Characterization of reactions catalysed by yeast phosphatidylinositol synthase

Conversion of phosphatidylinositol (PI) to PI4-phosphate (PI4P) and then to PI(4,5)P2 is essential for the cytosolic Ca2+ concentration under heat stress in Ganoderma lucidum

How cells drive the phospholipid signal response to heat stress (HS) to maintain cellular homeostasis is a fundamental issue in biology, but the regulatory mechanism of this fundamental process is unclear. Previous quantitative analyses of lipids showed that phosphatidylinositol (PI) accumulates after HS in Ganoderma lucidum, implying the inositol phospholipid signal may be associated with HS signal transduction. Here, we found that the PI-4-kinase and PI-4-phosphate-5-kinase activities are activated and that their lipid products PI-4-phosphate and PI-4,5-bisphosphate are increased under HS. Further experimental results showed that the cytosolic Ca²⁺ ([Ca²⁺ ]_c ) and ganoderic acid (GA) contents induced by HS were decreased when cells were pretreated with Li⁺ , an inhibitor of inositol monophosphatase, and this decrease could be rescued by PI and PI-4-phosphate. Furthermore, inhibition of PI-4-kinases resulted in a decrease in the Ca²⁺ and GA contents under HS that could be rescued by PI-4-phosphate but not PI. However, the decrease in the Ca²⁺ and GA contents by silencing of PI-4-phosphate-5-kinase could not be rescued by PI-4-phosphate. Taken together, our study reveals the essential role of the step converting PI to PI-4-phosphate and then to PI-4,5-bisphosphate in [Ca²⁺ ]_c signalling and GA biosynthesis under HS.

Feruloyl esterase immobilization in mesoporous silica particles and characterization in hydrolysis and transesterification

Background

Enzymes display high reactivity and selectivity under natural conditions, but may suffer from decreased efficiency in industrial applications. A strategy to address this limitation is to immobilize the enzyme. Mesoporous silica materials offer unique properties as an immobilization support, such as high surface area and tunable pore size.

Results

The performance of a commercially available feruloyl esterase, E-FAERU, immobilized on mesoporous silica by physical adsorption was evaluated for its transesterification ability. We optimized the immobilization conditions by varying the support pore size, the immobilization buffer and its pH. Maximum loading and maximum activity were achieved at different pHs (4.0 and 6.0 respectively). Selectivity, shown by the transesterification/hydrolysis products molar ratio, varied more than 3-fold depending on the reaction buffer used and its pH. Under all conditions studied, hydrolysis was the dominant activity of the enzyme. pH and water content had the greatest influence on the enzyme selectivity and activity. Determined kinetic parameters of the enzyme were obtained and showed that K_m was not affected by the immobilization but k_cat was reduced 10-fold when comparing the free and immobilized enzymes. Thermal and pH stabilities as well as the reusability were investigated. The immobilized biocatalyst retained more than 20% of its activity after ten cycles of transesterification reaction.

Conclusions

These results indicate that this enzyme is more suited for hydrolysis reactions than transesterification despite good reusability. Furthermore, it was found that the immobilization conditions are crucial for optimal enzyme activity as they can alter the enzyme performance.

Electronic supplementary material

The online version of this article (10.1186/s12858-018-0091-y) contains supplementary material, which is available to authorized users.

Ubiquitous distribution of phosphatidylinositol phosphate synthase and archaetidylinositol phosphate synthase in Bacteria and Archaea, which contain inositol phospholipid

In Eukarya, phosphatidylinositol (PI) is biosynthesized from CDP-diacylglycerol (CDP-DAG) and inositol. In Archaea and Bacteria, on the other hand, we found a novel inositol phospholipid biosynthetic pathway. The precursors, inositol 1-phosphate, CDP-archaeol (CDP-ArOH), and CDP-DAG, form archaetidylinositol phosphate (AIP) and phosphatidylinositol phosphate (PIP) as intermediates. These intermediates are dephosphorylated to synthesize archaetidylinositol (AI) and PI. To date, the activities of the key enzymes (AIP synthase, PIP synthase) have been confirmed in only three genera (two archaeal genera, Methanothermobacter and Pyrococcus, and one bacterial genus, Mycobacterium). In the present study, we demonstrated that this novel biosynthetic pathway is universal in both Archaea and Bacteria, which contain inositol phospholipid, and elucidate the specificity of PIP synthase and AIP synthase for lipid substrates. PIP and AIP synthase activity were confirmed in all recombinant cells transformed with the respective gene constructs for four bacterial species (Streptomyces avermitilis, Propionibacterium acnes, Corynebacterium glutamicum, and Rhodococcus equi) and two archaeal species (Aeropyrum pernix and Sulfolobus solfataricus). Inositol was not incorporated. CDP-ArOH was used as the substrate for PIP synthase in Bacteria, and CDP-DAG was used as the substrate for AIP synthase in Archaea, despite their fundamentally different structures. PI synthase activity was observed in two eukaryotic species, Saccharomyces cerevisiae and Homo sapiens; however, inositol 1-phosphate was not incorporated. In Eukarya, the only pathway converts free inositol and CDP-DAG directly into PI. Phylogenic analysis of PIP synthase, AIP synthase, and PI synthase revealed that they are closely related enzymes.

