Degradation of red cell membrane phospholipids by sea anemone nematocyst venom

Cytolytic peptide and protein toxins from sea anemones (Anthozoa: Actiniaria)

More than 32 species of sea anemones have been reported to produce lethal cytolytic peptides and proteins. Based on their primary structure and functional properties, cytolysins have been classified into four polypeptide groups. Group I consists of 5-8 kDa peptides, represented by those from the sea anemones Tealia felina and Radianthus macrodactylus. These peptides form pores in phosphatidylcholine containing membranes. The most numerous is group II comprising 20 kDa basic proteins, actinoporins, isolated from several genera of the fam. Actiniidae and Stichodactylidae. Equinatoxins, sticholysins, and magnificalysins from Actinia equina, Stichodactyla helianthus, and Heteractis magnifica, respectively, have been studied mostly. They associate typically with sphingomyelin containing membranes and create cation-selective pores. The crystal structure of equinatoxin II has been determined at 1.9A resolution. Lethal 30-40 kDa cytolytic phospholipases A(2) from Aiptasia pallida (fam. Aiptasiidae) and a similar cytolysin, which is devoid of enzymatic activity, from Urticina piscivora, form group III. A thiol-activated cytolysin, metridiolysin, with a mass of 80 kDa from Metridium senile (fam. Metridiidae) is a single representative of the fourth family. Its activity is inhibited by cholesterol or phosphatides. Biological, structure-function, and pharmacological characteristics of these cytolysins are reviewed.

Enzymatic characterization of the major phospholipase A2 component of sea anemone (Aiptasia pallida) nematocyst venom

The purified beta phospholipase A2 (PLA2; EC 3.1.1.4) (PLA2) from sea anemone (Aiptasia pallida) nematocysts is larger and more labile than other known venom PLA2s. In common with all other known venoms and most secretory PLA2s, the beta PLA2 requires mM Ca2+ for optimal activity and is surface-activated by aggregated lipids such as mixed micelles of detergent and phospholipid. The beta PLA2 exhibits an unusually steep and narrow pH optimum of activity at pH 7.7. The effects of changes in pH on the activity of the enzyme suggest that the active site contains functional groups having a pKs of about 7.0 and 8.0. The effects of temperature on beta PLA2 activity show a marked decrease in the energy of activation above the pre-transition temperature, suggesting that the enzyme "melts" both fatty chains in order for catalysis to occur.

Purification and partial characterization of the phospholipase A2 and co-lytic factor from sea anemone (Aiptasia pallida) nematocyst venom

Functional nematocysts of one specific morphological class, the penetrant microbasic mastigophores, were isolated from the sea anemone, Aiptasia pallida. These nematocysts contain a multicomponent venom composed of several proteins, including those with neurotoxic, hemolytic, and lethal activities. Hemolytic activity is produced by at least three synergistic venom proteins. One of these proteins is identified as a phospholipase A2 (EC 3.1.1.4) which exists in two isozymic forms, alpha and beta, with molecular weights of 45,000 and 43,000, respectively. The beta isozyme has been purified to homogeneity. It is a single-chained glycoprotein with an isoelectric point (pI) of 8.8 and represents 70% of the phospholipase activity of the venom. The activity of the beta isozyme is relatively labile and is inactivated by 3.5 M urea or by heating at 45 degrees C. It is most stable at pH 4.0 and loses 50% of its activity at pH values below 3.5 and above 8.0. A second venom protein has also been purified. It is essential for the hemolytic activity of the venom and is termed co-lytic factor (CLF). It is a monomeric glycoprotein having a pI of 4.5. CLF has a molecular weight of approximately 98,000, a sedimentation coefficient of 4.8 S, and is prolate in shape, having a frictional ratio of about 1.6. CLF constitutes about 1.25% of the total venom protein and is assayed by reversing fatty acid inhibition of the venom hemolysis activity.

