Reactivity of a sulfhydryl group of the ras oncogene product p21 modulated by GTP binding

Guanine nucleotide stimulation of norepinephrine secretion from permeabilized PC12 cells: effects of Mg2+, other nucleotide triphosphates and N-ethylmaleimide

The addition of either Ca2+ or guanosine 5'-O-3-(thiotriphosphate), GTP gamma S, to digitonin-permeabilized rat pheochromocytoma PC12 cells stimulates norepinephrine release. Unlike Ca(2+)-stimulated release, there is a delay between the time of addition of GTP gamma S to digitonin-permeabilized PC12 cells and stimulation of norepinephrine release. Preincubation of the permeabilized cells in the absence of Mg2+ eliminates this lag and increases the initial rate of GTP-gamma S-stimulated norepinephrine secretion. This suggests that the rate of GDP dissociation from the GTP-binding protein responsible for this stimulation is faster in the absence of Mg2+ than in its presence. While an equimolar concentration of GTP gives 50% inhibition of GTP gamma S-stimulated release, 100-fold excesses of ITP, ATP, UTP and CTP gave no inhibition of GTP gamma S-stimulated release. Both the inability of ITP to inhibit GTP gamma S-stimulated secretion and the increase in GTP gamma S-stimulated secretion caused by preincubation in the absence of Mg2+ indicate that some of the properties of the GTP-binding protein responsible for this stimulation are more like those of the low molecular weight GTP-binding proteins rap1 and ras than those of a heterotrimeric G-protein. Low concentrations of N-ethylmaleimide gave more inhibition of GTP gamma S-stimulated release than Ca(2+)-stimulated release which suggests that the mechanisms by which Ca2+ and GTP gamma S stimulate norepinephrine release are at least in part distinct.

Experimental studies on the mechanism of action of 4-hydroxy-2,3-trans-nonenal, a lipid peroxidation product displaying chemotactic activity toward rat neutrophils

The effects of 4-hydroxy-2,3-trans-nonenal (HNE) and nonanal on the activity of phosphoinositide-specific phospholipase C of rat neutrophils have been studied in parallel with their action on neutrophil oriented migration. Concentrations of HNE ranging from 10(-7) to 10(-5) M significantly stimulated the oriented migration of rat polymorphonuclear leukocytes. HNE stimulated both the basal and GTP gamma S-induced phospholipase C activity when used at concentrations between 10(-8) and 10(-6) M. Nonanal was devoid both of chemotactic activity and of any action on phospholipase C activity. The effect of GTP gamma S on the stimulation of phospholipase C induced by HNE was higher when the lowest dose of the aldehyde was used; the finding of an additive effect between 10(-8) M HNE and 2 x 10(-5) M GTP gamma S suggests that the two compounds may share a final common pathway of action. These results suggest that the chemotactic activity of HNE might be mediated, like that of other more well-known chemoattractants, by the stimulation of phosphoinositide-specific phospholipase C.

Heterogeneity of guanine nucleotide binding proteins in human red blood cell membranes

Membranes from human erythrocytes bind radioactive GTP and GTP analogs according to apparently homogeneous patterns. In spite of this uniform type of association, multiple guanine nucleotide binding proteins have been identified both by SDS-PAGE analysis of native and of variously ADP-ribosylated membrane preparations and by FPLC chromatography of solubilised erythrocyte membranes preliminarily incubated with [alpha-32P] GTP in the presence of 5 mM MgCl2. From eight to nine peak fractions of pronase-digestible GTP-binding activity were separated on a MA7Q anion exchange column, this pattern being highly reproducible with different membrane preparations. Prior incubation of membranes with [alpha-32P] GTP in the presence of excess unlabeled GDP resulted in displacement of bound labeled nucleotide from all FPLC fractions. The patterns of GTP binding were also markedly modified by preliminary treatment of membranes with N-ethylmaleimide. Detectable GTPase activity was present in each of the FPLC peak fractions. This wide heterogeneity of guanine nucleotide binding proteins raises so far unanswered questions as to their physiological significance in the mature erythrocyte.