Studies of inositol 1-phosphate analogues as inhibitors of the phosphatidylinositol phosphate synthase in mycobacteria

We previously reported a novel pathway for the biosynthesis of phosphatidylinositol in mycobacteria via phosphatidylinositol phosphate (PIP) [Morii H., Ogawa, M., Fukuda, K., Taniguchi, H., and Koga, Y (2010) J. Biochem. 148, 593-602]. PIP synthase in the pathway is a promising target for the development of new anti-mycobacterium drugs. In the present study, we evaluated the characteristics of the PIP synthase of Mycobacterium tuberculosis. Four types of compounds were chemically synthesized based on the assumption that structural homologues of inositol 1-phosphate, a PIP synthase substrate, would act as PIP synthase inhibitors, and the results confirmed that all synthesized compounds inhibited PIP synthase activity. The phosphonate analogue of inositol 1-phosphate (Ino-C-P) had the greatest inhibitory effect among the synthesized compounds examined. Kinetic analysis indicated that Ino-C-P acted as a competitive inhibitor of inositol 1-phosphate. The IC(50) value for Ino-C-P inhibition of the PIP synthase activity was estimated to be 2.0 mM. Interestingly, Ino-C-P was utilized in the same manner as the normal PIP synthase substrate, leading to the synthesis of a phosphonate analogue of PIP (PI-C-P), which had a structure similar to that of the natural product, PIP. In addition, PI-C-P had high inhibitory activity against PIP synthase.

A revised biosynthetic pathway for phosphatidylinositol in Mycobacteria

For the last decade, it has been believed that phosphatidylinositol (PI) in mycobacteria is synthesized from free inositol and CDP-diacylglycerol by PI synthase in the presence of ATP. The role of ATP in this process, however, is not understood. Additionally, the PI synthase activity is extremely low compared with the PI synthase activity of yeast. When CDP-diacylglycerol and [(14)C]1L-myo-inositol 1-phosphate were incubated with the cell wall components of Mycobacterium smegmatis, both phosphatidylinositol phosphate (PIP) and PI were formed, as identified by fast atom bombardment-mass spectrometry and thin-layer chromatography. PI was formed from PIP by incubation with the cell wall components. Thus, mycobacterial PI was synthesized from CDP-diacylglycerol and myo-inositol 1-phosphate via PIP, which was dephosphorylated to PI. The gene-encoding PIP synthase from four species of mycobacteria was cloned and expressed in Escherichia coli, and PIP synthase activity was confirmed. A very low, but significant level of free [(3)H]inositol was incorporated into PI in mycobacterial cell wall preparations, but not in recombinant E. coli cell homogenates. This activity could be explained by the presence of two minor PI metabolic pathways: PI/inositol exchange reaction and phosphorylation of inositol by ATP prior to entering the PIP synthase pathway.

Alternative metabolic fates of phosphatidylinositol produced by phosphatidylinositol synthase isoforms in Arabidopsis thaliana

PtdIns is an important precursor for inositol-containing lipids, including polyphosphoinositides, which have multiple essential functions in eukaryotic cells. It was previously proposed that different regulatory functions of inositol-containing lipids may be performed by independent lipid pools; however, it remains unclear how such subcellular pools are established and maintained. In the present paper, a previously uncharacterized Arabidopsis gene product with similarity to the known Arabidopsis PIS (PtdIns synthase), PIS1, is shown to be an active enzyme, PIS2, capable of producing PtdIns in vitro. PIS1 and PIS2 diverged slightly in substrate preferences for CDP-DAG [cytidinediphospho-DAG (diacylglycerol)] species differing in fatty acid composition, PIS2 preferring unsaturated substrates in vitro. Transient expression of fluorescently tagged PIS1 or PIS2 in onion epidermal cells indicates localization of both enzymes in the ER (endoplasmic reticulum) and, possibly, Golgi, as was reported previously for fungal and mammalian homologues. Constitutive ectopic overexpression of PIS1 or PIS2 in Arabidopsis plants resulted in elevated levels of PtdIns in leaves. PIS2-overexpressors additionally exhibited significantly elevated levels of PtdIns(4)P and PtdIns(4,5)P(2), whereas polyphosphoinositides were not elevated in plants overexpressing PIS1. In contrast, PIS1-overexpressors contained significantly elevated levels of DAG and PtdEtn (phosphatidylethanolamine), an effect not observed in plants overexpressing PIS2. Biochemical analysis of transgenic plants with regards to fatty acids associated with relevant lipids indicates that lipids increasing with PIS1 overexpression were enriched in saturated or monounsaturated fatty acids, whereas lipids increasing with PIS2 overexpression, including polyphosphoinositides, contained more unsaturated fatty acids. The results indicate that PtdIns populations originating from different PIS isoforms may enter alternative routes of metabolic conversion, possibly based on specificity and immediate metabolic context of the biosynthetic enzymes.

