The relative orientations of adjacent structural
elements without many well-defined NOE contacts between
them are typically poorly defined in NMR structures. For
apo-S100B(ββ) and the structurally homologous protein
calcyclin, the solution structures determined by conventional
NMR exhibited considerable differences and made it impossible
to draw unambiguous conclusions regarding the Ca2+-induced
conformational change required for target protein binding.
The structure of rat apo-S100B(ββ) was recalculated
using a large number of constraints derived from dipolar
couplings that were measured in a dilute liquid crystalline
phase. The dipolar couplings orient bond vectors relative
to a single-axis system, and thereby remove much of the
uncertainty in NOE-based structures. The structure of apo-S100B(ββ)
indicates a minimal change in the first, pseudo-EF-hand
Ca2+ binding site, but a large reorientation
of helix 3 in the second, classical EF-hand upon Ca2+
binding.