Properties of a novel GTP-binding protein which is associated with soluble phosphoinositides-specific phospholipase C

Biochemical characterization of a phosphatidylinositol 4,5-bisphosphate-specific phospholipase C activity in gills and digestive gland of the marine mussel Mytilus galloprovincialis Lam

1. Polyphosphoinositide-specific phosphodiesterase (phospholipase C, PLC) activity against phosphatidylinositol 4,5-bisphosphate, present in gill and digestive gland homogenates of mussel (Mytilus galloprovincialis Lam.), has been biochemically characterized. 2. The enzyme was strictly modulated by free calcium ion concentration in both tissues and maximally activated at 10(-5) M Ca2+ (19 +/- 4 and 11 +/- 2 nmol phosphatidylinositol 4,5-bisphosphate hydrolysed/min/mg of protein for gill and digestive gland PLC, respectively, at 19 degrees C). Optimum pH at 10(-5) M Ca2+ was around 7.0 in both cases. The Ca(2+)-stimulated PLC activity showed high specificity for PIP2; the KMa for PIP2 were 150 and 170 microM for the gills and digestive gland, respectively. 3. Good substrate dispersion was obtained in the presence of sodium deoxycholate; the concentration routinely used in the assay (0.08%) produced a 9-fold activation of both gill and digestive gland PLC, consistent with previous reports. 4. The possible biochemical and physiological role of the enzyme in mussel tissues is discussed.

Multiple forms of the G protein-related beta subunit in Daudi lymphoblastoid cells

We have explored the forms of the G protein-related beta subunit which are present in Daudi lymphoblastoid cells. Northern blotting with labeled beta-1 and beta-2 probes indicates that two messages of 3.3 kb and 1.7 kb are present for both beta-1 and beta-2, implying that multiple forms of the beta subunit are present. Antibodies were raised against two peptides of the beta subunit (residues 1-23 and 127-145). Both antibodies detected subunits at 35 kDa and 31 kDa, of which the 35-kDa form predominates in the membrane fraction and the 31-kDa one in the cell cytosol. Crosslinking of the membrane fraction with the cleavable crosslinker (DTSSP) caused a simultaneous diminution in the 31-kDa form while increasing the amount of the 35-kDa form--a pattern which was reversed upon the reduction of these crosslinks with DTT. Studies of the soluble form indicate that this is truly a soluble protein since centrifugation at 200,000 x g for 2 h did not diminish the levels of the protein in the soluble fraction. Sedimentation analysis indicates that the soluble beta-homologue is found in fractions which overlap with those which contain the mu chain of immunoglobulin at a position clearly distinct from the expected positions of free mu or free beta. Our results suggest that at least two forms of a subunit which is closely related to, or identical with, the beta subunit of G proteins are present in Daudi cells.

G proteins in T cell signal transduction

This review summarises recent data on G protein implication in receptor signalling in T cells. The data show that PPI-specific PLC in T cell membranes is under G protein control. Some evidence indicates that a G protein couples PLC to TCR. Differences are revealed between the effects induced by direct G protein activators, such as GTP gamma S or AlF4-, and TCR ligands, which imply that TCR ligands may trigger some G-protein-independent signals. An analysis of the conflicting results on the action of PTX and CTX, one of the main tools in studying G proteins, has shown that the toxins produce both G protein-dependent and independent effects. The G protein which couples PLC to TCR appears insensitive to both PTX and CTX. Some findings suggest G protein involvement in signalling induced by interleukins; however, in this case the effector molecules implicate often remain unknown. Scarce data on G protein involvement in signalling from differentiation antigens, on direct G protein regulation of ion channels, and on identification of G proteins in T cells, are also discussed.

Characteristics of phosphoinositide-specific phospholipase C activity from mouse pancreatic islets

In pancreatic islets the bulk of phosphoinositide-specific phospholipase C (PI-PLC) activity was cytosolic. The soluble enzyme was activated by submicromolar concentrations of Ca2+, independent of calmodulin. It was unaffected by glucose and a series of glycolytic intermediates, including glyceraldehyde 3-phosphate. These observations lend support to the hypothesis that glucose-stimulated inositol triphosphate production in islets may be secondary to and provoked by glucose-mediated Ca2+ influx. All four pyridine nucleotides stimulated PI-PLC. Phosphatidylinositol hydrolysis was also stimulated by dioleine and arachidonic acid, and by the polyamines, putrescine and spermine. Phosphatidylinositol hydrolysis was inhibited by chlorpromazine, tetracaine, ATP, 5'-AMP, inorganic pyrophosphate and by phosphatidylinositol 4,5-bisphosphate, phosphatidylcholine and phosphatidylserine--but not affected by phosphatidylethanolamine. The cyclic nucleotides, cAMP and cGMP had no effect on the enzyme, and GTP-gamma-S did not activate the enzyme event at very low Ca2+ concentrations. The diglyceride lipase inhibitor, RHC 80267, and the cyclooxygenase inhibitor, indomethacin, had no effect on PI-PLC activity.

