title: Allosteric effects of the antipsychotic drug trifluoperazine on the energetics of calcium binding by calmodulin.
abstract: Trifluoperazine (TFP; Stelazine) is an antagonist of calmodulin (CaM), an essential regulator of calcium-dependent signal transduction. Reports differ regarding whether, or where, TFP binds to apo CaM. Three crystallographic structures (1CTR, 1A29, and 1LIN) show TFP bound to (Ca(2+))(4)-CaM in ratios of 1, 2, or 4 TFP per CaM. In all of these, CaM domains adopt the "open" conformation seen in CaM-kinase complexes having increased calcium affinity. Most reports suggest TFP also increases calcium affinity of CaM. To compare TFP binding to apo CaM and (Ca(2+))(4)-CaM and explore differential effects on the N- and C-domains of CaM, stoichiometric TFP titrations of CaM were monitored by (15)N-HSQC NMR. Two TFP bound to apo CaM, whereas four bound to (Ca(2+))(4)-CaM. In both cases, the preferred site was in the C-domain. During the titrations, biphasic responses for some resonances suggested intersite interactions. TFP-binding sites in apo CaM appeared distinct from those in (Ca(2+))(4)-CaM. In equilibrium calcium titrations at defined ratios of TFP:CaM, TFP reduced calcium affinity at most levels tested; this is similar to the effect of many IQ-motifs on CaM. However, at the highest level tested, TFP raised the calcium affinity of the N-domain of CaM. A model of conformational switching is proposed to explain how TFP can exert opposing allosteric effects on calcium affinity by binding to different sites in the "closed," "semi-open," and "open" domains of CaM. In physiological processes, apo CaM, as well as (Ca(2+))(4)-CaM, needs to be considered a potential target of drug action.
authors.affiliation: Department of Biochemistry, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, Iowa 52242-1109, USA.
authors.forename: Michael D
authors.initials: MD
authors.lastname: Feldkamp
authors.forename: Susan E
authors.initials: SE
authors.lastname: O'Donnell
authors.forename: Liping
authors.initials: L
authors.lastname: Yu
authors.forename: Madeline A
authors.initials: MA
authors.lastname: Shea
mesh_terms: D000818:Animals; D014150:Antipsychotic Agents / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites / Q000187:drug effects; D002118:Calcium / Q000737:chemistry* / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects; D002147:Calmodulin / Q000037:antagonists & inhibitors / Q000097:blood / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D019295:Computational Biology; D030562:Databases, Protein; D007700:Kinetics; D008024:Ligands; D008958:Models, Molecular; D019906:Nuclear Magnetic Resonance, Biomolecular; D009994:Osmolar Concentration; D010247:Paramecium / Q000378:metabolism; D010446:Peptide Fragments / Q000097:blood / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding / Q000187:drug effects; D011487:Protein Conformation; D054730:Protein Interaction Domains and Motifs; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D014268:Trifluoperazine / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D014150:Antipsychotic Agents; D002147:Calmodulin; D008024:Ligands; D010446:Peptide Fragments; D011994:Recombinant Proteins; C552406:calmodulin (1-80); C552405:calmodulin (76-148); D014268:Trifluoperazine; D002118:Calcium