4.11: GTPases and their Regulation

Guanine nucleotide-binding proteins (G-proteins), also known as GTPases, are a superfamily of proteins that regulate many cellular processes, such as cell signaling, vesicular transport, and the regulation of cell shape and motility. Mutation or dysfunction of these proteins can lead to disease. There are around 40,000 known G-proteins that can broadly be classified into two groups ‒ small G-proteins consisting of a single domain and large multi-domain G-proteins.

Large G-proteins, also known as heterotrimeric G-proteins, consist of three subunits —α, β, and γ. The α subunit has a conserved domain that interacts with G-protein-coupled receptors to mediate transmembrane signaling

Small G-proteins are a single subunit and signal throughout the cell in a variety of pathways. They are divided into five subfamilies based on sequence and function ‒ Ras, Rho, Rab, Ran, and Arf. Mutations in the Ras subfamily lead to the formation of cancerous tumors in the lungs, colon, and pancreas. The Rho subfamily regulates actin reorganization and microtubule cytoskeleton dynamics. The Rab subfamily, the largest small G-protein family, regulates vesicle transport and membrane trafficking in secretory and endocytic pathways. The Ran subfamily regulates nucleocytoplasmic transport of RNA and proteins through the nuclear pore and mitotic spindle assembly and function. The Arf subfamily is involved in vesicle transport and membrane trafficking.

G-proteins are regulated by GTP/GDP binding and have intrinsic GTPase activity, meaning they can hydrolyze GTP to GDP. When GTP is bound, the G-protein is in the “ON” state, that is the protein will promote signaling cascades in the cell. When GDP is bound, it is in the “OFF” state, causing the signaling to stop. Regulation of G-protein activation further regulated by guanine nucleotide exchange factors (GEFs), which assist in GDP dissociation, and GTPase activating proteins (GAPs) that stimulate GTP hydrolysis. Additionally, guanine dissociation inhibitors (GDIs) can bind to small GTPases and regulate their location in the membrane or cytoplasm.

Guanosine triphosphate, GTP, a close relative of ATP, is a small molecule important for the regulation of protein function.  

G-proteins are proteins regulated by GTP binding. These proteins have intrinsic GTPase activity, that is when GTP is bound, they can catalyze its hydrolysis to guanosine diphosphate, GDP.  

G-proteins are divided into two categories, small and large. The small, or monomeric, G-proteins are a single protein subunit that is activated by various intracellular signaling pathways.

In contrast, the large, or heterotrimeric, G-proteins contain three subunits and are activated by membrane-bound G-protein coupled receptors. 

GTPases act as a molecular switch, where the GDP bound state usually is inactive while the GTP bound state is active. GTP binding followed by GTP to GDP hydrolysis is part of the GDP/GTP cycle.

The cycle starts when a guanine exchange factor, GEF, induces a conformational change in the G-protein that causes the release of GDP.  

GTP quickly binds to the now-empty nucleotide-binding site as GTP is abundant in the cytoplasm. The G-protein is now switched into its active state with GTP bound.  

G-proteins have intrinsic GTPase activity to hydrolyze the bound molecule; however, the GTP breakdown by the enzyme is a slow process without additional cellular signals.

Therefore, when it is time for the G-protein to be switched off, a GTPase activating protein, or GAP, will bind and enhance the GTPase activity of the protein. The GTP is broken down into GDP and inorganic phosphate and the G-protein returns to its inactive state completing the cycle.