Calcium-Binding Protein Protocols Calcium-Binding Protein Protocols

Solution Scattering 151
Fig. 2. Models derived from the neutron scattering data for the 4Ca 2+ /CaM/MLCK
complexes with (right, 12) and without (left, 11) substrates. The conserved portion of the
inhibited kinase catalytic core (13) and the NMR structure of CaM complexed with
MLCK-I (19) are fit within the dimensions of the larger and the smaller ellipsoids, respectively,
that were derived from modeling the neutron scattering data. The upper and lower
lobes of the catalytic core are represented as gray and black ribbon drawings, respectively,
with the catalytic cleft between them labeled. The empty spaces in the ellipsoid representing
MLCK are occupied by N- and C-terminal sequence segments whose structures are not
known at this time. CaM is represented as a gray ribbon drawing, with its bound MLCK-I
peptide in black, and a CPK representation of its hydrophobic Trp residue near the
N-terminal end. This Trp residue is key to recognition and binding by the C-terminal CaM
domain. The models show that CaM binds to the kinase such that there must be a significant
movement of the CaM-binding and autoinhibitory sequences away from the surface
of the catalytic core. Upon substrate binding there is a movement of the CaM approx 12 Å
closer to the catalytic cleft accompanied by a reorientation that brings the N-terminal helix
of CaM into contact with the surface of the kinase. At the same time the catalytic cleft of
MLCK closes, presumably about its substrate. This contraction is inferred from the overall
shortening of the MLCK ellipsoid seen in the neutron model derived from the plus substrate
experiment. Figure adapted from ref. 12.