The mechanism of signal transduction in G-protein-coupled receptors (GPCRs) is a crucial step in cell signaling. However, the molecular details of this process are still largely undetermined. Carrying out submicrosecond molecular dynamics simulations of beta-adrenergic receptors, we found that cooperation between a number of highly conserved residues is crucial to alter the equilibrium between the active state and the inactive state of diffusible ligand GPCRs. In particular, "ionic-lock" formation in beta-adrenergic receptors is directly correlated with the protonation state of a highly conserved aspartic acid residue [Asp(2.50)] even though the two sites are located more than 20 angstrom away from each other. Internal polar residues, acting as local microswitches, cooperate to propagate the signal from Asp(2.50) to the G-protein interaction site at the helix III-helix VI interface. Evolutionarily conserved differences between opsin and non-opsin GPCRs in the surrounding of Asp(2.50) influence the acidity of this residue and can thus help in rationalizing the differences in constitutive activity of class A GPCRs. (C) 2010 Elsevier Ltd. All rights reserved.