part_of part of signaling receptor binding signal transduction signaling receptor activator activity receptor ligand activity The activity of a gene product that interacts with a receptor to effect a change in the activity of the receptor. Ligands may be produced by the same, or different, cell that expresses the receptor. Ligands may diffuse extracellularly from their point of origin to the receiving cell, or remain attached to an adjacent cell surface (e.g. Notch ligands). receptor agonist activity signaling molecule signaling receptor ligand activity vitamin D receptor activator activity