Effects of specific fatty acid acylation of phospholipase A2 on its interfacial binding and catalysis

Detection of polychlorinated biphenyls using an antibody column in tandem with a fluorescent liposome column

Phospholipase A2 (PLA2)-catalyzed membrane leakage can be detected by immobilized liposomes containing a self-quenching fluorescent dye, 3,3-bis[N,N-di(carboxymethyl)aminomethyl]fluorescein (calcein). This enzymatic reaction was applied as signal amplification for biosensor detection of low concentrations of polychlorinated biphenyls (PCBs). In order to increase the fluorescent signal for improvement of PCBs detection, the effect of BSA on optimal lipid composition for PLA2-catalyzed membrane leakage from fluorescent liposomes has been investigated in this report. Various kinds of calcein-entrapped liposomes were immobilized in Sephacryl S1000 gel beads using avidin-biotin binding. In a contrast, free calcein was removed by size exclusion chromatography on Sephacryl S300 for free liposome suspensions. The PLA2-catalyzed membrane leakage was detected both in these gel-bead-immobilized liposomes and in free liposome suspensions. In both systems, the fluorescent release from the liposomes by PLA2 hydrolytic action significantly increased with increasing albumin concentration. The most rapid and greatest membrane leakage by PLA2 hydrolysis was found in anionic liposomes in the presence of albumin, both in free liposome suspensions and gel-bead-immobilized liposomes. Finally, the stabilities of various free liposomes and gel-bead-immobilized liposomes were monitored. Immobilized 1-palmitoy-2-oleoylphosphatidylcholine (POPC)/1-palmitoy-2-oleoylphosphatidylglycerol (POPG) liposome gel was chosen due to its excellent stability and large dye leakage by PLA2. A concentration of PCBs as low as 0.1 ng/mL was detectable using this tandem column system.

Membrane-perturbing activity of Viperidae myotoxins: an electrostatic surface potential approach to a puzzling problem

Phospholipase-like myotoxins are a class of proteins present in Viperidae venom. Despite the high level of amino acid and structural homology with soluble phospholipases A(2), myotoxins are devoid of enzymatic activity and share cytolytic activity by means of a totally unknown mechanism involving the lipid bilayer perturbation. The distribution of electrostatic surface potentials of four myotoxins and seven phospholipases A(2) has been compared. The charge distribution is similar in all active non-cytolytic phospholipases with a strongly positive side corresponding to the domain interacting with the micellar substrate and with the opposite side negatively charged. In contrast, all myotoxins examined are positively charged on both sides. Myotoxin III, the only known example of a myotoxin sharing enzymatic activity, displays the same electrostatic surface potential as other related toxins. Using liposomes made with non-hydrolysable phospholipids, we demonstrate that myotoxin III perturbs the lipid bilayer like other myotoxins. Based on these results, a molecular model for myotoxin-membrane perturbing activity is proposed. In this model, potential double-face binding of myotoxic phospholipases A(2) to lipid surfaces could trigger a lipid bilayer destabilization and could generate a stable fusion pore, probably because of the presence of hydrophobic moieties that flank the cationic sites.

Roles of Trp31 in high membrane binding and proinflammatory activity of human group V phospholipase A2

Group V phospholipase A2 is a recently discovered secretory phospholipase A2 (PLA2) that has been shown to be involved in eicosanoid formation in inflammatory cells, such as macrophages and mast cells. We have demonstrated that human group V PLA2 (hsPLA2-V) can bind phosphatidylcholine (PC) membranes and hydrolyze PC substrates much more efficiently than human group IIa PLA2, which makes it better suited for acting on the outer plasma membrane (Han, S.-K., Yoon, E. T., and Cho, W. (1998) Biochem. J. 331, 353-357). In this study, we demonstrate that exogenous hsPLA2-V has much greater activity than does group IIa PLA2 to release fatty acids from various mammalian cells and to elicit leukotriene B4 formation from human neutrophils. To understand the molecular basis of these activities, we mutated two surface tryptophans of hsPLA2-V to alanine (W31A and W79A) and measured the effects of these mutations on the kinetic activity toward various substrates, on the binding affinity for vesicles and phospholipid-coated beads, on the penetration into phospholipid monolayers, and on the activity to release fatty acids and elicit eicosanoid formation from various mammalian cells. These studies show that the relatively high ability of hsPLA2-V to induce cellular eicosanoid formation derives from its high affinity for PC membranes and that Trp31 on its putative interfacial binding surface plays an important role in its binding to PC vesicles and to the outer plasma membrane.

