Reptile & Amphibian Forums

Hi all

Here's a link to our latest paper. This one examines the molecular evolution of the 3FTx (three-finger toxins) in the elapid snake venoms. This paper was written before we started pulling out the 3FTx from colubrid venoms so no colubrid sequences are included. However it does hammer the elapid 3FTx rigorously. Its a pretty technical paper so might not be terribly understandable at times to some. The main gist of it is that there is a tremendously greater diversity in these toxins than has been previously appreciated. This diversification means that new activities exist for these toxins, besides the well chararacterised ones such as alpha-neurotoxic, muscarinic, cytotoxic etc.

Cheers
B
Elapid venom evolution

Great stuff! If this greater diversity in 3FT structure is indeed accompanied with more diversified 3FT functions (perhaps mediated through 3FT interaction with currently uncharacterized receptors?), these toxins could prove to be productive hunting grounds for new pharmaceuticals.

It is interesting that so much structural and functional diversity exists within snake-venom toxin families. Another family of proteins that displays great diversity among its members is the immunoglobulin (antibody) family. Like many of the snake-toxin families, the immunoglobulins are a multi-gene family. And, like snake toxins, they evolved to interact with foreign organisms in a manner that is advantageous to their owners. Structural diversity in immunoglobulins allows them to effectively interact with just about any foreign, potentially disease-causing organisms like bacteria, viruses, and fungi. The mechanisms underlying this necessary diversity in antibody structure are varied, including things like gene duplication / diversification, gene rearrangement, terminal NT addition, and targeted mutagenesis / somatic hypermutation. These processes have made it so a finite number of immunoglobulin genes produce the seemingly infinite number of structurally distinct immunoglobulins needed to fend off environmental pathogens.

Wouldn’t it be interesting if mechanisms similar to those underlying antibody diversity were behind some of this snake-venom toxin diversity?

It really does boggle the mind!

Cheers,
WK

That is a very crazy connection. I never thought about venom in that aspect. It is completely mind-boggling. Thinking on the molecular level of an inflammatory response to envenomation is really is pretty weird. The same cytokines that play an important role in inflammation, such as TNF-a, also play a part in cardiac toxicity, in some cases. I think it would be cool to some in vitro studies of cytokine levels and types when exposed to elapid venom as compared to vipers. This may have already been done though, as I am not up to date on toxinology as I am with other areas.

That is a very crazy connection…

LOL! Are you saying I missed my calling as a science fiction writer? It is quite interesting to speculate on this, however. I do not know a great deal about venom evolution, but I believe a fundamental driving force behind venom / venom toxin evolution is the ongoing competition between predator and prey. Snakes must recruit and evolve toxins to subdue varied and changing targets. An analogy can be drawn, in this respect, with the immune system in that it must evolve to deal with varied and changing pathogens. Components of the immune system that must change rapidly (on an evolutionary time scale) to keep up with the never-ending parade of nasties that invade our bodies daily, include the immunoglobulin gene family (antibodies, T & B cell receptors, etc.) Antibodies react in a very specific way with a very specific target, meaning they are effective against only that target. If we hold to the paradigm that one gene results in one protein product (antibody), the number of genes required to produce the required repertoire of antibodies would exceed the capacity of an organism's genome (doesn’t contain enough genes!). To solve this problem, the immune system has devised ways to introduce structural diversity in antibodies (and thereby increase target diversity) beyond gene duplication. I was just wondering if anyone has looked into the possibility of these types of mechanisms playing a role in venom toxin diversity. It seems to me that it might be advantageous to a venomous snake to have structural diversity in its toxins in an a priori fashion. Any thoughts, anyone?

Cheers,
WK

Hi mate

Yes, alternative mRNA splicing has been looked at for the toxins and it turns out that the dizzying array of toxins is not through alternate splicing but rather a massive amount of gene duplication and diversification. Some genes drop out through errors such as unequal cross-over or degenerate into pseudogenes through the promoters getting damaged. Thus, its a moveable feast and the 'birth/death' model of gene evolution at its most extreme.

Cheers
B
Venomdoc Homepage

Thanks mate. Next time you can just say, “Finish reading the paper I sent you!” Having just done this, I see that you mentioned immunoglobulins / MHC genes and the similarities between these and toxin genes with respect to evolution (bottom of page 118). You also mentioned the analogy between immune system-antigen and snake venom-prey interactions. Believe it or not, my apparent plagiarism in my last post was unintentional. I’m embarrassed!