Purification and characterization of a pore-forming protein from the marine sponge Tethya lyncurium

A pore-forming protein was detected and purified for the first time from a marine sponge (Tethya lyncurium). The purified protein has a polypeptide molecular mass of 21 kDa and a pI of 6.4. Tethya pore-forming protein (also called Tethya hemolysin) rapidly lysed erythrocytes from a variety of organisms. After binding to target membranes, the hemolysin resisted elution with EDTA, salt or solutions of low ionic strength and hence resembled an integral membrane protein. Erythrocytes could be protected from hemolysis induced by Tethya hemolysin by addition of 30 mM dextran 4 (4-6 kDa; equivalent hydrodynamic diffusion radius, 1.75-2.3 nm) to the extracellular medium, but not by addition of uncharged molecules of smaller size [sucrose, raffinose and poly(ethylene glycol) 1550; equivalent hydrodynamic diffusion radii, 0.46, 0.57 and 1.2 nm, respectively]. This result indicates that hemolysin is able to form stable transmembrane pores with an effective diameter of about 2-3 nm. Treatment of osmotically protected erythrocytes with Tethya hemolysin caused a rapid efflux of intracellular K+ and ATP, and a rapid influx of extracellularly added Ca2+ and sucrose. In negative-staining electron microscopy, target erythrocyte membranes exposed to purified Tethya hemolysin displayed ultrastructural lesions but without visible pores.

Polypeptide cytolytic toxins from sea anemones (Actiniaria)

Biochemical and biological properties of 30 cytolytic polypeptide toxins isolated from 18 species of sea anemones (Actiniaria) are presented and classified into three groups according to their molecular mass, isoelectric points and the molecular mechanism of action. Phospholipase A2-like toxins (30 kDa) from Aiptasia pallida are dissimilar to acidic metridiolysin (80 kDa) from Metridium senile and the group of about 27 predominantly basic toxins, having a molecular mass of 16-20 or 10 kDa, inhibited by sphingomyelin. They are lethal for both invertebrates and vertebrates, cardiotoxic, cytolytic and cytotoxic. Pharmacological activities, cytotoxic and cytolytic properties are mediated, at least in part, by forming pores in lipid membranes. Channels, 1-2 nm in diameter, formed in planar lipid membranes are cation selective and rectified. The mechanisms and some characteristics of ion channel formation by the toxins in the cells as well as in artificial lipid membranes are summarized and discussed in view of the structure-function studies of the toxins. Putative biological roles of toxins, based on their channel-forming activity, in the capture and killing of prey, digestion, repelling of predators and intraspecific spatial competition are suggested.

Interactions between membranes and cytolytic peptides

The physico-chemical and biological properties of cytolytic peptides derived from diverse living entities have been discussed. The principal sources of these agents are bacteria, higher fungi, cnidarians (coelenterates) and the venoms of snakes, insects and other arthropods. Attention has been directed to instances in which cytolytic peptides obtained from phylogenetically remote as well as from related sources show similarities in nature and/or mode of action (congeneric lysins). The manner in which cytolytic peptides interact with plasma membranes of eukaryotic cells, particularly the membranes of erythrocytes, has been discussed with emphasis on melittin, thiolactivated lysins and staphylococcal alpha-toxin. These and other lytic peptides are characterized in Table III. They can be broadly categorized into: (a) those which alter permeability to allow passage of ions, this process eventuating in colloid osmotic lysis, signs of which are a pre-lytic induction or latent period, pre-lytic leakage of potassium ions, cell swelling and inhibition of lysis by sucrose. Examples of lysins in which this mechanism is involved are staphylococcal alpha-toxin, streptolysin S and aerolysin; (b) phospholipases causing enzymic degradation of bilayer phospholipids as exemplified by phospholipases C of Cl. perfringens and certain other bacteria; (c) channel-forming agents such as helianthin, gramicidin and (probably) staphylococcal delta-toxin in which toxin molecules are thought to embed themselves in the membrane to form oligomeric transmembrane channels.