Mutagenesis of the C2 domain of protein kinase C-alpha. Differential roles of Ca2+ ligands and membrane binding residues

The C2 domains of conventional protein kinase C (PKC) have been implicated in their Ca2+-dependent membrane binding. The C2 domain of PKC-alpha contains several Ca2+ ligands that bind multiple Ca2+ ions and other putative membrane binding residues. To understand the roles of individual Ca2+ ligands and protein-bound Ca2+ ions in the membrane binding and activation of PKC-alpha, we mutated five putative Ca2+ ligands (D187N, D193N, D246N, D248N, and D254N) and measured the effects of mutations on vesicle binding, enzyme activity, and monolayer penetration of PKC-alpha. Altered properties of these mutants indicate that individual Ca2+ ions and their ligands have different roles in the membrane binding and activation of PKC-alpha. The binding of Ca2+ to Asp187, Asp193, and Asp246 of PKC-alpha is important for the initial binding of protein to membrane surfaces. On the other hand, the binding of another Ca2+ to Asp187, Asp246, Asp248, and Asp254 induces the conformational change of PKC-alpha, which in turn triggers its membrane penetration and activation. Among these Ca2+ ligands, Asp246 was shown to be most essential for both membrane binding and activation of PKC-alpha, presumably due to its coordination to multiple Ca2+ ions. Furthermore, to identify the residues in the C2 domain that are involved in membrane binding of PKC-alpha, we mutated four putative membrane binding residues (Trp245, Trp247, Arg249, and Arg252). Membrane binding and enzymatic properties of two double-site mutants (W245A/W247A and R249A/R252A) indicate that Arg249 and Arg252 are involved in electrostatic interactions of PKC-alpha with anionic membranes, whereas Trp245 and Trp247 participate in its penetration into membranes and resulting hydrophobic interactions. Taken together, these studies provide the first experimental evidence for the role of C2 domain of conventional PKC as a membrane docking unit as well as a module that triggers conformational changes to activate the protein.

Acylation of Escherichia coli hemolysin: a unique protein lipidation mechanism underlying toxin function

The pore-forming hemolysin (HlyA) of Escherichia coli represents a unique class of bacterial toxins that require a posttranslational modification for activity. The inactive protoxin pro-HlyA is activated intracellularly by amide linkage of fatty acids to two internal lysine residues 126 amino acids apart, directed by the cosynthesized HlyC protein with acyl carrier protein as the fatty acid donor. This action distinguishes HlyC from all bacterial acyltransferases such as the lipid A, lux-specific, and nodulation acyltransferases, and from eukaryotic transferases such as N-myristoyl transferases, prenyltransferases, and thioester palmitoyltransferases. Most lipids directly attached to proteins may be classed as N-terminal amide-linked and internal ester-linked acyl groups and C-terminal ether-linked isoprenoid groups. The acylation of HlyA and related toxins does not equate to these but does appear related to a small number of eukaryotic proteins that include inflammatory cytokines and mitogenic and cholinergic receptors. While the location and structure of lipid moieties on proteins vary, there are common effects on membrane affinity and/or protein-protein interactions. Despite being acylated at two residues, HlyA does not possess a "double-anchor" motif and does not have an electrostatic switch, although its dependence on calcium binding for activity suggests that the calcium-myristoyl switch may have relevance. The acyl chains on HlyA may provide anchorage points onto the surface of the host cell lipid bilayer. These could then enhance protein-protein interactions either between HlyA and components of a host signal transduction pathway to influence cytokine production or between HlyA monomers to bring about oligomerization during pore formation.

