Associative GTP

Dynamin is a microtubule-associated GTP-binding protein. Dynamins are activated by proteinkinase C. They have a PH, a pleckstrin-homology domain and bind inositol phospholipids which aaivate the GTPase of dynamin. Dynamins are involved in microtubule assembly and vesicular traffic. 

ADP-ribosylation completely blocks the actin ATPase activity and increases the rate of ATP exchange by about twofold (Geipel ef al., 1990 Geipel ef al., 1989). This effect is not due to inhibition of polymerization, because the basal ATPase activity of G-actin is also inhibited. Moreover, the ATPase activity of actin is blocked even in the quasi-monomeric actin-DNAse I complex after stimulation with the mycotoxin cytochalasin (Geipel ef al., 1990). Thus, by analogy with the ADP-ribosylation of G-proteins by cholera toxin, which inhibits G-protein-associated GTP hydrolysis, the ADP-ribosylation of actin inhibits its intrinsic ATPase activity. 

Schrezenmeier Ff, Ahnert-Hilger G, Fleischer B (1988) AT cell receptor-associated GTP-binding protein triggersTcell receptor-mediated granule exocytosis in cytotoxic T lymphocytes. J. Immunol. 141 3785-3790. 

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... 

FIGURE 15.21 Hormone (H) binding to its receptor (R) creates a hormone receptor complex (H R) that catalyzes GDP-GTP exchange on the o -subunit of the heterotrimer G protein (G ), replacing GDP with GTP. The G -subunit with GTP bound dissociates from the /37-subunits and binds to adenylyl cyclase (AC). AC becomes active upon association with G GTP and catalyzes the formation of cAMP from ATP. With time, the intrinsic GTPase activity of the G -subunit hydrolyzes the bound GTP, forming GDP this leads to dissociation of G GDP from AC, reassociation of G with the /Sy subunits, and cessation of AC activity. AC and the hormone receptor H are integral plasma membrane proteins G and G are membrane-anchored proteins. 

Stimulation and inhibition of the enzyme by the GPCR-G-protein cycle occur by analogous mechanisms. Agonists induce hormone receptors to increase a Ga-GDP-GTP exchange and subsequent Ga 3y dissociation (GDP-a py + GTP GTP-ax + [3y + GDP) (Fig. 4). Consequently, agents that affect either the dissociation of either G or Gs, or the association of their respective as, a , or (3y subunits with adenylyl cyclase could affect rates of cAMP formation in enzyme preparations or in intact cells and tissues. There are several important examples. Gas is stably activated by poorly hydrolyzable analogs of GTP, e.g. GTPyS... 

Membrane-bound GTP rabs recruit effectors to the membrane. In neurons and neuroendocrine cells, the vesicle-associated Rab3 binds to rabphilin and to RIM. RIM is a component of the presynaptic cytomatrix and may thus serve as a docking receptor for synaptic vesicles at the active zone. 

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. 

The first step in this process involves the binding of GTP by eIF-2. This binary complex then binds to met-tRNAf a tRNA specifically involved in binding to the initiation codon AUG. (There are two tRNAs for methionine. One specifies methionine for the initiator codon, the other for internal methionines. Each has a unique nucleotide sequence.) This ternary complex binds to the 40S ribosomal subunit to form the 43S preinitiation complex, which is stabilized by association with eIF-3 and elF-lA. 

The binding of the 60S ribosomal subunit to the 48S initiation complex involves hydtolysis of the GTP bound to elF-2 by elF-5. This teaction tesults in telease of the initiation factots bound to the 48S initiation complex (these factots then ate tecycled) and the tapid association of the 40S and 60S subunits to fotm the 80S ribosome. At this point, the met-tRNA is on the P site of the ribosome, ready for the elongation cycle to commence. 

St 1 Coat assembly is initiated when ARF is activated by binding GTP, which is exchanged for GDP. This leads to the association of GTP-bound ARF with its putative receptor (hatched in Figure 46-7) in the donor membrane. 

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