The function of glycosyl phosphoinositides in hormone action

S-phase DNA damage checkpoint in budding yeast

Eukaryotic cells must be able to coordinate DNA repair, replication and cell cycle progression in response to DNA damage. A failure to activate the checkpoints which delay the cell cycle in response to internal and external cues and to repair the DNA lesions results in an increase in genetic instability and cancer predisposition. The use of the yeast Saccharomyces cerevisiae has been invaluable in isolating many of the genes required for the DNA damage response, although the molecular mechanisms which couple this regulatory pathway to different DNA transactions are still largely unknown. In analogy with prokaryotes, we propose that DNA strand breaks, caused by genotoxic agents or by replication-related lesions, trigger a replication coupled repair mechanism, dependent upon recombination, which is induced by the checkpoint acting during S-phase.

Mutagenesis study of the glycosylphosphatidylinositol phospholipase C of Trypanosoma brucei

The glycosylphosphatidylinositol phospholipase C (GPI-PLC) from Trypanosoma brucei is particularly effective in hydrolysing the GPI-anchors of some proteins. The enzyme is inhibited by Zn2+ and p-chloromercurylphenylsulphonic acid, both of which can act as sulphydryl reagents, suggesting that a cysteine residue may be important in catalysis. Single cysteine to serine mutants have been produced for all eight cysteines in GPI-PLC; all the mutants were fully active in vitro and were still susceptible to p-chloromercurylphenylsulphonic acid inhibition. In contrast, a single histidine 34 to glutamine mutation totally inactivated GPI-PLC. The histidine was chosen after a sequence alignment with the Bacillus cereus phosphatidylinositol phospholipase C (PI-PLC) suggested a conservation of active site residues, including histidine 34 which is central to the proposed reaction mechanism (Heinz D.W., Ryan M., Bullock T.L., Griffith O.H. EMBO J 1995;14:3855-3863). The results suggest that the GPI-PLC and bacterial PI-PLCs have conserved active sites and that the inhibition of GPI-PLC by sulphydryl reagents can occur through more than one residue.

Cell signalling by inositol phosphoglycans from different species

The discovery of glycosyl-phosphatidylinositol (GPI) molecules and their products has given new insight into the field of signal transduction. In the last decade a novel mechanism of protein attachment to membranes has emerged, which involves a covalent linkage of the protein to the glycan moiety of a GPI. The discovery that GPI-anchored proteins are ubiquitous throughout the eukaryotes was followed by the observation that uncomplexed GPI molecules are implicated in signal transduction for a diversity of hormones and growth factors. The hydrolysis of free-GPI generates a novel second messenger: the inositol phosphoglycan (IPG). The aim of this article is to review the role of IPG and IPG-like molecules in signal transduction and to discuss future research directions.

Plasmodium falciparum and Plasmodium chabaudi: characterization of glycosylphosphatidylinositol-degrading activities

Merozoites of malaria parasites have a membrane-bound serine protease whose solubilization and subsequent activity depend on a parasite-derived glycosylphosphatidylinositol-phospholipase C (GPI-PLC). The GPI-degrading activities from both Plasmodium falciparum and Plasmodium chabaudi have been characterized and partially purified by phenylboronate chromatography. They are membrane-bound, developmentally regulated, calcium-independent enzymes and as such they resemble GPI-PLC of Trypanosoma brucei. Furthermore, a T. brucei GPI-PLC-specific monoclonal antibody (mAT3) immunoprecipitates the plasmodial GPI-degrading activity. Thin-layer chromatography is suggestive of two activities: a GPI-PLC and a phospholipase A.

The synthesis and resolution of (+/-)-1,5,6-tri-O-benzyl-myo-inositol

Racemic 1,5,6-tri-O-benzyl-myo-inositol was prepared by five routes and converted into 1,5,6-tri-O-benzyl-2,3-O-isopropylidene-myo-inositol, the camphanates of which were readily separated by chromatography. The absolute configurations of the chiral derivatives were established by their conversion into the known chiral 1,4,5,6-tetra-O-benzyl-myo-inositols. 1D-1,5,6-Tri-O-benzyl-2,3-O-isopropylidene-myo-inositol was converted into 1D-1,3,5,6-tetra-O-benzyl-myo-inositol and thence into 1D-2,4-di-O-methyl-myo-inositol. 1D-1,5,6-Tri-O-benzyl-myo-inositol was converted into 1D-1,2,5,6-tetra-O-benzyl-myo-inositol, the diacetate of which is a chiral analogue of "thermosalient crystals". The potential of the above compounds for the synthesis of natural products is surveyed.

Isolation and characterization of a phosphatidylinositol-glycan-anchor-specific phospholipase D from bovine brain

In recent years an increasing number of proteins has been shown to be membrane-anchored by a covalently attached PtdIns-glycan residue. In mammalian cells little is known about PtdIns-glycan-specific phospholipases which might play a role in the metabolism of PtdIns-glycan-anchored proteins. In order to identify PtdIns-glycan-specific phospholipases, a rapid and sensitive assay for such enzymes was developed using the PtdIns-glycan-anchored amphiphilic membrane form of acetylcholinesterase as substrate. The rate of product formation was monitored by the increase in soluble hydrophilic acetylcholinesterase in the aqueous phase after separation in Triton X-114. With this assay we established the presence of a PtdIns-glycan-specific phospholipase in bovine brain. This enzyme was soluble and could be partially purified by a heat step followed by chromatography on DEAE-cellulose and by gel filtration on Sepharose CL-6B. PtdIns-glycan-specific phospholipase had a high affinity for the PtdIns-glycan anchor of the substrate (Km = 52 nM) and did not degrade either PtdCho or PtdIns. Hydrophobic labeling of the anchor of the substrate with 3-trifluoromethyl-3-(m-[125I]iodophenyl)diazirine [( 125I]TID) caused a marked decrease in the cleavage rate and methylation of the amino group of the glucosamine residue of the anchor decreased the cleavage rate to zero. Using [125I]TID-labeled substrate, diradylglycerol phosphate was identified as the second product showing that the cleavage specificity of PtdIns-glycan-specific phospholipase was that of a phospholipase D. PtdIns-glycan-specific phospholipase D was inhibited by mercurials, omicron-phenanthroline and EGTA. It was stimulated by Ca2+ in micromolar concentrations indicating that PtdIns-glycan-phospholipase D is a Ca2(+)-regulated enzyme.

Lipid modulators of cell function

Lipids have surfaced as potent and diverse modulators of cell functions, as determinants of membrane structure, as ligands for cell-surface receptors, as anchors for membrane-associated proteins, and as "second messengers." Some of these functions involve the complex lipids directly, as exemplified by the alteration of receptor behavior by gangliosides. However, many other functions entail cleavage of membrane lipids to yield (as examples): unsaturated fatty acids, which are converted to prostaglandins, prostacyclins, thromboxanes, and other compounds; diacylglycerols, which activate protein kinase C; inositol phosphates, which stimulate release of calcium from intracellular stores; and lysoalkylphosphatidylcholine, which is converted to platelet-activating factor. New roles for membrane lipids are constantly appearing, such as the inhibition of protein kinase C by sphingosine and the release of phosphatidylinositol-linked proteins in response to hormones. Dietary modification of these lipid systems could have important implications for normal cell function and disease.