Hydrolysis-resistant GTP analogs stimulate catecholamine release from digitonin-permeabilized PC12 cells

The effect of the hydrolysis-resistant GTP analogs, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) and guanylyl imidodiphosphate (GMPPNP), on norepinephrine (NE) secretion from digitonin-permeabilized rat pheochromocytoma cells, PC12, was examined. Although secretion in the presence of saturating Ca2+ (10 microM) was not affected by GTP gamma S or GMPPNP, secretion in the absence of Ca2+ was stimulated by these GTP analogs. Secretion induced by saturating concentrations of GTP gamma S or GMPPNP was approximately 80% of that induced by 10 microM Ca2+. Half-maximum stimulation was induced by 30 microM GTP gamma S or GMPPNP. Both Ca2(+)-stimulated and GTP gamma S-stimulated secretion were ATP dependent and inhibited by N-ethylmaleimide. The GTP gamma S-stimulated secretion of NE from permeabilized PC12 cells does not appear to result from either the release of Ca2+ or the activation of protein kinase C. Activation of protein kinase C by pretreatment of intact cells with 12-O-tetradecanoylphorbol 13-acetate caused a 50% increase in both Ca2(+)-stimulated and GTP gamma S-stimulated secretion. Cholera and pertussis toxins did not affect Ca2(+)-stimulated or GTP gamma S-stimulated NE secretion. Guanosine 5'-O-(2-thiodiphosphate) (GDP beta S) and GTP inhibited GTP gamma S-stimulated secretion but not Ca2(+)-stimulated secretion. The inability of GDP beta S to inhibit Ca2(+)-stimulated secretion indicates that the process affected by GTP gamma S is not an essential step in the Ca2(+)-stimulated pathway.

Conformational alterations detected by circular dichroism induced in the normal ras p21 protein by activating point mutations at position 12, 59, or 61

Activation of the oncogenic potential of ras oncogenes occurs by point mutations at codons 12, 13, 59, 61, and 63 of the sequences that codify for its product, a 21-kDa protein designated as p21. This activation has been postulated by computer models as modifiers of the structure of the protein, which may alter its biochemical and biological activities. We have expressed in bacteria the normal ras p21 and five mutated p21 proteins with mutations at positions 12, 59, 61, 12 plus 59, and 12 plus 61. Purification was carried out by solubilization from bacterial pellets in 7 M urea and chromatography through a Sephadex G-100 column to obtain greater than 95% purified proteins. Circular dichroic (CD) spectra showed that the normal protein and that activated by substitution of Ala59 to Thr59 are very similar in their overall structure. By contrast, point mutations affecting either 12 or 61 residues substantially altered the structure of the proteins. When the parameters of Chen et al. [Biochemistry II, 4120-4131 (1972)] were applied to the CD spectra, both normal and thr59-mutated ras proteins showed a less organized structure than mutated proteins at position 12 or 61. Since the Thr59 mutant has more similar transforming activity than other activated proteins, but a GTPase activity similar to that of the normal protein, our results support the hypothesis that there is more than one mechanism of activation of the ras p21 protein. One of these mechanisms involves important structural alterations by point mutations at position 12 or 61 which reduce the GTPase activity of the protein. Another mechanism will be that induced by a substitution of Ala59 to Thr59 which does not substantially alter the protein conformation. A putative alternative mechanism for the activation of this mutant is discussed.

Study of a temperature-sensitive mutant of the ras-related YPT1 gene product in yeast suggests a role in the regulation of intracellular calcium

Intragenic mutations were isolated that suppressed the dominant-lethal phenotype of the YPT1ile121 mutant gene in a temperature-dependent fashion. Among different amino acid substitutions resulting from single point mutations, two, Ala161----Val (A161V) and Met165----Ile (M165I), restored the function of the YPT1ile121 mutant protein. Mutants expressing the YPT1ile121/val161 allele (ypt1ts) only, grew normally at temperatures up to 30 degrees C but were arrested at 37 degrees C. At the restrictive temperature, ypt1ts mutants accumulated ER membranes, small vesicles, and unprocessed invertase, and they exhibited cytoskeletal defects and an enhanced 45Ca2+ uptake. Similar alterations were seen in YPT1-depleted cells. The ypt1ts mutant cells could be rescued from growth arrest by increasing extracellular Ca2+, and, even at the permissive temperature, they displayed increased trifluoperazine sensitivity.