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Squid Teeth and Reasons for Repetition

APF_SRTspecies (2).jpg

Do squid have teeth? No, but inside the suckers on their tentacles are the next best thing: proteins with tough, crystalline properties. Intrigued by the toughness and flexibility of these “squid ring teeth” (SRTs), scientists at Penn State tried to imitate them.

They programmed bacteria to generate sequences of amino acids similar to the proteins in SRTs. What they discovered and published in the Proceedings of the National Academy of Sciences is a story of design imitating design.

Jung et al. are not advocates of intelligent design, as far as we know. Yet in the article, “design” got 11 mentions, while “evolution” received only one, in the Introduction. Here it is:

Proteins are diverse but often display substantial similarity in sequence and 3D structure. Duplication of structural units is a natural evolutionary strategy for increasing the complexity of both globular and fibrous/structural proteins (3). For example, collagen has polyproline- and glycine-rich helices, whereas silk and elastin have β-spiral [GPGXX], linker [GP(S,Y,G)], and 310-helix [GGX] repeats. These repetitions are advantageous because of the intrinsic promotion of stability through the periodic recurrence of favorable interactions. [Emphasis added.]

Notice that they relied on an outside reference to support the claim, an old 1972 paper. After that off-hand reference to “evolutionary strategy” (isn’t that an oxymoron?) they had no more use for Darwinian thinking and turned, instead, to “design strategy.”

In general, our design strategy uses three parameters to modulate the properties of the protein: (i) the composition of the crystalline/ordered or amorphous regions, (ii) the length (L = La + Lc) and fraction (f = La/Lc) of the amorphous (La) and crystalline regions (Lc), and (iii) the repeat number n: the number of tandem copies of the amorphous plus crystalline unit.

This approach requires the efficient construction of DNA sequences that encode artificial TR proteins.

In intelligent design theory, we often note that repetitive structures differ from information-rich structures, which tend to be non-repetitive, or aperiodic, like the letters in sentences. Repetition can be designed, however; rhythm patterns in music are often repetitive, but that doesn’t imply they aren’t designed. When you find a repetitive structure, you simply need more information to make a design inference. Natural repetitive patterns include pulsars and crystals. Designed repetitive patterns include rhythms in music and stripes on a highway. So what about the squid? Are the repeats there for a reason?

The repetitive stretches of amino acids in SRTs are called tandem repeats. They are “advantageous” to the squid, the authors said, because when interspersed with amorphous (aperiodic) stretches within the protein, the repetitive or crystalline portions give springiness to the material, while the amorphous portions provide toughness. A diagram of the protein’s structure shown in the paper resembles one of those old chest exercise devices, with handles on the end (the amorphous part) and springs in between (the repetitive part).

Fine-tuning the lengths of repetitive and non-repetitive regions gives the desired combination of springiness and toughness. The authors describe the differences in material properties in a Penn State news item titled, “Programmable materials find strength in molecular repetition“:

The researchers suggest that “the repetitions in native squid proteins could have a genetic advantage for increased toughness and flexibility.”

“We found that the shortest polypeptide chains were brittle,” said Demirel. “As they get longer, they are stretchy.”

The structural properties in this material are highly programmable. Extremely elastic materials, like the amorphous portion of these proteins, absorb energy and are useful in things like automobile bumpers, while the crystalline portion acts like a spring and is more like the material in a car’s dashboard. The proper balance of each could provide the desired materials characteristics.

By following the design strategy of the squid, they created artificial proteins that could be tuned to specific applications.

Similar to their natural and recombinant counterparts, synthetic SRT mimics such as those described here can be processed to form any of a variety of 3D shapes, including but not necessarily limited to ribbons, lithographic patterns, and nanoscale objects, such as nanotube arrays. The ability to easily manufacture protein-based materials with tunable self-healing properties will find applications in a broad array of useful applications, including textiles, cosmetics, and medicine.

No one would describe the scientists’ work as anything other than intelligent design, but what about the squid species they used as models? They obtained four species of squid from around the world, and sampled the proteins in their suction cups. Once they had the sequences, they were able to program bacteria to produce the SRT proteins made to their specs. No more harvesting of live squid was needed after that.

Squid have been in the news lately. Cephalopods, among which squid are members, are enjoying a population explosion in the oceans due to global warming, Live Science says. And National Geographic says that some giant squid lurking in deep waters may be as long as a school bus. To imagine what the ring teeth on those monsters could do, watch Voyage to the Bottom of the Sea.

Seriously, we need to ask who taught the squid to invent ideal materials out of protein. How did these animals learn to program the right amounts of repetitive and aperiodic stretches of amino acids to get them to fold into functional proteins? And who taught them how to place these proteins into rings within their tentacle suckers? Penn State scientists had to do a lot of intelligent work to mimic the process. Can they really believe the squid hit upon this design strategy by chance?

Squid and their relatives, cuttlefish and octopuses, are among the most fascinating animals in the sea. If these proteins provide enough evidence of origin by design, how much more the camouflaging skin, the advanced eyes, and the complex behaviors they exhibit? The octopus genome is no help to evolution, Casey Luskin showed last year (see also this article). If genomics doesn’t support an evolutionary origin for cephalopods, and if these scientists didn’t need evolutionary theory for their work, why even go that way?

Photo credit: Demirel Lab via Penn State.

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