Calmodulin structure refined at 1.7 Å resolution

We have determined and refined the crystal structure of a recombinant calmodulin at 1.7 Å resolution. The structure was determined by molecular replacement, using the 2.2 Å published native bovine brain structure as the starting model. The final crystallographic R-factor, using 14,469 reflections in the 10.0 to 1.7 Å range with structure factors exceeding 0.5σ, is 0.216. Bond lengths and bond angle distances have root-mean-square deviations from ideal values of 0.009 Å and 0.032 Å, respectively. The final model consists of 1279 non-hydrogen atoms, including four calcium ions, 1130 protein atoms, including three Asp118 side-chain atoms in double conformation, 139 water molecules and one ethanol molecule. The electron densities for residues 1 to 4 and 148 of calmodulin are poorly defined, and not included in our model, except for main-chain atoms of residue 4. The calmodulin structure from our crystals is very similar to the earlier 2.2 Å structure described by Babu and coworkers with a root-mean-square deviation of 0.36 Å. Calmodulin remains a dumb-bell-shaped molecule, with similar lobes and connected by a central α-helix. Each lobe contains three α-helices and two Ca 2+ binding EF hand loops, with a short antiparallel β-sheet between adjacent EF hand loops and one non-EF hand loop. There are some differences in the structure of the central helix. The crystal packing is extensively studied, and facile crystal growth along the z-axis of the triclinic crystals is explained. Herein, we describe hydrogen bonding in the various secondary structure elements and hydration of calmodulin.