Calbindin D28k eExhibits properties characteristic of a Ca2+ sensor

Calbindin D-28k is a member of the calmodulin superfamily of Ca2+-binding proteins and contains six EF-hands. The protein is generally believed to function as a Ca2+ buffer, but the studies presented in this work indicate that it may also act as a Ca2+ sensor. The results show that Mg2+ binds to the same sites as Ca2+ with an association constant of similar to1.4.10(3) M-1 in 0.15 M KCl. The four high affinity sites in calbindin D-28k bind Ca2+ in a non-sequential, parallel manner. In the presence of physiological concentrations of Mg2+ the Ca2+ affinity is reduced by a factor of 2, and the cooperativity, which otherwise is modest, increases. Based on the binding constants determined in the presence of physiological salt concentrations, we estimate that at the Ca2+ concentration in a resting cell calbindin D-28k is saturated to 40-75% with Mg2+ but to less than 9% with Ca2+. In contrast, the protein is expected to be nearly fully saturated with Ca2+ at the Ca2+ level of an activated cell. A substantial conformational change is observed upon Ca2+ binding, but only minor structural changes take place upon Mg2+ binding. This suggests that calbindin D-28k undergoes Ca2+-induced structural changes upon Ca2+ activation of a cell. Thus, calbindin D-28k displays several properties that would be expected for a protein involved in Ca2+-induced signal transmission and hence may function not only as a Ca2+ buffer but also as a Ca2+ sensor. Digestion patterns resulting from limited proteolysis of the protein suggest that the loop of EF-hand 2, a variant site that does not bind Ca2+, becomes exposed upon Ca2+ binding.