different subcellular localizations that dynamically change in the course of signal transduction. PLD activity has been observed within the plasma membrane, cytosol, ER, and Golgi complex. PLD1 PLD1 is a 120 kDa protein that is mainly located on the inner membranes of cells. It is primarily present at the Golgi complex, endosomes, lysosomes, and secretory granules. Upon the binding of an extracellular stimulus, PLD1 is transported to the plasma membrane. Basal PLD1 activity is low however, and in order to transduce the extracellular signal, it must first be activated by proteins such as Arf, Rho, Rac, and protein kinase C. Regulation The activity of phospholipase D is extensively regulated by hormones, neurotransmitters, lipids, small monomeric GTPases, and other small molecules that bind to their corresponding domains on the enzyme. In most cases, signal transduction is mediated through production of phosphatidic acid, which functions as a secondary messenger. Specific phospholipids are regulators of PLD activity in plant and animal cells. Most PLDs require phosphatidylinositol 4,5-bisphosphate (PIP2), as a cofactors for activity. PIP2 and other phosphoinositides are important modifiers of cytoskeletal dynamics and membrane transport and can traffic PLD to its substrate PC. PLDs regulated by these phospholipids are commonly involved in intracellular signal transduction. Their activity is dependent upon the binding of these phosphoinositides near the active site. In plants and animals, this binding site is characterized by the presence of a conserved sequence of basic and aromatic amino acids. In plants such as Arabidopsis thaliana, this sequence is constituted by a RxxxxxKxR motif together with its inverted repeat, where R is arginine and K is lysine. Its proximity to the active site ensures high level of PLD1 and PLD2 activity, and promotes the translocation of PLD1 to target membranes in response to extracellular signals. C2 domain Calcium acts as a cofactor in PLD isoforms that contain the C2 domain. Binding of Ca2+ to the C2 domain leads to conformational changes in the enzyme that strengthen enzyme-substrate binding, while weakening the association with phosphoinositides. In some plant isoenzymes, such as PLDβ, Ca2+ may bind directly to the active site, indirectly increasing its affinity for the substrate by strengthening the binding of the activator PIP2. PX domain The pbox consensus sequence (PX) is thought to mediate the binding of additional phosphatidylinositol phosphates, in particular, phosphatidylinositol 5-phosphate (PtdIns5P), a lipid thought to be required for endocytosis, may help facilitate the reinternalization of PLD1