Role of GTP

The role of ATP in muscular contraction has parallels to the role of GTP in G-protein activation... 

The reaction is complex and involves an intermediate in which a phosphate is attached to a histidine residue of the succinate thiokinase enzyme. Probably CoA is first displaced by inorganic phosphate, forming succinyl phosphate. A nitrogen atom of a specific histidine residue then attacks phosphorus, displacing succinate and forming an A-phos-phoryl derivative. In the final step GDP attacks the phosphorus atom of that derivative forming GTP. The role of GTP in this reaction is played by ATP in some organisms. 

Hyman, A. A., Salser, S., Drechsel, D. N., Unwin, N., and Mitchison, T. J. (1992). Role of GTP hydrolysis in microtubule dynamics Information from a slowly hydrolyzable analogue, GMPCPP. Mol. Biol. Cell 3, 1155-1167. 

Litosch I, Wallis C, Fain JN. 1985. 5-Hydroxytryptamine stimulates inositol phosphate production in a cell-free system from blowfly salivary glands. Evidence for a role of GTP in coupling receptor activation to phosphoinositide breakdown. J Biol Chem 260 5464-5471. 

The first evidences for a stimulatory role of GTP in regulation of adenylyl cyclase systems were published in 1971 [1,2]. By 1980 a separate component, responsible for mediation of hormonal stimulation of adenylyl cyclases, had been purified [3]. This component, initially referred to as G/F and Ns, is now called Gs. It is a het-erotrimeric complex composed of a subunits that migrate on SDS-PAGE at 42 and 52 kDa [3], (3 subunits of ca. 35 kDa [3], and y subunits of 6-10 kDa [4] (For reviews see Refs. 5 and 6). Its a subunits are ADP-ribosylated by CTX [7], dissociate from the holocomplex after activation [8,9] and hydrolyze GTP [10]. The a subunits have been cloned in several laboratories [11-17] and their amino acid composition has been deduced from the cDNA nucleotide sequence. The amino acid sequence of one of two types of /3 subunits, called /336 [18], has also been deduced from its cDNA [19-21]. The amino acid sequence of the y subunit is not yet known. 

Role of GTP-binding proteins in receptor-response coupling... 

Kowluru, A., et al. (1996). Glucose- and GTP-dependent stimulation of the carboxyl methylation of CDC42 in rodent and human pancreatic islets and pure beta cells. Evidence for an essential role of GTP-binding proteins in nutrient-induced insulin secretion. J Clin Invest 98 540-555. 

Achyuthan KE, Greenberg CS. Identification of a guanosine triphosphate-binding site on guinea pig liver transglutaminase. Role of GTP and calcium ions in modulating activity. J. Biol. Chem. 1987 262 1901-1906. 

Lin MC, Welton AF, Berman MF (1978) Essential role of GTP in the expression of adenylate cyclase activity after cholera toxin treatment. In J. Cycl. Nuc. Res. 4 159-168. 

Winder, B.S., Strandgaard, C.S., and Miller, M.G. 2001. The role of GTP binding and microtubule-associated proteins in the inhibition of microtubule assembly by carbendazim. Toxicol. Sci., 59, 138-146. 

Indicate the steps of the cycle that yield CO2, NADH, FADH2, and GTP. Note the biological roles of GTP. 

The progression of benign tumors to malignant cancer is the most critical step in carcinogenesis, since malignant lesions are capable of metastatic spread that eventually results in fatal consequences (Athar et al., 1991). Studies were conducted to assess the possible role of GTP in prevention against the conversion of chemically induced benign... 

Figure 13.20 Diagrams for eukaryotic translocations across the ER membrane. The mammalian co-tanslational translocation (a) and yeast post-translational translocation (b) of polypeptide chain are diagrammatically represented. Abbreviations used are SRP, signal recognition particle SR, SRP receptor and TRAM, translocating chain-associated membrane protein. Sec61p spans the ER membrane multiple times and likely forms the translocation channel. The cytosolic components, SsalP and Ydjlp which maintain the nascent polypeptide chain in the unfolded state in the post-translational translocation. The nascent polypeptide-associated complex (NAC) which maintains the fidelity of co-translational precursor and the role of GTP are not shown...

