CAP-23/NAP-22, a neuron-specific protein kinase C substrate,
is Nα-myristoylated and interacts with calmodulin
(CaM) in the presence of Ca2+ ions. Takasaki et al.
(1999, J Biol Chem 274:11848–11853) have recently
found that the myristoylated N-terminal nonapeptide of CAP-23/NAP-22
(mC/N9) binds to Ca2+-bound CaM (Ca2+/CaM).
In the present study, small-angle X-ray scattering was used to
investigate structural changes of Ca2+/CaM induced by
its binding to mC/N9 in solution. The binding of one mC/N9 molecule
induced an insignificant structural change in Ca2+/CaM.
The 1:1 complex appeared to retain the extended conformation much
like that of Ca2+/CaM in isolation. However, it could
be seen that the binding of two mC/N9 molecules induced a drastic
structural change in Ca2+/CaM, followed by a slight
structural change by the binding of more than two but less than
four mC/N9 molecules. Under the saturated condition (the molar ratio
of 1:4), the radius of gyration (Rg) for
the Ca2+/CaM-mC/N9 complex was 19.8 ± 0.3 Å.
This value was significantly smaller than that of Ca2+/CaM
(21.9 ± 0.3 Å), which adopted a dumbbell structure and
was conversely 2–3 Å larger than those of the complexes
of Ca2+/CaM with the nonmyristoylated target peptides of
myosin light chain kinase or CaM kinase II, which adopted a compact
globular structure. The pair distance distribution function had no
shoulder peak at around 40 Å, which was mainly due to the
dumbbell structure. These results suggest that Ca2+/CaM
interacts with Nα-myristoylated CAP-23/NAP-22
differently than it does with other nonmyristoylated target proteins.
The N-terminal amino acid sequence alignment of CAP-23/NAP-22 and
other myristoylated proteins suggests that the protein myristoylation
plays important roles not only in the binding of CAP-23/NAP-22 to
Ca2+/CaM, but also in the protein–protein interactions
related to other myristoylated proteins.