Post-translational processing of rac p21s is important both for their interaction with the GDP/GTP exchange proteins and for their activation of NADPH oxidase

rac1 and rac2 p21s are ras p21-like small GTP-binding proteins which are implicated in the NADPH oxidase-catalyzed superoxide generation in phagocytes. rac1 and rac2 p21s have a Cys-A-A-Leu (A = aliphatic amino acid) structure in their C-terminal region which may undergo post-translational processing including prenylation, proteolysis, and carboxyl methylation. We studied the function of this post-translational processing of rac p21s in their interaction with the stimulatory and inhibitory GDP/GTP exchange proteins for rac p21s, named smg GDS and rho GDI, and in their NADPH oxidase activation. We produced human recombinant rac1 and rac2 p21s in insect cells and purified them from the membrane and soluble fractions as the post-translationally processed and unprocessed forms, respectively. Post-translationally processed rac1 and rac2 p21s were sensitive to both smg GDS and rho GDI, but post-translationally unprocessed rac1 and rac2 p21s were insensitive to them. The GTP gamma S (guanosine 5'-(3-O-thio)triphosphate)-bound form of post-translationally processed rac1 and rac2 p21s stimulated the NADPH oxidase activity, but post-translationally unprocessed rac1 and rac2 p21s were far less effective. These results indicate that both rac1 and rac2 p21s stimulate the NADPH oxidase activity and that their post-translational processing is important not only for their interaction with smg GDS and rho GDI but also for their NADPH oxidase activation.

The identification and characterization of a GDP-dissociation inhibitor (GDI) for the CDC42Hs protein

The ras-related protein, CDC42Hs, is a 22-kDa GTP-binding protein which is the human homolog of a Saccharomyces cerevisiae yeast-cell-division cycle protein. In attempting to isolate and biochemically characterize mammalian proteins capable of regulating various activities of CDC42Hs, we have identified an activity in bovine brain cytosol which effectively inhibits the dissociation of [3H]GDP from the platelet- or the Spodoptera frugiperda-expressed CDC42Hs protein. The purification of this activity was achieved by a series of steps which included ammonium sulfate fractionation, DEAE-Sephacel, Mono-Q, and Mono-S chromatographies. The purified CDC42Hs regulatory protein has an apparent molecular weight of 28,000, and cyanogen bromide-generated peptide sequences of this protein were identical to sequences from the carboxyl-terminal portion of rho-GDP-dissociation inhibitor (rho-GDI) (Fukumoto, Y., Kaibuchi, K., Hori, Y., Fujioka, H., Araki, S., Ueda, T., Kikuchi, A., and Takai, Y. (1990) Oncogene 5, 1321-1328). In addition, an Escherichia coli-expressed, glutathione S-transferase-rho-GDI fusion protein fully substitutes for the GDI which we have purified from bovine brain in its ability to inhibit GDP dissociation from CDC42Hs. These findings suggest either that a common regulatory protein (GDI) is capable of inhibiting GDP dissociation from the rho and CDC42Hs proteins or that these two GTP-binding proteins interact with GDI proteins of very similar structure. The purified brain GDI protein shows little ability to inhibit GDP dissociation from the E. coli-expressed CDC42Hs and is capable of only a very weak inhibition of the dissociation of [35S]guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) from the Spodoptera frugiperda-expressed CDC42. However, brain GDI very effectively inhibits the ability of the human dbl oncogene product to catalyze GDP dissociation from CDC42Hs. In addition to influencing guanine nucleotide association with CDC42Hs, the purified brain GDI protein also appears to catalyze the dissociation of CDC42Hs from the plasma membranes of human placenta and human epidermoid carcinoma (A431) cells. This effect by the GDI protein is observed whether the membrane-associated CDC42Hs is preincubated with GDP, GTP gamma S, or no guanine nucleotides, and occurs over a similar concentration range as that necessary for the inhibition of the intrinsic GDP dissociation.