A GTP-dependent acyl-CoA synthetase of broad substrate specificity was once described and characterized from rat liver (Galzigna et a/., 1967) but the rates of GTP-dependent activation of fatty acid in most tissues including liver are only a very small fraction of those observed with ATP (Pande and Mead, 1968). The occurrence and the role of GTP-dependent activation in fatty acid metabolism continue to remain uncertain (Groot et a/., 1976). 

The fate of EF-G after translocation is not clear, but the protein is believed to be released and recycled. The role of GTP hydrolysis in protein synthesis remains unknown. To date, no functional phosphorylation or guanylation of any of the factors involved has been described. 

Sato, K., and Nakano, A. (2004). Reconstitution of coat protein complex II (COPII) vesicle formation from cargo-reconstituted proteoliposomes reveals the potential role of GTP hydrolysis by Sarlp in protein sorting. J. Biol. Chem. 279, 1330-1335. 

The EF-Tu-promoted, GTP-dependent binding of aminoacyl-tRNA may be considered as a process analogous to active transport. In this reaction, EF-Tu functions as a carrier protein which promotes the energy-dependent, unidirectional transport of aminoacyl-tRNA to ribosomes. On the other hand, the translocation reaction may be compared to muscle contraction since mRNA moves or slides on the 308 ribosomal subunit. What I wish to show here is that both reactions are mediated through essentially similar mechanisms and that the role of GTP in these two reactions is quite comparable. 

Role of GTP in the F-Ta-promoted binding of aminoacyl-tRNA to ribosomes... 

The role of GTP in EF-Tu-promoted binding of aminoacyl-tRNA could be formulated as follows. First, a unique conformation of EF-Tu induced by GTP can select exclusively the aminoacylated form of tRNA in preference to its deacylated form. Second, the ternary complex, aminoacyl-tRNA-EF-Tu-GTP, is transferred to a precise location on the 508 ribosomal subunit through the conformation of EF-Tu-GTP favorable for interaction with ribosomes. The conformational change of aminoacyl-tRNA induced by complexing with EF-Tu-GTP may also serve for this interaction. Third, after the transfer of aminoacyl-tRNA to the A site of ribosomes, EF-Tu is to be released from ribosomes to reinitiate a new cycle of reactions. This could be accomplished by the hydrolysis of bound GTP to GDP. An additional advantage of the split of GTP is to shift the equilibrium irreversibly... 

As has been described above and also elsewhere, the role of GTP in EF-Tu- and EF-G-promoted reactions appears to be quite analogous. In both cases, the conformation as well as reactivity of the protein molecules are reversibly and qualitatively altered by the chan of its nucleotide ligands. A single turnover reaction is accomplished utilizing the specific conformation induced by GTP, and the splitting of GTP is required to shift the protein to an alternate conformation. 

Yokosawa, H., Kawakita, M., Arai, K., Inoue-Yokosawa, N. and Kaziro, Y. (1975) Binding of aminoacyl-tRNA to ribosomes promoted by elongation factor Tu further studies on the role of GTP hydrolysis. J. Biochem. Tokyo), 77, 719-728. 

Kaziro, Y. (1973) The role of GTP in the pol)rpeptide elongation reaction in E. coli. In Organiiation of Energy Transducing Membranes (M. Nakao and L. Packer, eds.), pp. 187-200, University of Tokyo Press, Tokyo. 

Subsequently, other investigators established that the release of some initiation and termination factors from the ribosome also depends on GTP cleavage. Further insight into the role of GTP and its cleavage in protein synthesis has been provided by the studies of... 

Fusidic acid inhibits EFG activity [eqn. (10)] since the stable fusidic acid complex prevents EFG from acting catalytically in translocation. A tight binding of EFG to the ribosome can also be achieved with GDPCP. In this case a SOS subunit-EFG-GDPCP complex is formed. The data indicate that since hydrolysis does not occur with this analog, EFG cannot recycle and translocation is inhibited. In retrospect, the studies with fusidic acid have focused our thinking on the overall role of GTP hydrolysis in protein synthesis. 

Ertel, R., Redfield, B. and Weissbach, H. (1968) Role of GTP in protein synthesis interaction of GTP with soluble transfer factors from E. coli. Arch. Biochem. Biophys. 128, 331-338. 

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