Lipid-induced aggregation of phospholipase A2: sucrose density gradient ultracentrifugation and crosslinking studies

Assembly of the herpes simplex virus capsid: preformed triplexes bind to the nascent capsid

The herpes simplex virus type 1 (HSV-1) capsid is a T=16 icosahedral shell that forms in the nuclei of infected cells. Capsid assembly also occurs in vitro in reaction mixtures created from insect cell extracts containing recombinant baculovirus-expressed HSV-1 capsid proteins. During capsid formation, the major capsid protein, VP5, and the scaffolding protein, pre-VP22a, condense to form structures that are extended into procapsids by addition of the triplex proteins, VP19C and VP23. We investigated whether triplex proteins bind to the major capsid-scaffold protein complexes as separate polypeptides or as preformed triplexes. Assembly products from reactions lacking one triplex protein were immunoprecipitated and examined for the presence of the other. The results showed that neither triplex protein bound unless both were present, suggesting that interaction between VP19C and VP23 is required before either protein can participate in the assembly process. Sucrose density gradient analysis was employed to determine the sedimentation coefficients of VP19C, VP23, and VP19C-VP23 complexes. The results showed that the two proteins formed a complex with a sedimentation coefficient of 7.2S, a value that is consistent with formation of a VP19C-VP23(2) heterotrimer. Furthermore, VP23 was observed to have a sedimentation coefficient of 4.9S, suggesting that this protein exists as a dimer in solution. Deletion analysis of VP19C revealed two domains that may be required for attachment of the triplex to major capsid-scaffold protein complexes; none of the deletions disrupted interaction of VP19C with VP23. We propose that preformed triplexes (VP19C-VP23(2) heterotrimers) interact with major capsid-scaffold protein complexes during assembly of the HSV-1 capsid.

Inactivation of secretory phospholipase A2 by ionizing radiation

The extracellular phospholipase A2s (PLA2) from cobra venom, rattlesnake venom, and porcine pancreas were analyzed by radiation inactivation to determine their functional aggregation states. The analysis was performed in the presence of the protein transferrin at two different concentrations of PLA2: 5 micrograms/ml. The small size of these proteins necessitated the use of high radiation dosages. The catalytic activity of all samples decreased as a single exponential as a function of radiation dosage, to > 97% inactivation. Target size analysis of these curves yielded sizes corresponding to dimers for all three PLA2s, indicating that all three enzymes exist as dimers or larger aggregates under the conditions studied. An analysis of the amount of intact protein remaining by sodium dodecyl sulphate-polyacrylamide gel electrophoresis showed that the loss of protein also followed a dimeric size for all three PLA2s. The loss of protein as a dimer indicates that transfer of radiation energy is occurring between polypeptides.

A study on the functional subunits of phospholipases A2 by enzyme immobilization

Pancreatic and venom phospholipases A2 have complex and distinct oligomerization behaviour. Pancreatic enzymes are monomeric in solution, but their quaternary structure at interfaces is unknown. On the other hand, certain crotalid venom phospholipases A2 are dimeric in solution, and different reports have proposed either the monomer or the dimer as the catalytically functional subunit. In this study, enzyme immobilization was used as a tool for determining the functional subunits of these enzymes. The dimeric Crotalus atrox phospholipase A2 was covalently attached to agarose beads, via either the amine or the carboxylic groups of the protein. In the first case immobilization led to an 80% loss of activity as compared with the soluble form, and measured by using micellar diheptanoylphosphocholine. Inclusion of micellar protectants in the coupling media did not improve the activity. Enzyme immobilized via carboxylic groups was 2-3-fold more active than the amine-coupled form. In a second approach, Crotalus atrox enzyme was immobilized with single-subunit attachment. The removal, with denaturating washes, of the non-covalently bound units involved in monomer-monomer interactions, caused a large decrease in specific activity of the support-bound enzyme. This suggests the dimeric form as the fully active one. Similar procedures were also carried out with pig pancreatic and Naja naja phospholipases A2. The results indicated that these enzymes are active as monomers.

