Monomeric GTP-binding proteins

Elongation factor EF-G and translocation. The third step in the elongation sequence on ribosomes (Fig. 29-12, step g) depends upon EF-G, a monomeric GTP-binding protein with a sequence homologous with that of other members of the G protein family. It apparently utilizes the Gibbs energy of hydrolysis of GTP to GDP to drive translocation of the peptidyl-tRNA from the A site to the P site (Fig. 29-12) and of the previously utilized (de-acylated) tRNA to the exit site. 

Kadono-Okuda, K. and Andres, D. A. (1997) An expression cloning method to identify monomeric GTP-binding proteins by GTP overlay. Anal. Biochem. 254, 187-191. 

The Ras superfamily of small monomeric GTP-binding proteins consists of at least 13 small GTPases. We have already mentioned the enormous functional diversity of the monomeric G proteins, of which many aspects are still unclear (see Chapter 3). For example, the Rab GTPases regulate vesicular protein traffic and direct the transport vesicles to their targets in eukaryotic cells . Another monomeric G protein. Ran, regulates transport in and out of the nucleus. Moreover, Ran in the active, GTP-bound form... 

The first human oncogene found by this approach was the ras gene, which encodes a monomeric GTP-binding protein involved in signal transduction. It was also possible to mutate NIH 3T3 cells with chemical... 

Small GTPases are monomeric 20 to 40 kD GTP-binding proteins that interconvert between an active (GTP-bound) and an inactive (GDP-bound) state. As molecular switches they are involved in the regulation of complex cellular processes. 

Most transport vesicles bud off as coated vesicles, with a unique set of proteins decorating their cytosolic surface 141 GTP-binding proteins, such as the small monomeric GTPases and heterotrimeric GTPases (G proteins) facilitate membrane transport 142 SNARE proteins and Rabs control recognition of specific target membranes 143... 

The pleckstrin-homology domain is named after pleckstrin, where it was found first (pleckstrin is a major platelet protein which is a substrate of protein kinase Pleckstrin and brain spectrin contain two domains, 2 about 120 residues each, with similar sequences in their amino- and carboxy-terminal parts. To date, at least 90 such PH-domain sequences have been described in proteins with very diverse functions, such as spectrin,43 cytosolic protein kinases, phospholipase C isoforms (PLC-P, -y, and the GTPase dynamin,44.45 guanine nucleotide exchange factors for monomeric GTP-bind-ing proteins, the GTPase-acdvating protein for Ras, the Ras-GAP, and cytoskeletal and several other proteins. 

Ras is a monomeric GTP-binding switch protein that, like the G subunits in trimeric G proteins, alternates between an active on state with a bound GTP and an inactive off state... 

A further dimension to the ethylene transduction process was provided by the work of Novikova et al. [59] which demonstrated that in Triton X-lOO-solubilised membrane fractions ethylene increased GTP binding markedly. The effect was observable after 20 min and reached a maximum at 40 min before declining. The G-proteins affected appear to be of the small monomeric type (SMG) which are immunoprecipitated by antibodies to the small GTP-binding protein of Ras superfamily. A similar situation has now been observed in Arabidopsis [60]. As with protein phosphorylation the effect is antagonised by c)dokinin and the ethylene-insensitive mutant eti5 shows higher levels of... 

Protein-protein interactions. Enzyme activity can be modulated through the reversible binding of a modulator protein, such as Ca calmodulin. Monomeric G proteins (GTP-binding proteins) activate target proteins through reversible binding. 

S-acylated proteins include many GTP-binding regulatory proteins (G proteins), including most a subunits of heterotrimeric G-proteins and also many members of the Ras superfamily of monomeric G proteins, a number of G protein-coupled receptors, several nonreceptor tyrosine kinases, and a number of other signaling molecules, -acylation is posttranslational and reversible, a property that allows the cell to control... 

Table 1-9-2 is a summary of the major components of water-soluble hormone pathways reviewed in this section. There are several different G proteins (GTP-binding) involved. Tfimeric G pro- teins include G, Gj, G, and in the photoreceptor pathway reviewed in Chapter 10, G (trans- ducin). Receptors that engage these aU have the 7-helix membrane-spanning structure. Receptor stimulation causes the Ga subunit to bind GTP and become active. The Ga subunit subsequently hydrolyzes the GTP to GDP, terminating the signal. The p21 G protein is monomeric. 

Receptors for ADP, epinephrine, thromboxane, thrombin and PAF have been well characterized (27, 31, reviewed in other chapters). Membrane spanning receptors of epinephrine, t mbin and thromboxane are coupled to the ubiquitous GTP-hinding proteins. Platelets contain monomeric, low molecular weight G proteins as well as heterotrimeric membrane associated G roteins. GTP binding to e a-subunit of G-proteins fiicilitates die interaction with effector enzymes, resulting in the hydrolysis of GTP to GDP, which terminates its stimulatory role (29). 

Fig. 9.14. The monomeric G protein Ras. When SOS is activated, it binds to Ras, a monomeric G protein anchored to the plasma membrane. SOS is a guanine nucleotide exchange protein that activates the exchange of GTP for bound GDP on Ras. Activated Ras containing GTP binds the target enzyme Raf, thereby activating it.

Fig. 10.22. Nuclear import. Proteins with the nuclear localization signal bind to importins, which carry them through the nuclear pore into the nucleus. The monomeric G protein Ran containing bound GTP binds to one of the subunits of importin. This causes dissociation of the importin subunits and release of the imported protein in the nucleus. The Ran-importin complex exits a nuclear pore. On the cytoplasmic side, a RanGAP (GTPase activating protein) activates the hydrolysis of GTP to GDP, which causes dissociation of the complex. RanGDP is subsequently returned to the nucleus, where an accessory protein activates dissociation of GDP and association of GTP.

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