I plan to get the referenced 1997 Nei paper, and this question may be addressed therein, but I was also wondering if mutations in toxin genes seem to occur more frequently or in a non-random manner in exons coding for surface regions whose residues actually contact the target? If so, could this represent evidence that these sequences are intentionally mucked with to generate diversity?

Cheers,
WK

G'day mate

Yeah, finish reading the paper you schmuck

>ut I was also wondering if mutations in toxin genes seem to occur more frequently or in a non-random manner in exons coding for surface regions whose residues actually contact the target? If so, could this represent evidence that these sequences are intentionally mucked with to generate diversity?

Thats a good question. It seems that mutations happen in pretty much any part of the molecule but that the cysteines and residues flanking them (which could also be considered structural residues and like the cysteines be essential for proper folding of the protein) are highly conserved. However, whether they are conserved through genetic protection (which would require some sort of stencil protein) or alternately when the proteins are produced in the rough endoplasmic reticulum of the cell, misfolded proteins are more sensitive to degradation by housekeeping proteases. The latter scenario is the most likely since it fits with what we already known about cell biology and is also the simplest scenario (which is also a good measure of plausibility). Protection by stencil proteins would be an extraordinarily complex system and also one that has not been discovered.

What is curious though is that the introns consistently undergo less mutation than the exons. This to me seems curious since it would require some sort of way of discriminating between the two at the DNA level. Very intriguing.

Cheers
B

Yeah, finish reading the paper you schmuck

LOL! Now, is that any way to talk to someone who could one day be looking at you down the barrel of a hypodermic?

What is curious though is that the introns consistently undergo less mutation than the exons. This to me seems curious since it would require some sort of way of discriminating between the two at the DNA level. Very intriguing.

Very intriguing indeed! This could be evidence of targeted mutation. Recently, a family of proteins was discovered that inflicts deamination upon cytosine in nucleic acids, creating uracil in its place. The presence of uracil in DNA is a problem (but OK in RNA, as you know), so mechanisms are in place to quickly recognize and correct this lesion. In immunoglobulin producing cells, the dC-->dU lesions produced by one of these deaminase proteins are repaired in ways that can produce somatic hypermutation, gene conversion (using nearby pseudogenes as repair templates!), and class-switch recombination. This, except for C-SR, is targeted to genes encoding variable regions on immunoglobulins (the regions that actually contact antigen), apparently by recognizing CG dinucleotide pairs that occur in specific contexts.

Since you and Wolfgang have uncovered the existence of high structural diversity within venom toxin families, I would not be surprised if a mutation-targeting mechanism is discovered "dancing among the toxin genes". Damn fine stuff!

Cheers,
WK

>>Yeah, finish reading the paper you schmuck
>>
>>LOL! Now, is that any way to talk to someone who could one day be looking at you down the barrel of a hypodermic?
>>

LOL! My humblest regards kind sir

>>What is curious though is that the introns consistently undergo less mutation than the exons. This to me seems curious since it would require some sort of way of discriminating between the two at the DNA level. Very intriguing.
>>
>>Very intriguing indeed! This could be evidence of targeted mutation. Recently, a family of proteins was discovered that inflicts deamination upon cytosine in nucleic acids, creating uracil in its place. The presence of uracil in DNA is a problem (but OK in RNA, as you know), so mechanisms are in place to quickly recognize and correct this lesion. In immunoglobulin producing cells, the dC-->dU lesions produced by one of these deaminase proteins are repaired in ways that can produce somatic hypermutation, gene conversion (using nearby pseudogenes as repair templates!), and class-switch recombination. This, except for C-SR, is targeted to genes encoding variable regions on immunoglobulins (the regions that actually contact antigen), apparently by recognizing CG dinucleotide pairs that occur in specific contexts.
>>
>>Since you and Wolfgang have uncovered the existence of high structural diversity within venom toxin families, I would not be surprised if a mutation-targeting mechanism is discovered "dancing among the toxin genes".

Yes, the degree of seemingly targeted hyper-variability is extraordinary we find it extremely interesting.

>Damn fine stuff!

Fangs for that

Cheers
B
-----
Dr. Bryan Grieg Fry
Deputy Director
Australian Venom Research Unit
University of Melbourne

www.venomdoc.com

Hi Bryan,

thanks for the paper. Quite a study!

Best,

Klaus

Fangs for that mate.

Ciao.
B
-----
Dr. Bryan Grieg Fry
Deputy Director
Australian Venom Research Unit
University of Melbourne

www.venomdoc.com