Residue-residue interactions regulating the Ca2+-induced EF-hand conformation changes in calmodulin.

Calmodulin (CaM) is a Ca2+-binding messenger protein having four Ca2+-binding motifs named 'EF-hand'; the EF-hand motifs undergo a conformation change induced by Ca2+-binding. In order to study how Ca2+-binding induces the conformation change of EF-hand motifs and which residues are involved in the reaction, two 1μ second long MD simulations were independently performed from the apo- and holo-CaM and their structures and interactions were compared. The Ca2+-binding weakens the helix-helix interaction in all EF-hand, however, the holo-CaM MD adopted the close-like form. The correlation coefficients obtained from the two MDs show the residues comprising interactions being involved in their close-open conformation changes; most of these residues are hydrophobic amino acids but some of them are hydrophilic (T34, H107, N111 and Q143). The hydrophilic residues are expected to lock the EF-hands by their side-chains and main-chain carbonyl oxygen of another hydrophobic residue. Furthermore, the interaction pattern of EF-hand3 and 4 are similar to each other. On the other hand, the interaction pattern of EF-hand2 is different from others; its polar residues are expected to play an important role in regulating the EF-hand2 conformation.