A GDP dissociation inhibitor that serves as a GTPase inhibitor for the Ras-like protein CDC42Hs

Members of the family of Ras-related guanosine triphosphate (GTP) binding proteins appear to take part in the regulation of a number of biological processes, including cell growth and differentiation. Three different classes of proteins that regulate the GTP binding and GTP hydrolytic activities of the Ras family members have been identified. These different regulatory proteins inhibit guanosine diphosphate (GDP) dissociation (designated as GDIs), stimulate GDP dissociation and GDP-GTP exchange (designated as GDSs), or stimulate GTP hydrolysis (designated as GAPs). In the case of the Ras-like protein CDC42Hs, which is the human homolog of a Saccharomyces cerevisiae cell division cycle protein, the GDI protein also inhibited both the intrinsic and GAP-stimulated hydrolysis of GTP. These findings establish an additional role for the GDI protein--namely, as a guanosine triphosphatase (GTPase) inhibitory protein for a Ras-like GTP binding protein.

Cooperative function of rho GDS and rho GDI to regulate rho p21 activation in smooth muscle

The GDP/GTP exchange reaction of rho p21, a member of ras p21-related small GTP-binding protein superfamily, is regulated by two stimulatory GDP/GTP exchange proteins (GEPs), named smg GDS and rho GDS, and by one inhibitory GEP, named rho GDI. In bovine aortic smooth muscle, rho GDS and rho GDI were major GEPs for rho p21, and the rho GDI activity on the GDP/GTP exchange reaction of rho p21 was stronger than the rho GDS activity in their simultaneous presence. Moreover, in the crude cytosol, the GDP-bound form of rho p21 was complexed with rho GDI but not with rho GDS. These results, together with our recent finding that rho p21 is involved in the vasoconstrictor-induced Ca2+ sensitization of smooth muscle contraction, suggest that there is some mechanism to release the inhibitory action of rho GDI and to make rho p21 sensitive to the stimulatory action of rho GDS, eventually leading to the rho p21 activation, in the signaling pathways of the vasoconstrictor receptors in smooth muscle.

A novel type of regulatory protein for the GDP/GTP exchange reaction of rho p21, a ras p21-like small GTP-binding protein, in rabbit intestine

A novel regulatory protein for rho p21, a ras p21-like GTP-binding protein (G protein), was purified from the cytosol fraction of rabbit intestine. This protein, designated as rho GDP dissociation inhibitor (GDI), regulated the GDP/GTP exchange reaction of rho p21 by inhibiting the dissociation of GDP from and subsequent binding of GTP to it. rho GDI did not affect the GTPase activity of rho p21. rho GDI formed a complex with the GDP-bound form of rho p21 but not the GTP-bound form. rho GDI was inactive for other small G proteins including ras p21, smg p21 and smg p25A. These results indicate that rho GDI is a novel type of regulatory protein for the GDP/GTP exchange reaction of rho p21.

Molecular cloning of the human cDNA for a stimulatory GDP/GTP exchange protein for c-Ki-ras p21 and smg p21

We have previously purified smg GDP dissociation stimulator (GDS) from bovine brain and isolated its cDNA from a bovine brain cDNA library. smg GDS stimulates the GDP/GTP exchange reaction of a group of small GTP-binding proteins (G proteins), including at least c-Ki-ras p21, smg p21A, smg p21B, rhoA p21 and rhoB p21, by stimulating the dissociation of GDP from and the subsequent binding of GTP to each small G protein. In this study, we have isolated and sequenced the cDNA of smg GDS from a human brain cDNA library using the cloned bovine smg GDS cDNA. The cDNA has an open reading frame encoding a protein of 558 amino acids with a calculated Mr value of 61,122. Human smg GDS shares 93% nucleotide and 96% amino acid sequence homologies with bovine smg GDS. The isolated cDNA is expressed in Escherichia coli, and the encoded protein shows the physical and functional properties similar to those of bovine smg GDS.

