ATP is involved in signal transduction by serving as substrate for kinases, enzymes that transfer phosphate groups.  Kinases are the most common ATP-binding proteins.  They share a small number of common folds.  Phosphorylation of a protein by a kinase can activate a cascade such as the mitogen-activated protein kinase cascade.
ATP is also a substrate of adenylate cyclase, most commonly in G protein-coupled receptor signal transduction pathways and is transformed to second messenger, cyclic AMP, which is involved in triggering calcium signals by the release of calcium from intracellular stores. This form of signal transduction is particularly important in brain function, although it is involved in the regulation of a multitude of other cellular processes.