45 research outputs found
The structure of the complex of calmodulin with KAR-2: a novel mode of binding explains the unique pharmacology of the drug
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 Ca2+-calmodulin has
been characterized both in solution using NMR and when crystallized
using x-ray diffraction. Heterocorrelation (H-1-N-15 heteronuclear
single quantum coherence) spectra of N-15-labeled calmodulin indicate a
global conformation change (closure) of the protein upon its binding to
KAR-2. The crystal structure at 2.12-Angstrom 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