The alpha, beta, and gamma subunits (or beta-gamma dimers) of heterotrimeric guanine nucleotide binding (G) proteins are intracellular membrane-associated proteins activated by various receptors (e.g., beta adrenergic). They serve as transducers of the receptor-initiated response linked to intracellular elements, such as enzymes, that generate second messengers to initiate an effect. These proteins have a high affinity for guanine nucleotides and hence are named "G" proteins. [ ]