Binds the second messenger bis-(3'-5') cyclic dimeric guanosine monophosphate (c-di-GMP). Can bind two c-di-GMP molecules per monomer. May play a role in bacterial second-messenger regulated processes. Binding to c-di-GMP induces a conformational change of the C- and N-termini resulting in the exposure of a highly negative surface on one side of the protein to a possible effector protein. Monomer in both c-di-GMP-bound and free forms. Consists of a six-stranded anti-parallel beta-barrel core structure with an unstructured N-terminus in apo-form and a C-terminal alpha helix. In the holo-form the C-terminal helix is displaced by the ligand, thereby opening one side of the beta-barrel as a binding site, and the N-terminus containing the RXXXR motif wraps around the ligand and in turn ties the C-terminal helix in a loose conformation. The structural rearrangement upon ligand binding creates a significant change in surface charge distribution.