+)-binding proteins, regulates carcinoma cell motility via interactions with myosin-IIA. Numerous studies indicate that S100A4 is not simply a marker for metastatic disease, but rather has a direct role in metastatic progression. These observations suggest that S100A4 is an excellent target for therapeutic intervention. Using a unique biosensor-based assay, trifluoperazine (TFP) was identified as an inhibitor that disrupts the S100A4/myosin-IIA interaction. To examine the interaction of S100A4 with TFP, we determined the 2.3 A crystal structure of human Ca(2+)-S100A4 bound to TFP. Two TFP molecules bind within the hydrophobic target binding pocket of Ca(2+)-S100A4 with no significant conformational changes observed in the protein upon complex formation. NMR chemical shift perturbations are consistent with the crystal structure and demonstrate that TFP binds to the target binding cleft of S100A4 in solution. Remarkably, TFP binding results in the assembly of five Ca(2+)-S100A4/TFP dimers into a tightly packed pentameric ring. Within each pentamer most of the contacts between S100A4 dimers occurs through the TFP moieties. The Ca(2+)-S100A4/prochlorperazine (PCP) complex exhibits a similar pentameric assembly. Equilibrium sedimentation and cross-linking studies demonstrate the cooperative formation of a similarly sized S100A4/TFP oligomer in solution. Assays examining the ability of TFP to block S100A4-mediated disassembly of myosin-IIA filaments demonstrate that significant inhibition of S100A4 function occurs only at TFP concentrations that promote S100A4 oligomerization. Together these studies support a unique mode of inhibition in which phenothiazines disrupt the S100A4/myosin-IIA interaction by sequestering S100A4 via small molecule-induced oligomerization.

Investigation of the Ca2(+)-dependent interaction of trifluoperazine with S100a: a 19F NMR and circular dichroism study

S100a is a heterodimeric, acidic calcium-binding protein that interacts with calmodulin antagonists in a Ca2(+)-dependent manner. In order to study the behavior of the hydrophobic domain on S100a when bound to Ca2+, its interaction with trifluoperazine (TFP) was investigated using 16F nuclear magnetic resonance (NMR) and circular dichroism (CD) spectroscopy. The dissociation constant (Kd) values of TFP, as estimated from the chemical shifts of 19F NMR, were 191 and 29 microns in the absence and presence of Ca2+, respectively, and were similar to those previously reported for S100b. However, the TFP linewidth in the presence of Ca2(+)-bound S100a was 65 Hz greater than in the presence of Ca2(+)-bound S100b. This suggests a slower TFP exchange rate for S100a than for S100b. Thus, the TFP linewidths observed for each isoform may reflect differences in structural and modulatory properties of the Ca2(+)-dependent hydrophobic domains on S100a and S100b. Additionally, the presence of magnesium had no effect on the observed Ca2(+)-induced TFP spectral changes in S100a solutions. Circular dichroism studies indicate that Ca2+ induces a small transition from alpha-helix to random coil in S100a; in contrast, the opposite transition is reported for calmodulin (Hennessey et al., 1987). However, TFP did not significantly alter the secondary structure of Ca2(+)-bound S100a; this observation is similar to the effect of TFP on Ca2(+)-bound calmodulin and troponin C (Shimizu and Hatano, 1984; Gariépy and Hodges, 1983). It is, therefore, proposed that TFP binds to a hydrophobic domain on S100a in a fashion similar to other calcium-modulated proteins.