Structure of Paramecium tetraurelia calmodulin at 1.8 A resolution

The crystal structure of calmodulin (CaM; M(r) 16,700, 148 residues) from the ciliated protozoan Paramecium tetraurelia (PCaM) has been determined and refined using 1.8 A resolution area detector data. The crystals are triclinic, space group P1, a = 29.66, b = 53.79, c = 25.49 A, alpha = 92.84, beta = 97.02, and gamma = 88.54 degrees with one molecule in the unit cell. Crystals of the mammalian CaM (MCaM; Babu et al., 1988) and Drosophila CaM (DCaM; Taylor et al., 1991) also belong to the same space group with very similar cell dimensions. All three CaMs have 148 residues, but there are 17 sequence changes between PCaM and MCaM and 16 changes between PCaM and DCaM. The initial difference in the molecular orientation between the PCaM and MCaM crystals was approximately 7 degrees as determined by the rotation function. The reoriented Paramecium model was extensively refitted using omit maps and refined using XPLOR. The R-value for 11,458 reflections with F > 3 sigma is 0.21, and the model consists of protein atoms for residues 4-147, 4 calcium ions, and 71 solvent molecules. The root mean square (rms) deviations in the bond lengths and bond angles in the model from ideal values are 0.016 A and 3 degrees, respectively. The molecular orientation of the final PCaM model differs from MCaM by only 1.7 degrees. The overall Paramecium CaM structure is very similar to the other calmodulin structures with a seven-turn long central helix connecting the two terminal domains, each containing two Ca-binding EF-hand motifs. The rms deviation in the backbone N, Ca, C, and O atoms between PCaM and MCaM is 0.52 A and between PCaM and DCaM is 0.85 A. The long central helix regions differ, where the B-factors are also high, particularly in PCaM and MCaM. Unlike the MCaM structure, with one kink at D80 in the middle of the linker region, and the DCaM structure, with two kinks at K75 and I85, in our PCaM structure there are no kinks in the helix; the distortion appears to be more gradually distributed over the entire helical region, which is bent with an apparent radius of curvature of 74.5(2) A. The different distortions in the central helical region probably arise from its inherent mobility.