Snake Venom:
Bungarotoxins
Evolution of snake neurotoxins
The evolution of venom toxins has been key to the diversification of snakes. Venom toxins evolved from the recruitment of body proteins, which then underwent significant variation in both sequence and structure, while maintaining the molecular scaffold of the ancestral protein. In this way, snakes have been able to obtain functional biological molecules that they can then adjust for use as toxic weaponry. Several different families of venom toxins were recruited for use in snake venom before the diversification of advanced snakes, including Kunitz-type protease inhibitors; others arose from independent recruitment events, such as phospholipase A2. Snake venom phospholipase A2 peptides show remarkable functional diversity: some enzymes are capable of myotoxic effects, causing rapid necrosis of skeletal muscle; others evoke an inflammatory response; while others, like b-bungarotoxin, destroy nerve termini.
b-Bungarotoxin
b-Bungarotoxin is a heterodimeric neurotoxin, consisting of a phospholipase A2 peptide subunit connected by a disulphide bond to a K+-channel-binding subunit, the latter being a member of the Kunitz protease inhibitor superfamily. These two subunits are structurally and functionally distinct; therefore, in InterPro they have been given different classifications. The two subunits are discussed separately below.
P00617 Krait b-bungarotoxin
phospholipase A2-chain entry from InterPro
|
InterPro entry |
Method accession |
Graphical match |
Method name |
|
IPR001211 |
PD000303 |
|
PhospholipaseA2 |
|
IPR001211 |
PF00068 |
|
Phospholip_A2_1 |
|
IPR001211 |
PR00389 |
|
PHPHLIPASEA2 |
|
IPR001211 |
PS00118 |
|
PA2_HIS |
|
IPR001211 |
PS00119 |
|
PA2_ASP |
|
IPR001211 |
SM00085 |
|
PA2c |
Classification
|
PDB Chain/Domain ID |
PDB Chain/Structural Domains |
|
|
1bun |
1buna |
|
|
|
1.20.90.10.2 |
1bunA0 |
|
|
|
a.133.1.2 |
d1buna_ |
|
|
The A-chain subunit of b-bungarotoxin displays phospholipase A2 activity (EC 3.1.1.4), and different isoforms exist in Bungarus venom. The A-chain shares similarity in structure and catalytic function with non-toxic mammalian phospholipase A2 (PLA2) enzymes, which play important roles in fertilisation, cell proliferation, smooth muscle contraction, signal transduction, prostaglandin biosynthesis and membrane homeostasis. By contrast, venom PLA2 activity is responsible for a variety of toxic biological effects by interfering in normal physiological processes; all known pre-synaptic neurotoxins from snake venom are PLA2 enzymes. The PLA2 A-chain is the active subunit of b-bungarotoxin, and is responsible for both the neurotoxic and phospholipase activities of the protein.
In the graphical match above, you can see that the signatures in InterPro for b-bungarotoxin A-chain are found in one entry, IPR001211, which represents the family of phospholipase A2 proteins found not only in toxins, but as endogenous proteins in animals and plants as well. It is represented by six signatures: PD000303 from the PRODOM database, PF00068 from the PFAM database, the 5-element fingerprint PR00389 from the PRINTS database, PS00118 and PS00119 (based on an active site histidine and aspartic acid, respectively) from the PROSITE database, and SM00085 from the SMART database. The remaining three entries are from the structural database PDB, and from the structural classification databases CATH and SCOP (the names such as d1buna_ are derived from the PDB entry upon which they are based: here PDB entry 1bun, chain a). The graphical match for the PDB entry, 1buna, displays the length of the original PDB structure. Both the SCOP (d1buna_) and the CATH (1bunA0) databases describe the structural features and classification of the protein structure. These last three entries are not signatures, but direct structural links.
P00989 Krait b-bungarotoxin
B2-chain entry from InterPro
|
InterPro entry |
Method accession |
Graphical match |
Method name |
|
IPR002223 |
PD000222 |
|
Kunitz_BPTI |
|
IPR002223 |
PF00014 |
|
Kunitz_BPTI |
|
IPR002223 |
PR00759 |
|
BASICPTASE |
|
IPR002223 |
PS00280 |
|
BPTI_KUNITZ_1 |
|
IPR002223 |
PS50279 |
|
BPTI_KUNITZ_2 |
|
IPR002223 |
SM00131 |
|
KU |
Classification
|
PDB Chain/Domain ID |
PDB Chain/Structural Domains |
|
|
1bun |
1bunb |
|
|
|
4.10.410.10.2 |
1bunB0 |
|
|
|
g.8.1.1 |
d1bunb_ |
|
|
The B-chain of b-bungarotoxin has no detectable phospholipase activity. Instead, the B-chain is homologous to Kunitz-type mammalian pancreatic protease inhibitors; however, the B-chain does not have any intrinsic protease inhibitory activity. Instead, the B-chain is thought to act as an affinity probe to guide the toxin to its target potassium channel on nerve terminals. The ion channel-binding region may mimic the regulatory interaction of endogenous neuropeptides. There are several different isoforms of B-chains found in Bungarus venom, which may account for differences in lethality found between different isolated proteins.
In the graphical match above, you can see that the signatures in InterPro for b-bungarotoxin B-chain are found in one entry, IPR002223, which represents the Kunitz-type protein inhibitor I2 domain found in several animal proteins besides toxins. It is represented by six signatures: PD000222 from PRODOM, PF00014 from the PFAM database, the 3-element fingerprint PR00759 from the PRINTS database, PS00280 and PS50279 from the PROSITE database, and SM00131 from the SMART database. The remaining three entries are from the structural database PDB, 1bunb, and from the structural classification databases CATH (1bunB0) and SCOP (d1bunb_).
What the structure tells us
Several of the bungarotoxins have structures in PDB, including the two domains of b-bungarotoxin mentioned above. A description and visualisation of the structural features of endogenous acetylcholinesterase and its snake toxin counter-part can be found at the PDB database. This enzyme plays a key role in the regulation of cholinergic transmission, and a homologous protein can be found in Bungarus venom.