GTPase switch proteins

The GTPase family of proteins is a set of intracellular trigger proteins or on–off molecular switches of that there are two major classes that are as follows: the Gα subunit of the trimeric G proteins and the monomeric G proteins like as the Ras, Rho and Rab families. Rho Ras, and Rab proteins which couple to the activated receptor by adaptor scaffold proteins behave as bifurcation and transducers signaling proteins modifying the nature of the signal and sending it along numerous downstream path. In case of Ras a downstream serine or threonine   phosphorylation    cascade   is   activated,   which   involve   the mitogen-activated protein kinase (MAP-kinase).  Such as the Gα subunits, the Ras proteins are attached to the cytosolic face of the plasma membrane by a covalently attached lipid group. When the GTPase has guanosine diphosphate (GDP) bound it is in the ‘off’ state. Activation, through a cell surface receptor or a guanine nucleotide-exchange factor (GEF) leads to the GDP being exchanged for GTP, converting the GTPase to the ‘on’ state. The activated GTPase with its bound GTP then dissociates from the receptor and binds to and activates an effector enzyme for instance adenylyl cyclase that in turn catalyzes the formation of a second messenger example for cAMP. The GTPase then hydrolyzes the bound GTP causing it to revert back to the ‘off’ state. Cholera toxin acts through inhibiting its intrinsic GTPase activity, with the result which once activated to the GTP- bound state the GTPase cannot be turned off again.