Influence of chemistry in immobilization of cobra venom phospholipase A2: implications as to mechanism

Phospholipase A2 from Naja naja kaouthia venom was covalently coupled onto agarose beads using two different chemistries. The effect of micellar competitive inhibitors in the coupling media was evaluated. Enzyme bound to N-hydroxysuccinimide-activated agarose, which is reactive primarily toward epsilon-amino groups, had 20% activity retention against micellar diheptanoylphosphatidylcholine (DiC7-PC). Enzyme bound through carboxylic groups, using a modification of the carbodiimide method, had 50% retention. Similar relative activities were observed, for both conjugates, in monomeric dihexanoyl-PC and in mixed micelles of Triton X-100 with dipalmitoyl-PC or dioleoylphosphatidylethanolamine. The soluble form of the enzyme showed premicellar activation against monomeric DiC7-PC, while the immobilized form showed interfacial recognition at concentrations around the critical micellar concentration. These results suggest that the enzyme activity lost upon immobilization is a result of the inherent chemical modification of the enzyme and that enzyme oligomerization and interfacial recognition are not cause-effect phenomena.

1,25-(OH)2D3 and 24,25-(OH)2D3 regulation of arachidonic acid turnover in chondrocyte cultures is cell maturation-specific and may involve direct effects on phospholipase A2

Previous studies have shown that 1,25-(OH)2D3 stimulates phospholipase A2 (PA2) activity in growth zone chondrocytes (GC), but has no effect on the resting zone chondrocyte (RC) enzyme activity. 24,25-(OH)2D3 inhibits the RC enzyme but has no effect on the GC. This study examined whether the vitamin D metabolites affect arachidonic acid turnover in their contra-target cell populations. Incorporation and release of [14C]arachidonate was measured at various times following addition of hormone. Acylation and reacylation were measured independently by incubating with p-chloromercuribenzoate. The results demonstrated that 1,25-(OH)2D3 has no effect on arachidonic acid turnover in RC, but stimulates turnover in GC. In contrast, 24,25-(OH)2D3 stimulates arachidonic acid turnover in RC, but inhibits both incorporation and release in GC. To determine whether direct interaction with PA2 is one mechanism by which 1,25-(OH)2D3 and 24,25-(OH)2D3 regulate arachidonic acid release, snake venom (Niger niger) PA2 was incubated with the vitamin D metabolites. Enzyme specific activity was inhibited by 24,25-(OH)2D3 and stimulated by 1,25-(OH)2D3 in a time- and dose-dependent manner. These results suggest that at least part of the direct effect of vitamin D3 metabolites on cell membranes may be related to changes in PA2 activity. The regulation is related to the stage of differentiation of the target cell population. Changes in fatty acid acylation and reacylation may be one mode of vitamin D3 action in cartilage.

Purification and characterization of a mutant human platelet phospholipase A2 expressed in Escherichia coli. Cleavage of a fusion protein with cyanogen bromide

Both methionine residues in phospholipase A2 (PLA2) from porcine pancreas have been replaced by leucines with retention of full enzymatic activity. The methionine-less mutant has been expressed as a Cro-LacZ fusion protein in Escherichia coli, from which a pro-PLA2 was liberated by chemical cleavage with CNBr. The general applicability of CNBr cleavage of proteins lacking methionine residue(s) was demonstrated by replacing the single Met8 in human platelet phospholipase A2 (HP-PLA2) by a leucine residue, and the introduction of a methionine at a position just preceding the HP-PLA2 sequence. This protein was expressed in E. coli as a 68-kDa Cro-LacZ fusion protein. CNBr cleavage liberated the HP-PLA2 fragment which was reoxidized in vitro. The [Met8----Leu]HP-PLA2 is monomeric in aqueous solutions, requires calcium ions in the millimolar range for enzymatic activity and has optimal activity around pH 8. p-Bromophenacyl bromide rapidly inactivates the enzyme with calcium ions having a protective effect. The highest specific activities, 2400 U/mg and 9300 U/mg, were found with pure micelles of 1,2-dioctanoyl-sn-glycero-3-phosphoglycol and with mixed micelles of taurodeoxycholate and 1,2-dioctanoyl-sn-glycero-3-phosphoglycol, respectively. In mixed micelles the activity on dioleoyl phospholipids decreases in the order phosphatidylglycerol greater than phosphatidylethanolamine much greater than phosphatidylcholine. The enzyme has low activity on monomeric 1,2-diheptanoyl-sn-glycero-3-phosphocholine as a substrate, but high activity on micelles with a distinct jump in activity at the critical micellar concentration. The binding of the HP-PLA2, porcine pancreatic PLA2 and PLA2 from Naja melanoleuca venom to lipid/water interfaces was determined with micellar solutions of the substrate analog n-hexadecylphosphocholine. The HP-PLA2 has a high apparent Kd (2 mM) compared to pancreatic (0.2 mM) and venom (0.03 mM) PLA2. In mixed micelles of taurodeoxycholate and 1,2-didodecanoyl-sn-glycero-3-phosphocholine, the competitive inhibition of HP-PLA2 by the R and S enantiomers of 2-tetradecanoylaminohexanol-1-phosphocholine, its phosphoglycol, and its phosphoethanolamine derivatives were tested. The S enantiomers are only weak inhibitors, whereas the R enantiomers are potent inhibitors. The inhibitory power depends on the nature of the polar head group and increases in the order phosphocholine much less than phosphoethanolamine less than phosphoglycol. The best inhibitor, (R)-2-tetradecanoylaminohexanol-1-phosphoglycol, binds 2200 times stronger than the substrate to the HP-PLA2 active site.