Investigating lipid signalling: it's all about finding the right PI

Phosphoinositides are well-known components of cellular signal transduction pathways and, more recently, have been shown to play important roles in organelle identity and targeting determinants for various cytosolic proteins. Conversion of PtdIns into its various phosphorylated derivatives, such as PtdIns4P and PtdIns(4,5)P(2), is accomplished by a series of distinct lipid kinase and lipid phosphatase activities that are localized to specific subcellular membranes. As a result, production of distinct PtdIns forms is thought to be largely dependent on the access of these enzymes to their PtdIns or PtdInsP substrates. Interestingly, an investigation of two different PIS (PtdIns synthase) isoforms by Lofke et al. in this issue of the Biochemical Journal now indicates that the ability of PtdIns to be converted into downstream PtdInsPs may depend upon the PIS isoform from which it was synthesized.

Phosphatidylinositol synthesis is essential in bloodstream form Trypanosoma brucei

PI (phosphatidylinositol) is a ubiquitous eukaryotic phospholipid which serves as a precursor for messenger molecules and GPI (glycosylphosphatidylinositol) anchors. PI is synthesized either de novo or by head group exchange by a PIS (PI synthase). The synthesis of GPI anchors has previously been validated both genetically and chemically as a drug target in Trypanosoma brucei, the causative parasite of African sleeping sickness. However, nothing is known about the synthesis of PI in this organism. Database mining revealed a putative TbPIS gene in the T. brucei genome and by recombinant expression and characterization it was shown to encode a catalytically active PIS, with a high specificity for myo-inositol. Immunofluorescence revealed that in T. brucei, PIS is found in both the endoplasmic reticulum and Golgi. We created a conditional double knockout of TbPIS in the bloodstream form of T. brucei, which when grown under non-permissive conditions, clearly showed that TbPIS is an essential gene. In vivo labelling of these conditional double knockout cells confirmed this result, showing a decrease in the amount of PI formed by the cells when grown under non-permissive conditions. Furthermore, quantitative and qualitative analysis by GLC-MS and ESI-MS/MS (electrospray ionization MS/MS) respectively showed a significant decrease (70%) in cellular PI, which appears to affect all major PI species equally. A consequence of this fall in PI level is a knock-on reduction in GPI biosynthesis which is essential for the parasite's survival. The results presented here show that PI synthesis is essential for bloodstream form T. brucei, and to our knowledge this is the first report of the dependence on PI synthesis of a protozoan parasite by genetic validation.

Phosphatidylinositol synthesis and exchange of the inositol head are catalysed by the single phosphatidylinositol synthase 1 from Arabidopsis

In order to study some of its enzymatic properties, phosphatidylinositol synthase 1 (AtPIS1) from the plant Arabidopsis thaliana was expressed in Escherichia coli, a host naturally devoid of phosphatidylinositol (PtdIns). In the context of the bacterial membrane and in addition to de novo synthesis, the plant enzyme is capable of catalysing the exchange of the inositol polar head for another inositol. Our data clearly show that the CDP-diacylglycerol-independent exchange reaction can occur using endogenous PtdIns molecular species or PtdIns molecular species from soybean added exogenously. Exchange has been observed in the absence of cytidine monophosphate (CMP), but is greatly enhanced in the presence of 4 microm CMP. Our data also show that AtPIS1 catalyses the removal of the polar head in the presence of much higher concentrations of CMP, in a manner that suggests a reverse of synthesis. All of the PtdIns metabolizing activities require free manganese ions. EDTA, in the presence of low Mn2+ concentrations, also has an enhancing effect.

