A crucial event in calcium signaling is the transition of a calcium sensor from the apo (Ca2+ free) to the holo (Ca2+-saturated) state. Caldendrin (CDD) is a neuronal Ca2+-binding protein with two functional (EF3 and EF4) and two atypical (EF1 and EF2), non-Ca2+-binding EF-hand motifs. During the transition from the apo to the holo state, guided by the stepwise filling of Ca2+, the protein passes through distinct states and acquires a stable conformational state when only EF3 is occupied by Ca2+. This state is characterized by a Ca2+-derived structural gain in EF3 with destabilization of the EF4 motif. At higher Ca2+ levels, when Ca2+ fills in EF4, the motif regains stability. EF3 controls initial Ca2+ binding and dictates structural destabilization of EF4. It is likely that this unexpected intermotif communication will have an impact on Ca2+-dependent target interactions.