Both stimulatory and inhibitory GDP/GTP exchange proteins, smg GDS and rho GDI, are active on multiple small GTP-binding proteins

Six peaks of small GTP-binding proteins (G proteins) were separated by column chromatographies from the cytosol fraction of the differentiated HL-60 cells: two peaks of rho p21, one peak of smg/rap1 p21, two peaks of rac1 p21, and one peak of an unidentified small G protein with a Mr of about 20,000 (20 KG). smg GDS, previously thought to be a stimulatory GDP/GTP exchange protein for smg p21, Ki-ras p21, and rho p21, but not for Ha-ras p21 or smg p25A, was also active on rac1 p21. rho GDI, previously thought to be an inhibitory GDP/GTP exchange protein specific for rho p21, was also active on rac1 p21. These results indicate that both smg GDS and rho GDI are active on multiple small G proteins.

A mammalian inhibitory GDP/GTP exchange protein (GDP dissociation inhibitor) for smg p25A is active on the yeast SEC4 protein

Evidence is accumulating that smg p25A, a small GTP-binding protein, may be involved in the regulated secretory processes of mammalian cells. The SEC4 protein is known to be required for constitutive secretion in yeast cells. We show here that the mammalian GDP dissociation inhibitor (GDI), which was identified by its action on smg p25A, is active on the yeast SEC4 protein in inhibiting the GDP/GTP exchange reaction and is capable of forming a complex with the GDP-bound form of the SEC4 protein but not with the GTP-bound form. These results together with our previous findings that smg p25A GDI is found in mammalian cells with both regulated and constitutive secretion types suggest that smg p25A GDI plays a role in both regulated and constitutive secretory processes, although smg p25A itself may be involved only in regulated secretory processes. These results also suggest that a GDI for the SEC4 protein is present in yeast cells.

Tissue and subcellular distributions of an inhibitory GDP/GTP exchange protein (GDI) for smg p25A by use of its antibody

We have recently purified from bovine brain cytosol to near homogeneity a GDP/GTP exchange protein for smg p25A, named smg p25A GDI, that inhibits the dissociation of GDP from and the subsequent binding of GTP to smg p25A. In the present study, we made an antiserum against smg p25A GDI and studied its tissue distribution in rat and its subcellular distribution in rat cerebrum by use of this antiserum. smg p25A GDI was found in secretory cells with both regulated and constitutive secretion types. Since smg p25A was previously found in only secretory cells with a regulated secretion type, this result suggests that small GTP-binding proteins different from smg p25A but recognized by smg p25A GDI are present in secretory cells with a constitutive secretion type, and that smg p25A GDI is involved in both regulated and constitutive secretory processes. In subcellular fractionation analysis of rat cerebrum, smg p25A GDI was mostly found in the cytosol fraction of neuron body and synaptosome. In synaptosome, it was mainly found in the synaptic cytosol.

Purification and characterization from rat liver cytosol of a GDP dissociation inhibitor (GDI) for liver 24K G, a ras p21-like GTP-binding protein, with properties similar to those of smg p25A GDI

A regulatory protein for a liver GTP-binding protein (G protein) with a molecular weight value of 24,000 (24K G), which we have recently purified, was purified to near-homogeneity from rat liver cytosol and characterized. This regulatory protein, designated here as GDP dissociation inhibitor for 24K G (24K G GDI), inhibited the dissociation of GDP from and the subsequent binding of GTP to 24K G. 24K G GDI was inactive for other ras p21/ras p21-like small G proteins including c-Ha-ras p21, rhoB p20, smg p21B, and smg p25A. 24K G was, however, recognized by bovine brain smg p25A GDI which regulated the GDP/GTP exchange reaction of smg p25A. By analyses of sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE), immunoblotting with anti-smg p25A GDI antibody, two-dimensional PAGE, and C4 column chromatography, 24K G GDI showed physical properties very similar to those of smg p25A GDI. The peptide map and the partial amino acid sequences of 24K G GDI were not identical with those of smg p25A GDI. Among the 83 residues, 2 amino acids were different between rat liver 24K G GDI and bovine brain smg p25A GDI. These results indicate that there is a specific regulatory protein for 24K G, 24K G GDI, in rat liver cytosol and that 24K G GDI has close similarity to smg p25A GDI.

