Ca2+ and Mg2+ modulate conformational dynamics and stability of downstream regulatory element antagonist modulator

Downstream Regulatory Element Antagonist Modulator (DREAM) belongs to the family of neuronal calcium sensors (NCS) that transduce the intracellular changes in Ca(2+) concentration into a variety of responses including gene expression, regulation of Kv channel activity, and calcium homeostasis. Despite the significant sequence and structural similarities with other NCS members, DREAM shows several features unique among NCS such as formation of a tetramer in the apo-state, and interactions with various intracellular biomacromolecules including DNA, presenilin, Kv channels, and calmodulin. Here we use spectroscopic techniques in combination with molecular dynamics simulation to study conformational changes induced by Ca(2+) /Mg(2+) association to DREAM. Our data indicate a minor impact of Ca(2+) association on the overall structure of the N- and C-terminal domains, although Ca(2+) binding decreases the conformational heterogeneity as evident from the decrease in the fluorescence lifetime distribution in the Ca(2+) bound forms of the protein. Time-resolved fluorescence data indicate that Ca(2+) binding triggers a conformational transition that is characterized by more efficient quenching of Trp residue. The unfolding of DREAM occurs through an partially unfolded intermediate that is stabilized by Ca(2+) association to EF-hand 3 and EF-hand 4. The native state is stabilized with respect to the partially unfolded state only in the presence of both Ca(2+) and Mg(2+) suggesting that, under physiological conditions, Ca(2+) free DREAM exhibits a high conformational flexibility that may facilitate its physiological functions.