Hydrogen-Deuterium Exchange Mass Spectrometry Reveals Calcium Binding Properties and Allosteric Regulation of Downstream Regulatory Element Antagonist Modulator (DREAM)

Downstream regulatory element antagonist modulator (DREAM) is an EF-hand Ca²⁺-binding protein that also binds to a specific DNA sequence, downstream regulatory elements (DRE), and thereby regulates transcription in a calcium-dependent fashion. DREAM binds to DRE in the absence of Ca²⁺ but detaches from DRE under Ca²⁺ stimulation, allowing gene expression. The Ca²⁺ binding properties of DREAM and the consequences of the binding on protein structure are key to understanding the function of DREAM. Here we describe the application of hydrogen-deuterium exchange mass spectrometry (HDX-MS) and site-directed mutagenesis to investigate the Ca²⁺ binding properties and the subsequent conformational changes of full-length DREAM. We demonstrate that all EF-hands undergo large conformation changes upon calcium binding even though the EF-1 hand is not capable of binding to Ca²⁺. Moreover, EF-2 is a lower-affinity site compared to EF-3 and -4 hands. Comparison of HDX profiles between wild-type DREAM and two EF-1 mutated constructs illustrates that the conformational changes in the EF-1 hand are induced by long-range structural interactions. HDX analyses also reveal a conformational change in an N-terminal leucine-charged residue-rich domain (LCD) remote from Ca²⁺-binding EF-hands. This LCD domain is responsible for the direct interaction between DREAM and cAMP response element-binding protein (CREB) and regulates the recruitment of the co-activator, CREB-binding protein. These long-range interactions strongly suggest how conformational changes transmit the Ca²⁺ signal to CREB-mediated gene transcription.