GTPases

GTPase is a compound acronym for guanosine triphosphate (GTP) and hydrolase. GTPases refer to a large family of hydrolases that bind the nucleotide guanosine triphosphate (GTP ) and hydrolyze it to guanosine diphosphate (GDP). GTP binding and hydrolysis takes place in the highly conserved P-loop, the so-called "G-domain", a protein domain that is common to many GTPases.

The GTPases also include the G proteins (also known as small GTPases). These occupy a central position in signal transduction between the receptor and second messenger systems. By alternately binding the nucleotides GDP or GTP, they act as molecular "switch elements" in signal transduction chains.

GTPases act as switching elements, also as timers in many basic cellular processes. Examples of these functions are:

• Signal transduction in response to activation of cell surface receptors, including transmembrane receptors such as those mediating taste, smell and vision.
• Protein biosynthesis(translation) at ribosomes.
• Regulation of cell differentiation, proliferation, division and movement.
• Transduction of proteins through biological membranes.
• Transport of vesicles within the cell and vesicle-mediated secretion and uptake by GTPase control of the vesicle coat.

GTPases are active when bound to GTP. They are inactive when bound to GDP. This switch between inactive and active is due to conformational changes of the protein that distinguish these two forms, in particular the "switch" regions, which in the active state are able to make protein-protein contacts with partner proteins that alter the function of these effectors.

The hydrolytic cleavage of GTP leads to the deactivation of the signaling/timing function of the enzyme (Kahn et al. (1986). The hydrolysis of the third (gamma-) phosphate of GTP to form guanosine diphosphate (GDP) and inorganic phosphate is dependent on the presence of a magnesium ion Mg2+.

GTPase activity serves as a shut-off mechanism for the signaling functions of GTPases by returning the active, GTP-bound protein to the inactive, GDP-bound state. Most "GTPases" have a functional GTPase activity that allows them to remain active (i.e. bound to GTP) for only a short time before deactivating themselves by converting bound GTP to bound GDP (Kahn et al. 1986). However, many GTPases also utilize accessory proteins, GTPase-activating proteins (GAPs), to accelerate their GTPase activity. This further limits the active lifetime of signaling GTPases (Berman DM et al. 1998). Some GTPases have little or no intrinsic GTPase activity and are completely dependent on GAP proteins for deactivation (e.g. the ADP-ribosylation factor or ARF family of small GTP-binding proteins involved in vesicle-mediated transport within cells) (Kahn et al. 1986)

In order to be activated, GTPases must bind to GTP. Since the mechanisms for direct conversion of bound GDP to GTP are not known, the inactive GTPases are induced to release bound GDP by the action of certain regulatory proteins called guanine nucleotide exchange factors (GEFs). The nucleotide-free GTPase protein rapidly rebinds GTP, which in healthy cells is present in far greater excess than GDP, allowing the GTPase to enter the active conformational state and exert its effect on the cell (Gilman AG 1987).

In heterotrimeric G proteins and the many small GTP-binding proteins(small GTPases), GEF activity is stimulated by cell surface receptors in response to signals outside the cell (in heterotrimeric G proteins, the G protein-coupled receptors are themselves GEFs, whereas in receptor-activated small GTPases, the GEFs are distinct from the cell surface receptors).

Some GTPases also bind to accessory proteins, so-called guanine nucleotide dissociation inhibitors or GDIs, which stabilize the inactive GDP-bound state (Sasaki T et al. 1998).

1. Gilman AG (1987) G proteins: transducers of receptor-generated signals. Annual Review of Biochemistry 56: 615-649.
2. Rodbell M (1995). Nobel Lecture: Signal transduction: Evolution of an idea. Bioscience Reports 15: 117-133.
3. Berman DM et al. (1998). Mammalian RGS proteins: barbarians at the gate. Journal of Biological Chemistry 273: 1269-1272.
4. Kahn et al (1986). The protein cofactor necessary for ADP-ribosylation of Gs by cholera toxin is itself a GTP binding protein. Journal of Biological Chemistry 261: 7906-7911
5. Sasaki T et al.( 1998) The Rho Small G Protein Family-Rho GDI System as a Temporal and Spatial Determinant for Cytoskeletal Control. Biochemical and Biophysical Research Communications 245): 641-645.