Regulation of binding of rhoB p20 to membranes by its specific regulatory protein, GDP dissociation inhibitor

We have previously purified from bovine brain cytosol a novel regulatory protein for the GTP-binding proteins of the rho gene family. This regulatory protein, designated as rho GDP dissociation inhibitor (GDI), makes a complex stoichiometrically with the GDP-bound form of the rho gene products and thereby regulates the GDP/GTP exchange reaction by inhibiting the dissociation of GDP from and the subsequent binding of GTP to them. We show here that rho GDI also regulates the binding of rhoB p20, a member of the rho gene products, to various membranes including rat brain synaptic plasma membranes and human erythrocyte ghosts. Both the GDP- and GTP-bound forms of rhoB p20 bound to the membranes. This binding was not inhibited by prior treatment of the membranes with boiling or tryptic digestion, suggesting that a protein molecule of the membranes is not essential for the binding of rhoB p20 to the membranes. rho GDI inhibited the binding of the GDP-bound form of rhoB p20, but not that of the GTP-bound form, to the membranes. Moreover, rho GDI induced the dissociation of the GDP-bound form, but not that of the GTP-bound form, of rhoB p20 exogenously bound to the membranes from them. These results suggest that the rho gene products bind to the membranes, presumably in a reversible manner and that this binding is regulated by their specific GDI.

Molecular cloning and characterization of a novel type of regulatory protein (GDI) for the rho proteins, ras p21-like small GTP-binding proteins

We have recently purified to near homogeneity a novel type of regulatory protein for the rho proteins, ras p21-like small GTP-binding proteins, from bovine brain cytosol. This regulatory protein, named GDP dissociation inhibitor for the rho proteins (rho GDI), regulates the GDP/GTP exchange reaction of the rho proteins by inhibiting the dissociation of GDP from them, and the subsequent binding of GTP to them. In the present studies, we have isolated the cDNA of rho GDI from a bovine brain cDNA library using oligonucleotide probes designed from the partial amino acid sequences of the purified rho GDI and determined its complete nucleotide and deduced amino acid sequences. The cDNA contains an open reading frame encoding a protein of 204 amino acids with a calculated Mr value of 23,421. This Mr value is similar to those of the purified rho GDI estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and sucrose density gradient ultra-centrifugation, both of which are about 27,000. The rho GDI cDNA is expressed in Escherichia coli and COS7 cells and the encoded protein exhibits rho GDI activity. The 1.9-kilobase rho GDI mRNA corresponding to the isolated cDNA is detected in various rat tissues by Northern blot analysis. Hydropathy analysis indicates that rho GDI is overall hydrophilic except for one hydrophobic region. Computer homology search has revealed that rho GDI is a novel protein that does not share a high amino acid sequence homology with any known protein.

Rabbit intestine contains a protein that inhibits the dissociation of GDP from and the subsequent binding of GTP to rhoB p20, a ras p21-like GTP-binding protein

A novel regulatory protein for rhoB p20, a ras p21-like GTP-binding protein (G protein), was partially purified from the cytosol fraction of rabbit intestine. This protein, designated as rhoB p20 GDP dissociation inhibitor (GDI), inhibited the dissociation of GDP from rhoB p20. rhoB p20 GDI also inhibited the binding of guanosine 5'-(3-O-thio)triphosphate (GTP gamma S) to the GDP-bound form of rhoB p20 but not of that to the guanine nucleotide-free form. GDI did not affect the GTPase activity of rhoB p20 and by itself showed no GTP gamma S-binding activity. GDI was inactive for other ras p21/ras p21-like G proteins including c-Ha-ras p21, smg p21 and smg p25A. The Mr value of GDI was estimated to be about 27,000 from the S value. These results indicate that rabbit intestine contains a novel regulatory protein that inhibits the dissociation of GDP from and thereby the subsequent binding of GTP to rhoB p20.