Structure of Calmodulin in complex with KAR-2, a bis-indol alkaloid

Experimental Data Snapshot

  • Resolution: 2.12 Å
  • R-Value Free: 0.261 
  • R-Value Work: 0.218 
  • R-Value Observed: 0.220 

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

Horvath, I.Harmat, V.Perczel, A.Palfi, V.Nyitrai, L.Nagy, A.Hlavanda, E.Naray-Szabo, G.Ovadi, J.

(2005) J Biol Chem 280: 8266-8274

  • DOI: https://doi.org/10.1074/jbc.M410353200
  • Primary Citation of Related Structures:  

  • PubMed Abstract: 

    3'-(beta-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 Ca(2+)-calmodulin has been characterized both in solution using NMR and when crystallized using x-ray diffraction. Heterocorrelation ((1)H-(15)N heteronuclear single quantum coherence) spectra of (15)N-labeled calmodulin indicate a global conformation change (closure) of the protein upon its binding to KAR-2. The crystal structure at 2.12-A 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.

  • Organizational Affiliation

    Institute of Enzymology, Biological Research Center, Hungarian Academy of Sciences, Karolina út 29 Budapest, H-1113 Hungary.

Find similar proteins by:  (by identity cutoff)  |  3D Structure
Entity ID: 1
MoleculeChains Sequence LengthOrganismDetailsImage
Calmodulin148Bos taurusMutation(s): 0 
Gene Names: CALMCAM
Find proteins for P62157 (Bos taurus)
Explore P62157 
Go to UniProtKB:  P62157
Entity Groups  
Sequence Clusters30% Identity50% Identity70% Identity90% Identity95% Identity100% Identity
UniProt GroupP62157
Sequence Annotations
  • Reference Sequence
Small Molecules
Binding Affinity Annotations 
IDSourceBinding Affinity
KAR Binding MOAD:  1XA5 Kd: 316 (nM) from 1 assay(s)
PDBBind:  1XA5 Kd: 316 (nM) from 1 assay(s)
Experimental Data & Validation

Experimental Data

  • Resolution: 2.12 Å
  • R-Value Free: 0.261 
  • R-Value Work: 0.218 
  • R-Value Observed: 0.220 
  • Space Group: P 61 2 2
Unit Cell:
Length ( Å )Angle ( ˚ )
a = 37.573α = 90
b = 37.573β = 90
c = 356.662γ = 120
Software Package:
Software NamePurpose
XDSdata reduction
XDSdata scaling

Structure Validation

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Ligand Structure Quality Assessment 

Entry History 

Deposition Data

Revision History  (Full details and data files)

  • Version 1.0: 2004-12-21
    Type: Initial release
  • Version 1.1: 2008-04-30
    Changes: Version format compliance
  • Version 1.2: 2011-07-13
    Changes: Advisory, Version format compliance
  • Version 1.3: 2018-10-03
    Changes: Data collection
  • Version 1.4: 2023-10-25
    Changes: Data collection, Database references, Derived calculations, Refinement description