The structure of the complex of calmodulin with KAR-2: A novel mode of binding explains the unique pharmacology of the drug

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3′-(β-Chloroethyl)-2′,4′-dioxo-3, 5′-spiro-oxazolidino-4-deacetoxyvinblastine (KAR-2) is a potent anti-microtubular agent that arrests mitosis in cancer cells without significant toxic side effects. In this study we demonstrate that in addition to targeting microtubules, KAR-2 also binds calmodulin, thereby countering the antagonistic effects of trifluoperazine. To determine the basis of both properties of KAR-2, the three-dimensional structure of its complex with Ca2+-calmodulin has been characterized both in solution using NMR and when, crystallized using x-ray diffraction. Heterocorrelation (1H-15N heteronuclear single quantum coherence) spectra of 15N-labeled calmodulin indicate a global conformation change (closure) of the protein upon its binding to KAR-2. The crystal structure at 2.12-Å resolution reveals a more complete picture; KAR-2 binds to a novel structure created by amino acid residues of both the N- and C-terminal domains of calmodulin. Although first detected by x-ray diffraction of the crystallized ternary complex, this conformational change is consistent with its solution structure as characterized by NMR spectroscopy. It is noteworthy that a similar tertiary complex forms when calmodulin binds KAR-2 as when it binds trifluoperazine, even though the two ligands contact (for the most part) different amino acid residues. These observations explain the specificity of KAR-2 as an anti-microtubular agent; the drug interacts with a novel drug binding domain on calmodulin. Consequently, KAR-2 does not prevent calmodulin from binding most of its physiological targets.

Original languageEnglish
Pages (from-to)8266-8274
Number of pages9
JournalJournal of Biological Chemistry
Issue number9
Publication statusPublished - Mar 4 2005


ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology

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