Interactions of annexin V with phospholipid monolayers

To understand the mechanism of annexin V-membrane interactions, we measured the interaction of human recombinant annexin V with phospholipid monolayers with differing head group and acyl group structures. Annexin V interacted with anionic phospholipid monolayers via non-specific electrostatic interactions, which was highly dependent on the surface pressure of monolayer with a sharp maximum. The unique surface pressure dependence of the annexin V-monolayer binding is strikingly similar to that observed for the binding of Ca2+ to anionic phospholipid monolayers, which indicates that the annexin V-bound Ca2+ binds two phospholipids at the membrane surface and that factors governing the Ca(2+)-phospholipid complex formation regulate the overall annexin V-Ca(2+)-membrane interactions.

Enhancement of Agkistrodon piscivorus piscivorus venom phospholipase A2 activity toward phosphatidylcholine vesicles by lysolecithin and palmitic acid: studies with fluorescent probes of membrane structure

The activity of phospholipase A2 from snake venom to hydrolyze bilayers of phosphatidylcholines is greatly enhanced by the presence of the hydrolysis products, lysolecithin and fatty acid, in the bilayer. The fluorescence of several probes of membrane structure was used to monitor changes in bilayer physical properties during vesicle hydrolysis. These changes were compared to emission spectra and fluorescence polarization results occurring upon direct addition of lysolecithin and/or fatty acid to the bilayer. The excimer to monomer ratio of 1,3-bis(1-pyrene)propane was insensitive to vesicle hydrolysis, suggesting that changes in the order of the phospholipid chains were not relevant to the effect of the hydrolysis products on phospholipase activity. The fluorescence of 6-propionyl-2-(dimethylamino)-naphthalene (Prodan) suggested that the polarity of the bilayer in the region of the phospholipid head groups increases as the hydrolysis products accumulate in the bilayer. The fluorescence of 6-dodecanoyl-2-(dimethylamino)naphthalene (Laurdan) confirmed that such effects were restricted to the bilayer surface. Furthermore, the lysolecithin appeared to be the product most responsible for these changes. These results suggested that lysolecithin increases the activity of phospholipase A2 during vesicle hydrolysis by disrupting the bilayer surface, making the phospholipid molecules more accessible to the enzyme active site.

A structure-function study of bovine pancreatic phospholipase A2 using polymerized mixed liposomes

A new combinatorial approach that includes the genetic variation of protein structure and the chemical modification of phospholipid structure in polymerized mixed liposomes was used to delineate the structure-function relationships in the interfacial catalysis of bovine pancreatic phospholipase A2 (PLA2). Based on previous structural and mutational studies, several bovine PLA2 mutants were generated in which a positive charge of putatively important lysyl side chains was reversed (K10E, K53E, K56E, and K116E) or neutralized (K56Q and K116Q). Kinetic parameters of bovine wild type and mutant PLA2s determined using polymerized mixed liposomes consisting of 1-hexadecanoyl-2-(1-pyrenedecanoyl)-sn-glycero-3-phosphoethanolamine (or -phosphoglycerol) and 1,2-bis[12-(lipoyloxy)dodecanoyl]-sn-glycero-3-phosphoglycerol showed that Lys-53 is involved specifically in the interaction with a substrate bound in the active site. Also, these results showed that Lys-10 and Lys-116 are involved in the interaction of bovine PLA2 with anionic interfaces but not in the interaction with the active site-bound substrate. In particular, Lys-116 makes more significant contribution than Lys-10 by approximately 1.0 kcal/mol to the binding to anionic interfaces. Most importantly, Lys-56 was shown to participate in the interaction with both the active site-bound substrate and anionic interfaces. These findings establish Lys-56 and Lys-116 as essential residues for the binding of bovine pancreatic PLA2 to anionic interfaces. Lastly, our structure-function analysis based on the use of polymerized mixed liposomes was further supported by equilibrium binding measurements of these proteins using 1,2-bis[12-(lipoyloxy)dodecanoyl]-sn-glycero-3-phosphoglycerol polymerized liposomes and by kinetic analyses using monomeric substrates, 1,2-dihexanoyl-sn-glycero-3-phosphoethanolamine and -phosphoglycerol.