GTP is essential for signal transduction, especially with G proteins. G proteins are coupled with a cell membrane bound receptor. This whole complex is called a G protein-coupled receptor (GPCR). G proteins can bind either GDP or GTP. When bound to GDP, G proteins are inactive. When a ligand binds a GPCR, an allosteric change in the G protein is triggered, causing GDP to leave and be replaced by GTP. GTP activates the alpha subunit of the G protein, causing it to dissociate from the G protein and act as a downstream effector.
