Enzymatic synthesis of pyrene-labeled polyphosphoinositides and their behavior in organic solvents and phosphatidylcholine bilayers

A light-switching pyrene probe to detect phase-separated biomolecules

Biomolecules may undergo liquid-liquid phase separation (LLPS) to spatiotemporally compartmentalize and regulate diverse biological processes. Because the number of tools to directly probe LLPS is limited (ie. FRAP, FRET, fluorescence microscopy, fluorescence anisotropy, circular dichroism, etc.), the physicochemical traits of phase-separated condensates remain largely elusive. Here, we introduce a light-switching dipyrene probe (Pyr-A) that forms monomers in either hydrophobic or viscous environments, and intramolecular excimers in aqueous solutions. By exploiting their distinct fluorescence emission spectra, we used fluorescent microscopic imaging to study phase-separated condensates formed by in vitro protein droplets and membraneless intracellular organelles (centrosomes). Ratiometric measurement of excimer and monomer fluorescence intensities showed that protein droplets became hydrophobic and viscous as their size increased. Moreover, centrosomes became hydrophobic and viscous during maturation. Our results show that Pyr-A is a valuable tool to characterize LLPS and enhance our understanding of phase separation underlying biological functions.

Simple and rapid biochemical method to synthesize labeled or unlabeled phosphatidylinositol species

Phosphatidylinositol (PI) is the precursor of many important signaling molecules in eukaryotic cells and, most probably, PI also has important functions in cellular membranes. However, these functions are poorly understood, which is largely due to that i) only few PI species with specific acyl chains are available commercially and ii) there are no simple methods to synthesize such species. Here, we present a simple biochemical protocol to synthesize a variety of labeled or unlabeled PI species from corresponding commercially available phosphatidylcholines. The protocol can be carried out in a single vial in a two-step process which employs three enzymatic reactions mediated by i) commercial phospholipase D from Streptomyces chromofuscus, ii) CDP-diacylglycerol synthase overexpressed in E. coli and iii) PI synthase of Arabidopsis thaliana ectopically expressed in E. coli The PI product is readily purified from the reaction mixture by liquid chromatography since E. coli does not contain endogenous PI or other coeluting lipids. The method allows one to synthesize and purify labeled or unlabeled PI species in 1 or 2 days.Typically, 40-60% of (unsaturated) PC was converted to PI albeit the final yield of PI was less (25-35%) due to losses upon purification.

Electrostatic field effects on membrane domain segregation and on lateral diffusion

Natural membranes are organized structures of neutral and charged molecules bearing dipole moments which generate local non-homogeneous electric fields. When subjected to such fields, the molecules experience net forces that can modify the lipid and protein organization, thus modulating cell activities and influencing (or even dominating) the biological functions. The energetics of electrostatic interactions in membranes is a long-range effect which can vary over distance within r^-1 to r^-3. In the case of a dipole interacting with a plane of dipoles, e.g. a protein interacting with a lipid domain, the interaction is stronger than two punctual dipoles and depends on the size of the domain. In this article, we review several contributions on how electrostatic interactions in the membrane plane can modulate the phase behavior, surface topography and mechanical properties in monolayers and bilayers.

Pyrene-labeled lipids as tools in membrane biophysics and cell biology

Pyrene is one of the most frequently used lipid-linked fluorophores. Its most characteristic features are a long excited state lifetime and (local) concentration-dependent formation of excimers. Pyrene is also hydrophobic and thus does not significantly distort the conformation of the labeled lipid molecule. These characteristics make pyrene lipids well-suited for studies on a variety of biophysical phenomena like lateral diffusion, inter- or transbilayer movement of lipids and lateral organization of membranes. Pyrene lipids have also been widely employed to determine protein binding to membranes, lipid conformation and the activity of lipolytic enzymes. In cell biology, pyrene lipids are promising tools for studies on lipid trafficking and metabolism, as well as for microscopic mapping of membrane properties. The main disadvantage of pyrene lipids is the relatively large size of the fluorophore. Another disadvantage is that they require UV-excitation, which is not feasible with all microscopes.

Erythrocyte band 3 protein strongly interacts with phosphoinositides

85% of the phosphorus coisolated with band 3 protein during separation of the intrinsic proteins of the human erythrocyte membrane by zonal electrophoresis in high concentrations of acetic acid was found to be derived from phosphoinositides, mainly phosphatidylinositol 4,5-bisphosphate. When native band 3 protein and pyrene-labelled phospholipids were present in micelles of the nonionic detergent nonaethyleneglycol lauryl ether, strong resonance energy transfer was observed between the tryptophan residues and phosphatidylinositol 4,5-bisphosphate and, to a smaller degree, phosphatidylinositol 4-phosphate. We conclude that band 3 protein strongly interacts with phosphoinositides, in particular with phosphatidylinositol 4,5-bisphosphate.