Membrane structure, toxins and phospholipase A2 activity

The phospholipid-hydrolyzing enzyme phospholipase A2 (PLA2) (EC 3.1.1.4) exists in several forms which can be located in the cytosol or on cellular membranes. We review briefly cellular regulatory mechanisms involving covalent modification by protein kinase C and the action of Ca2+, cytokines, G proteins and other cellular proteins. The major focus is the role of phospholipid structure on PLA2 activity, including (1) the mechanism of PLA2 action on synthetic phospholipid bilayers, (2) perturbation of synthetic and cellular membranes with lipophilic agents and membrane-interactive peptides and (3) the ability of these agents to activate endogenous PLA2 activity, with emphasis on the venom and plant toxins melittin, cardiotoxin and Pyrularia thionein.

Crystal structure of cobra-venom phospholipase A2 in a complex with a transition-state analogue

The crystal structure of a complex between a phosphonate transition-state analogue and the phospholipase A2 (PLA2) from Naja naja atra venom has been solved and refined to a resolution of 2.0 angstroms. The identical stereochemistry of the two complexes that comprise the crystal's asymmetric unit indicates both the manner in which the transition state is stabilized and how the hydrophobic fatty acyl chains of the substrate are accommodated by the enzyme during interfacial catalysis. The critical features that suggest the chemistry of binding and catalysis are the same as those seen in the crystal structure of a similar complex formed with the evolutionarily distant bee-venom PLA2.

Hydrolytic action of phospholipase A2 in monolayers in the phase transition region: direct observation of enzyme domain formation using fluorescence microscopy

Phospholipase A2, a ubiquitous lipolytic enzyme highly active in the hydrolysis of organized phospholipid substrates, has been characterized optically in its action against a variety of phospholipid monolayers using fluorescence microscopy. By labeling the enzyme with a fluorescent marker and introducing it into the subphase of a Langmuir film balance, the hydrolysis of lipid monolayers in their liquid-solid phase transition region could be directly observed with the assistance of an epifluorescence microscope. Visual observation of hydrolysis of different phospholipid monolayers in the phase transition region in real-time could differentiate various mechanisms of hydrolytic action against lipid solid phase domains. DPPC solid phase domains were specifically targeted by phospholipase A2 and were observed to be hydrolyzed in a manner consistent with localized packing density differences. DPPE lipid domain hydrolysis showed no such preferential phospholipase A2 response but did demonstrate a preference for solid/lipid interfaces. DMPC solid lipid domains were also hydrolyzed to create large circular areas in the monolayer cleared of solid phase lipid domains. In all cases, after critical extents of monolayer hydrolysis in the phase transition region, highly stabile, organized domains of enzyme of regular sizes and morphologies were consistently seen to form in the monolayers. Enzyme domain formation was entirely dependent upon hydrolytic activity in the monolayer phase transition region and was not witnessed otherwise.