Heterotrimeric G protein, also sometimes referred to as the "large" G proteins (as opposed to the subclass of smaller, monomeric small GTPases) are membrane-associated G proteins that form a heterotrimeric complex. The biggest non-structural difference between heterotrimeric and monomeric G protein is that heterotrimeric proteins bind to their cell-surface receptors, called G protein-coupled receptors, directly. These G proteins are made up of alpha (α), beta (β) and gamma (γ) subunits. The alpha subunit is attached to either a GTP or GDP, which serves as an on-off switch for the activation of G-protein. When ligands bind a GPCR, the GPCR acquires GEF (guanine nucleotide exchange factor) ability, which activates the G-protein by exchanging the GDP on the alpha subunit to GTP. The binding of GTP to the alpha subunit results in a structural change and its dissociation from the rest of the G-protein. Generally, the alpha subunit binds membrane-bound effector proteins for the downstream signaling cascade, but the beta-gamma complex can carry out this function also. G-proteins are involved in pathways such as the cAMP/PKA pathway, ion channels, MAPK, PI3K. There are four main families of G proteins: Gi/Go, Gq, Gs, and G12/13. Alpha subunits Reconstitution experiments carried out in the early 1980s showed that purified Gα subunits can directly activate effector enzymes. The GTP form of the α subunit of transducin (Gt) activates the cyclic GMP phosphodiesterase from retinal rod outer segments, and the GTP form of the α subunit of the stimulatory G protein (Gs) activates hormone-sensitive adenylate cyclase. More than one type of G protein co-exist in the same tissue. For example, in adipose tissues, two different G-proteins with interchangeable beta-gamma complexes are used to activate or inhibit adenylyl cyclase. The alpha subunit of a stimulatory G protein activated by receptors for stimulatory hormones could stimulate adenylyl cyclase, which activates cAMP used for downstream signal cascades. While on the other hand, the alpha subunit of an inhibitory G protein activated by receptors of inhibitory hormones could inhibit adenylyl cyclase, which blocks downstream signal cascades. Gα subunits consist of two domains, the GTPase domain, and the alpha-helical domain. There exist at least 20 different Gα subunits, which are separated into four main groups. This nomenclature is based on their sequence homologies: G beta-gamma complex The β and γ subunits are closely bound to one another and are referred to as the G beta-gamma complex. Both beta and gamma subunits