The low-affinity lipid binding site of the non-specific lipid transfer protein. Implications for its mode of action

The non-specific lipid transfer protein (nsL-TP) from bovine liver was studied by using the following fluorescent lipid analogs: phosphatidylcholine species with a sn-2-pyrenylacyl-chain of different length [Pyr(x)PC], sn-2-pyrenyldecanoyl-labelled phosphatidylinositol [Pyr(10)PI], -phosphatidylinositol 4-phosphate [Pyr(10)PIP], -phosphatidylinositol 4,5-bisphosphate [Pyr(10)PIP2] and dehydroergosterol. These analogs provided information on the effect of hydrophobicity and charge on lipid binding and transfer by nsL-TP. Binding of the Pyr(x)PC species decreased with increasing sn-2 acyl-chain length. Under equilibrium conditions, the fraction of nsL-TP that carried a PC molecule did not exceed 8%, which is consistent with a low affinity binding site. Also nsL-TP-mediated transfer of the Pyr(x)PC species decreased with increasing sn-2 acyl-chain length and was highly correlated with spontaneous transfer. Binding of the phosphoinositides increased in the order Pyr(10)PI less than Pyr(10)PIP less than Pyr(10)PIP2, indicating that an increase in lipid negative charge stimulates binding. The transfer of the phosphoinositides, however, decreased in the same order, which suggests that a high negative charge impairs the dissociation of the phospholipid from nsL-TP. Cholesterol, at concentrations up to 50 mol% in the donor membrane, hardly affected binding and transfer of Pyr(6)PC, strongly suggesting that nsL-TP has no high binding affinity for cholesterol. In agreement with this, binding of dehydroergosterol to nsL-TP was not detectable. Despite this apparently negligible affinity, nsL-TP-mediated transfer of dehydroergosterol was in the same order as that of Pyr(6)PC. The results are interpreted to indicate that transfer of lipids by nsL-TP involves the formation of a putative low-affinity lipid-protein complex. This formation is enhanced when lipid hydrophobicity decreases or lipid negative charge increases. Based on the binding and transfer data, the mode of action of nsL-TP is discussed in terms of change in free energy.

A fluorescent substrate for the continuous assay of phosphatidylinositol-specific phospholipase C: synthesis and application of 2-naphthyl myo-inositol-1-phosphate

A fluorescent water-soluble substrate for phosphatidylinositol-specific phospholipase C was synthesized. The diacylglycerol moiety of the natural substrate, phosphatidylinositol, was replaced by the fluorescent moiety, 2-naphthol, resulting in the synthetic substrate, racemic 2-naphthyl myo-inositol-1-phosphate. The synthetic substrate provided a continuous fluorometric assay for the phosphatidylinositol-specific phospholipase C from Bacillus cereus. Initial rates of the cleavage of the 2-naphthyl substrate by the phospholipase measured by fluorometry were linear with time and the amount of enzyme added. The specific enzyme activity at pH 8.5 and 25 degrees C was about 0.04 mumol/min mg protein at an initial substrate concentration of 0.8 mM. 31P NMR experiments suggest that, as with phosphatidylinositol itself, cleavage of the fluorescent substrate proceeds in two steps via a myo-inositol-1,2-cyclic phosphate intermediate, and that only the D-isomer is a substrate for the B. cereus phospholipase. The synthetic substrate was stable during long-term storage as a solid in the dark at -20 degrees C. It was also stable for several weeks when stored in the dark frozen in aqueous solution near neutral pH.

Phosphoinositide-protein interactions of the plasma-membrane Ca2(+)-transport ATPase as revealed by fluorescence energy transfer

Fluorescence energy transfer has been used to study the interaction of various phospholipids with the erythrocyte (Ca2+ + Mg2+)-ATPase. The fluorescence energy transfer between tryptophan residues of the (Ca2+ + Mg2+)-ATPase purified from erythrocytes and pyrene-labelled analogues of phosphatidylcholine (Pyr-PC), phosphatidylinositol (Pyr-PI), phosphatidylinositol 4-phosphate (Pyr-PIP), phosphatidylinositol 4,5-bisphosphate (Pyr-PIP2), phosphatidylglycerol (Pyr-PG) and phosphatidic acid (Pyr-PA) was measured. A positive correlation was found between the number of negative charges on the phospholipids (PIP2 greater than PIP greater than PA greater than PI = PG greater than PC) and the potency of their pyrene-labelled analogues to act as quantum acceptors in fluorescence energy transfer from the tryptophan residues of the (Ca2+ + Mg2+)-ATPase. This is the first time that a physical interaction between PIP/PIP2 and an intrinsic membrane protein has been demonstrated. The dependence of the energy transfer on the number of negative charges of the phospholipids closely resembles the previously demonstrated charge dependence of the enzymatic activity of the (Ca2+ + Mg2+)-ATPase (Missiaen, L., Raeymaekers, L., Wuytack, F., Vrolix, M., Desmet, H. and Casteels, R. (1989) Biochem. J. 263, 687-694). It is concluded that the stimulation of the (Ca2+ + Mg2+)-ATPase activity by negatively charged phospholipids is based on a binding of these lipids to the (Ca2+ + Mg2+)-ATPase and that the negative charges are a major modulatory factor for this interaction.