The role of CDP-diacylglycerol synthetase and phosphatidylinositol synthase activity levels in the regulation of cellular phosphatidylinositol content

The regulation of phosphatidylinositol synthesis was examined by cloning and expressing in COS-7 cells the human cDNAs encoding the two enzymes in the biosynthetic pathway. Human CDP-diacylglycerol synthetase (cds1) and phosphatidylinositol synthase (pis1) clones were identified in the human expressed sequence-tagged (EST) data base, and full-length cDNAs were obtained by library screening. The cds1 cDNA did not possess a recognizable mitochondrial import signal, and the activity of the expressed Cds1 protein was stimulated by nucleoside triphosphates in vitro, indicating that cds1 did not encode the mitochondrial-specific isozyme. There were two mRNA species (3.9 and 5.6 kilobases) detected on Northern blots hybridized with the cds1 probe that were expressed at distinctly different levels in various human tissues. Consistent with the presence of the two mRNAs, a cDNA predicted to encode a second human CDP-diacylglycerol synthetase (cds2) was also uncovered in the EST data base. In contrast to the two cds mRNAs, a single, 2.1-kilobase pis1 mRNA was uniformly expressed in all human tissues examined. Expression of the pis1 gene led to the overproduction of both phosphatidylinositol synthase and phosphatidylinositol:inositol exchange reactions, indicating that the Pis1 polypeptide catalyzed both of these activities. Phosphatase treatment of cell extracts abolished the CMP-independent phosphatidylinositol:inositol exchange reaction, and exchange activity was completely restored by the addition of CMP. Overexpression of cds1 or pis1 alone or in combination did not enhance the rate of phosphatidylinositol biosynthesis. Also, overexpression did not result in a significant proportional increase in the cellular levels of CDP-diacylglycerol or phosphatidylinositol. These data illustrate that the levels of Cds1 and Pis1 protein expression are not critical determinants of cellular PtdIns content and argue against a determining role for the activity of either of these enzymes in the regulation of PtdIns biosynthesis.

Phosphatidylinositol synthase from yeast

PI is an important precursor for polyphosphoinositides and some sphingolipids and is also involved in the glycolipid anchoring of plasma membrane proteins. This lipid is synthesized from CDP-diacylglycerol and myo-inositol by PI synthase, an enzyme localized in the outer mitochondrial membranes and microsomes in yeast. PI synthase was highly purified from yeast microsomes after solubilization with Triton X-100. The activity is dependent on Mn2+ or Mg2+ and Triton X-100. The reaction follows a sequential Bi-Bi mechanism with binding to CDP-diacylglycerol before myo-inositol and releasing PI prior to CMP. Unlike most of the yeast phospholipid-synthesizing enzymes, PI synthase is a constitutive enzyme. Its expression is insensitive to the addition of myo-inositol and choline to culture medium or the transition of growth phase. The primary translate deduced from the encoding gene, PIS, comprises 220 amino acid residues with a calculated molecular mass of 23,613. The sequence contains several hydrophobic regions and resembles that of the human enzyme. The sequence also contains the local, conserved region found in enzymes catalyzing the transfer of the phosphoalcohol moiety from CDP-alcohol, such as phosphatidylserine synthase, cholinephosphotransferase and phosphatidylglycerolphosphate synthase. Substitution of amino acid at position 114 from His (CAC) to Gln (CAA) results in a 200-fold increase in Km of the enzyme for myo-inositol, making cells auxotrophic for myo-inositol. Disruption of the PIS locus in the genome is lethal, indicating that PI is essential for the survival and growth of yeast cells.

Inositol monophosphatase activity from the Escherichia coli suhB gene product

The suhB gene is located at 55 min on the Escherichia coli chromosome and encodes a protein of 268 amino acids. Mutant alleles of suhB have been isolated as extragenic suppressors for the protein secretion mutation (secY24), the heat shock response mutation (rpoH15), and the DNA synthesis mutation (dnaB121) (K. Shiba, K. Ito, and T. Yura, J. Bacteriol. 160:696-701, 1984; R. Yano, H. Nagai, K. Shiba, and T. Yura, J. Bacteriol. 172:2124-2130, 1990; S. Chang, D. Ng, L. Baird, and C. Georgopoulos, J. Biol. Chem. 266:3654-3660, 1991). These mutant alleles of suhB cause cold-sensitive cell growth, indicating that the suhB gene is essential at low temperatures. Little work has been done, however, to elucidate the role of the product of suhB in a normal cell and the suppression mechanisms of the suhB mutations in the aforementioned mutants. The sequence similarity shared between the suhB gene product and mammalian inositol monophosphatase has prompted us to test the inositol monophosphatase activity of the suhB gene product. We report here that the purified SuhB protein showed inositol monophosphatase activity. The kinetic parameters of SuhB inositol monophosphatase (Km = 0.071 mM; Vmax = 12.3 mumol/min per mg) are similar to those of mammalian inositol monophosphatase. The ssyA3 and suhB2 mutations, which were isolated as extragenic suppressors for secY24 and rpoH15, respectively, had a DNA insertion at the 5' proximal region of the suhB gene, and the amount of SuhB protein within mutant cells decreased. The possible role of suhB in E. coli is discussed.