signaling receptor binding receptor ligand receptor-associated protein activity Binding to one or more specific sites on a receptor molecule, a macromolecule that undergoes combination with a hormone, neurotransmitter, drug or intracellular messenger to initiate a change in cell function. Wikipedia:Ligand_(biochemistry) GO:0005102 Where appropriate, also consider annotating to 'receptor agonist activity ; GO:0048018'. receptor binding protein binding