Purification, properties, and phosphorylation by protein kinase C of two phosphoinositidase C isozymes from rat brain

Two forms of phosphoinositidase C have been purified from the soluble fraction of rat brain. The purification scheme included gel filtration followed by chromatography on cellulose phosphate, phenyl-Sepharose, and Mono Q. Gradient sodium dodecyl sulphate-polyacrylamide gel electrophoresis gave apparent molecular masses of 151 kDa and 147 kDa. Western blotting with monoclonal antibodies showed that the isozymes corresponded to PLC-beta-1 and PLC-gamma of bovine brain. With both enzymes phosphatidylinositol 4,5-bisphosphate was a better substrate than phosphatidylinositol at neutral pH and low calcium ion concentrations. Both enzymes produced a proportion of inositol 1:2-cyclic phosphates from each substrate, particularly at acid pH. Some GTPase activity was seen in the early stages of purification, but was separated from PLC-beta-1 and PLC-gamma on Mono Q. Purified rat brain protein kinase C phosphorylated PLC-gamma but not PLC-beta-1. Incubation with the kinase increased the activity of both enzymes however, possibly by phosphorylation of another protein in the preparations.

Guanosine 5'-[gamma-thio]triphosphate-stimulated hydrolysis of phosphatidylinositol 4,5-bisphosphate in HL-60 granulocytes. Evidence that the guanine nucleotide acts by relieving phospholipase C from an inhibitory constraint

Myeloid differentiated human leukaemia (HL-60) cells contain a soluble phospholipase C that hydrolysed phosphatidylinositol 4.5-bisphosphate and was markedly stimulated by the metabolically stable GTP analogue guanosine 5'-[gamma-thio]triphosphate (GTP[S]). Half-maximal and maximal (up to 5-fold) stimulation of inositol phosphate formation by GTP[S] occurred at 1.5 microM and 30 microM respectively. Other nucleotides (GTP, GDP, GMP, guanosine 5'-[beta-thio]diphosphate. ATP, adenosine 5'-[gamma-thio]triphosphate, UTP) did not affect phospholipase C activity, GTP[S] stimulation of inositol phosphate accumulation was inhibited by excess GDP, but not by ADP. The effect of GTP[S] on inositol phosphate formation was absolutely dependent on and markedly stimulated by free Ca2+ (median effective concn. approximately 100 nM). Analysis of inositol phosphates by anion-exchange chromatography revealed InsP3 as the major product of GTP[S]-stimulated phospholipase C activity. In the absence of GTP[S], specific phospholipase C activity was markedly decreased when tested at high protein concentrations, whereas GTP[S] stimulation of the enzyme was markedly enhanced under these conditions. As both basal and GTP[S]-stimulated inositol phosphate formation were linear with time whether studied at low or high protein concentration, these results suggest that (a) phospholipase C is under an inhibitory constraint and (b) GTP[S] relieves this inhibition, most likely by activating a soluble GTP-binding protein.

Histamine receptors: subclasses and specific ligands

In this review the three main types of histamine receptors are discussed together with their specific ligands. For the classical H1-receptors much emphasis is put on the mechanism by which the receptor is stimulated. For the H1- and H2-receptor the review includes information on the several models available for establishing agonistic or antagonistic activity. In the section on the H3-receptor the ligands are discussed as well as the possible physiological role of this receptor. In the final paragraphs some less well defined activities are presented.

G regulatory proteins and muscarinic receptor signal transduction in mucous acini of rat submandibular gland

The involvement of G regulatory proteins in muscarinic receptor signal transduction was examined in electrically permeabilized rat submandibular acinar cells. The guanine nucleotide analog, GTP gamma S, caused the dose dependent hydrolysis of membrane phosphatidylinositol 4,5-bisphosphate to release IP3. This response was insensitive to pertussis toxin treatment and was duplicated by NaF but not by GDP beta S. Enhanced IP3 synthesis was observed with a combination of GTP gamma S and carbachol. Exogenous IP3, as well as carbachol and GTP gamma S, provoked the release of sequestered 45Ca2+ from non-mitochondrial stores. In intact cells, carbachol significantly reduced the level of cyclic AMP induced by the beta-adrenergic agonist, isoproterenol, to 69% of its normal value. Pertussis toxin abolished this inhibitory action of carbachol on cyclic nucleotide levels. These results suggest that muscarinic receptors are coupled to two separate G regulatory proteins in submandibular mucous acini-the pertussis toxin-insensitive Gp of the phosphoinositide transduction pathway associated with elevated cytosolic calcium levels, and the pertussis toxin-sensitive Gi inhibitory protein of the adenylate cyclase complex.