GTP binding proteins

DAGs have been shown to stimulate a Ca2+-dependent protein kinase (PKC) by increasing the affinity of this enzyme for phospholipids and Ca2+ [95,96]. Initial evidence for a similar mechanism in the GnRH stimulus transmission was obtained from experiments in which phorbol dibutyrate and phorbol 12-myristate 13-acetate were able to stimulate LH release [53,97,98]. This stimulatory effect of phorbol esters is probably caused by binding and activation of PKC, thereby mimicking endogenous DAGs [99]. 

In order to obtain more direct evidence for an activational role of DAGs on PKC, water soluble DAGs were synthesized and added to pituitary cells [100]. Synthetic DAGs which had an acyl chain length between 4 and 10 carbons in the sn-1 and 2 position were able to stimulate PKC activity as well as LH release. Struc- 

Upon stimulation by an agonist but not an antagonist, PKC redistributes from the cytosolic pool to the particulate fraction. This redistribution could be measured in vivo [102] and in vitro [103-105] and was shown to be dependent upon Ca2+ influx [102]. These data suggest that a minimal intracellular concentration of Ca2+ is required for adequate PKC stimulation by DAGs. Since DAG production and translocation of PKC are parallel events it appears that PKC mediates endogenous DAG stimulation of LH secretion. 

Binding of GnRH to its plasma membrane receptor [108] results in regulation of receptors and of cellular responsiveness, as well as release of the gonadotropins, 

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G-Protein Coupling. The heterotrimeric guanosine triphosphate (GTP) binding proteins, known as G-proteins, are a principal family of proteins serving to couple membrane receptors of the G-protein family to ionic and biochemical processes. This topic is reviewed in References 63—67. 

Five of the six loop regions (G1-G5 in Figure 13.4) that are present at the carboxy end of the p sheet in the Ras structure participate in the GTP binding site. Three of these loops, G1 (residues 10-17), G3 (57-60), and G4 (116-119), contain regions of amino acid sequence conserved among small GTP-binding proteins and the Ga subunits of trimerlc G proteins. 

The hormonal stimulation of adenylyl cyclase is effected by a transmembrane signaling pathway consisting of three components, all membrane-associated. Binding of hormone to the external surface of a hormone receptor causes a conformational change in this transmembrane protein, which in turn stimulates a GTP-binding protein (abbreviated G protein). G proteins are heterotrimeric proteins consisting of a- (45-47 kD), /3- (35 kD), and y- (7-9 kD) subunits. The a-subunit binds GDP or GTP and has an intrinsic, slow... 

Costa, T., and Herz, A. (1989). Antagonists with negative intrinsic activity at 6-opioid receptors coupled to GTP-binding proteins. Proc. Natl. Acad. Sci. U.S.A. 86 7321-7325. 

GTP-binding protein at an arginine residue which is involved in GTP hydrolysis. ADP-ribosylation thus leads to constitutive activation of Gs. 

Heterotrimeric GTP-binding Proteins Small GTPases Bacterial Toxins... 

In general terms, cross talk refers to the interaction between signalling pathways, e.g. between pathways involving heterotrimeric GTP-binding proteins and tyrosine kinase pathways. 

Histidine phosphatases and aspartate phosphatases are well established in lower organisms, mainly in bacteria and in context with two-component-systems . Reversible phosphorylation of histidine residues in vertebrates is in its infancy. The first protein histidine phosphatase (PHP) from mammalian origin was identified just recently. The soluble 14 kD protein does not resemble any of the other phosphatases. ATP-citrate lyase and the (3-subunit of heterotrimeric GTP-binding proteins are substrates of PHP thus touching both, metabolic pathways and signal transduction [4]. 

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. 

The diversity of these subcellular actin structures is remarkable and appears to be determined by the interactions of many actin-binding proteins (ABPs) as well as by changes in the concentrations of intracellular signaling molecules such as Ca and cAMP, by small GTP-binding proteins, and by signals arising from mechanical stress. Approximately 50% of the actin molecules in most animal cells are unpolymerized subunits in the cytosolic pool and exist in a state of dynamic equilibrium with labile F-actin filamentous structures (i.e., new structures are formed while existing structures are renewed) (Hall, 1994). 

Hall, A. (1994). Small GTP-binding proteins and the regulation of the actin cytoskeleton. Ann. Rev. Cell Biol. 10,31-54. 

Ridley, A.J., Hall, A. (1992). The small GTP-binding protein rho regulates the assembly of focal adhesions and actin stress fibers in response to growth factors. Cell 70,389-399. 

The polyisoprenoids dolichol (Figure 14-20 and Chapter 47) and ubiquinone (Figure 12-5) are formed from farnesyl diphosphate by the further addition of up to 16 (dolichol) or 3-7 (ubiquinone) isopentenyl diphosphate residues, respectively. Some GTP-binding proteins in the cell membrane are prenylated with farnesyl or geranylgeranyl (20 carbon) residues. Protein prenylation is believed to facilitate the anchoring of proteins into lipoid membranes and may also be involved in protein-protein interactions and membrane-associated protein trafficking. 

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