Functional Domains of a Pore-forming Cardiotoxic Protein, Volvatoxin A2

Abstract

Volvatoxin A2 (VVA2), a novel pore-forming cardiotoxic protein was isolated from the mushroom Volvariella volvacea. We identified an N-terminal fragment (NTF) (1-127 residues) of VVA2 as a domain for oligomerization by limited tryptic digestion. On preincubation of NTF with VVA2, NTF was found to inhibit VVA2 hemolytic activity by inducing VVA2 oligomerization in the solution in the same manner as liposomes. By site-directed mutagenesis, the amphipathic ␣-helix B of NTF or VVA2 was shown to be indispensable for its biological functions. Interestingly, at a molar ratio of recombinant NTF (reNTF)/VVA2 as low as 0.01, reNTF was able to inhibit VVA2 hemolytic activity and induce VVA2 oligomerization. This indicates that reNTF can trigger VVA2 oligomerization by a seeding effect. Furthermore, the recombinant C-terminal fragment (128 -199 residues) was found to be a functional domain that mediates the membrane binding of VVA2. We found a fragment localized at the C-terminal half of VVA2 containing ␤6, -7, and -8, which is protected from protease digestion because of its insertion of a membrane. We also identified a putative heparin binding site (HBS) located in the VVA2 C terminus (166 -194 residues), which was conserved among 10 kinds of snake venom cardiotoxins. VVA2 or the reHBS fragment was shown to interact with sulfated glycoaminoglycans by affinity column chromatography. The finding of a higher number of glycoaminoglycans in the membrane of cardiac myocytes suggested that they could be the specific membrane target for VVA2. Taken together, these findings indicate that VVA2 contains two functional domains, NTF and CTF. The NTF domain is responsible for VVA2 oligomerization and the CTF domain for membrane binding and insertion. Our results support a model whereby the formation of VVA2 oligomeric pre-pore complexes precedes their membrane insertion.Volvatoxin A (VVA), 1 first isolated from the edible mushroom Volvariella volvacea (1, 2), is a novel pore-forming cardiotoxic protein that causes cardiac arrest by activation of Ca 2ϩ -dependent ATPase activity and inhibition of Ca 2ϩ accumulation in the microsomal fraction of the sarcoplasmic recticulum of the ventricular muscle (3). VVA is composed of VVA1 and VVA2, of which only VVA2 is endowed with hemolytic and cytotoxic activity (1). Our previous studies showed that VVA2 consists of 199 amino acid residues without cysteine residues, and that it forms pores in response to its oligomer formation in human erythrocyte membranes and liposomes. This toxic protein also permeabilized the cell membrane and allowed the passage of 70-kDa dextran rhodamine B into cultured Xenopus myocytes. Furthermore, conformational changes occurred when VVA2 interacted with liposomes (2).Naturally occurring hemolytic toxic proteins can be classified into three groups based on their mechanisms of lysis of red blood cells: (a) pore forming, (b) enzymatic degradation of membrane lipids, and (c) solubilizing cell membrane by a detergentlike action (4). Several hemolytic toxic proteins isolated...

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“…56 Similarly, volvatoxin A2, a novel pore-forming cardiotoxic protein was isolated from another edible mushroom, Volvariella volvacea. 57 Volvatoxin A causes cardiac arrest by activation of Ca 21 -dependent ATPase activity. It also causes inhibition of Ca 21 accumulation in microsomal fraction of the sarcoplasmic reticulum of guinea pig ventricular muscle.…”

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