Structure of Escherichia coli fragment TR₂C from calmodulin to 1.7 Å resolution

Fragment TR₂C is the C-terminal part of the calcium-binding protein calmodulin, including residues 78-148. The crystal structure of TR₂C was solved by molecular replacement and refined to a conventional R value of 21.8% (R_free = 22.0%), using all data in the resolution range 20.0-1.7 Å. This study shows that the secondary structure of TR₂C, a pair of EF-hand motifs with two calcium-binding sites, is similar to the corresponding motifs in intact calmodulin. However, it also indicates that the N-terminus of helix E is closer to the C-­terminus of helix H in TR₂C than in the intact protein and that the loop connecting the EF-hands shows different conformations in the two structures. The crystal structure of TR₂C was further found to be similar to the set of NMR structures of this fragment, although some pronounced differences exist.