S100B(ββ) is a dimeric Ca2+-binding
protein that interacts with p53, inhibits its phosphorylation
by protein kinase C (PKC) and promotes disassembly of the
p53 tetramer. Likewise, a 22 residue peptide derived from
the C-terminal regulatory domain of p53 has been shown
to interact with S100B(ββ) in a Ca2+-dependent
manner and inhibits its phosphorylation by PKC. Hence,
structural studies of Ca2+-loaded S100B(ββ)
bound to the p53 peptide were initiated to characterize
this interaction. Analysis of nuclear Overhauser effect
(NOE) correlations, amide proton exchange rates, 3JNH-Hα
coupling constants, and chemical shift index data show
that, like apo- and Ca2+-bound S100B(ββ),
S100B remains a dimer in the p53 peptide complex, and each
subunit has four helices (helix 1, Glu2–Arg20; helix
2, Lys29–Asn38; helix 3, Gln50–Asp61; helix
4, Phe70–Phe87), four loops (loop 1, Glu21–His25;
loop 2, Glu39–Glu49; loop 3, Glu62–Gly66; loop
4, Phe88–Glu91), and two β-strands (β-strand
1, Lys26–Lys28; β-strand 2, Glu67–Asp69),
which forms a short antiparallel β-sheet. However,
in the presence of the p53 peptide helix 4 is longer by
five residues than in apo- or Ca2+-bound S100B(ββ).
Furthermore, the amide proton exchange rates in helix 3
(K55, V56, E58, T59, L60, D61) are significantly slower
than those of Ca2+-bound S100B(ββ).
Together, these observations plus intermolecular NOE correlations
between the p53 peptide and S100B(ββ) support the
notion that the p53 peptide binds in a region of S100B(ββ),
which includes residues in helix 2, helix 3, loop 2, and
the C-terminal loop, and that binding of the p53 peptide
interacts with and induces the extension of helix 4.