GTP-binding protein; one of a superfamily of proteins that function in, for example, signal transduction (e.g. the G-protein associated with the b-adrenergic receptor), polymerization (e.g. tubulin), ribosomal protein synthesis (e.g. the translocase), cell differentiation (ras proteins) and intracellular transport of proteins, vesicles or cytoskeletal elements (e.g. dynamin). The common functional feature of the family is an affinity for a target protein in the presence of GTP which is lost upon hydrolysis to GDP.