Squid, octopus and squid are the masters of unnecessary deception and camouflage. Their extraordinary ability to change colors, textures and shapes is unmatched, even by modern technology.
Researchers at UC Santa Barbara's lab professor Daniel Morse have long been interested in the optical properties of animals that change color, and they are especially attracted by squid on a unified beach. Also known as the California market squid, these animals have evolved the ability to adjust their colors smoothly and continuously and sparkle at a level that is unmatched in other creatures. This allows them to communicate, and hide in plain sight in the sunny and often without features.
In previous research, the researchers found that a special protein, called reflectin, controlled the reflective pigment cells – iridocytes – which in turn contributed to altering the creature's overall visibility and appearance. But still a mystery is how reflectin actually works.
"We want to now understand how this extraordinary molecular machine works," said Morse, an Honorable Emeritus Professor in the Department of Molecular, Cellular and Developmental Biology, and lead author of a paper that appeared in Journal of Chemical Biology. Understanding this mechanism, he said, would provide insight into the melodious control of emerging properties, which could open the door for the next generation of bio-inspired synthetic materials.
Skin that reflects light
Like most cephalopods, squid on the beach in pairs, practicing their magic in a way that is perhaps the most sophisticated skin found anywhere in nature. Small muscles manipulate the texture of the skin while pigments and colorful cells affect its appearance. One group of cells controls their color by expanding and contracting cells in their skin that contain pigment sacks.
Behind these pigment cells is a layer of colorful cells – iridocytes – that reflect light and contribute to the color of animals across the visible spectrum. Squid also has leucophores, which control the reflection of white light. Together, these layers of cells that contain pigments and light reflectors give squid the ability to control the brightness, color, and color of their skin through a vast palette.
In contrast to the color of the pigments, a very dynamic hue from terrestrial squid is produced from changes in the structure of the iridocyte itself. Light bounces between nanometer-sized features of the same size as the wavelengths in the visible spectrum, producing colors. When these structures change their dimensions, the color changes. Reflectin proteins are behind the ability of these features to change shape, and the researchers' task is to find out how they do the work.
Thanks to a combination of genetic engineering and biophysical analysis, the scientists found the answer, and it turned out to be a mechanism that was far more elegant and powerful than previously imagined.
"The results are very surprising," said first author Robert Levenson, a postdoctoral researcher in the Morse lab. The group hopes to find one or two spots on the protein that control its activity, he said. "Instead, our evidence shows that the reflector features that control the detection of the signal and the resulting assemblies are scattered throughout the protein chain."
Reflectin, which is contained in dense membrane layers in iridocytes, looks somewhat like a series of beads on a rope, the researchers found. Usually, the bond between charged beads is very positive, so they repel each other, straightening proteins like raw spaghetti.
Morse and his team found that nerve signals to reflective cells triggered the addition of phosphate groups to the link. This negatively charged phosphate group neutralizes the link repulsion, allowing the protein to fold. The team was very pleased to find that these folds revealed a new sticky surface in parts like bead reflectin, allowing them to clump together. Up to four phosphates can bind to each reflectin protein, providing squid with a process that can be adjusted: the more phosphate is added, the more protein is folded, the more hydrophobic surfaces appear, and the greater the growing family.
When these clumps grow, many, single, smaller proteins in solution become fewer, larger groups of several proteins. This changes the fluid pressure in the membrane pile, releasing water – a type of "osmotic motor" that responds to the slightest change caused by neurons, which are connected to the spotting of thousands of leucophores and connected iridocytes. The resulting dehydration reduces the thickness and distance of the membrane stack, which shifts the wavelength of reflected light progressively from red to yellow, then green and finally blue. A more concentrated solution also has a higher refractive index, which increases cell brightness.
"We didn't know that the mechanism we discovered would turn out to be very complex but contained and so elegantly integrated in a multifunctional molecule – the copolymeric block reflector – with opposing domains so carefully that they act like metastable machines, continue to feel and responding to neuron signals by adjusting the osmotic pressure of intracellular nanostructures precisely to perfect the color and brightness of the reflected light, "Morse said.
What's more, the researchers found, the whole process can be reversed and can be recycled, allowing squid to continue to perfect whatever optical properties the situation requires.
New Design Principles
Researchers have succeeded in manipulating reflectin in previous experiments, but this study marks the first demonstration of the underlying mechanism. Now this can provide new ideas for scientists and engineers who design materials with tunable properties. "Our findings reveal a fundamental relationship between the properties of biomolecular materials produced in living systems and highly engineered synthetic polymers that are now being developed at the frontiers of industry and technology," Morse said.
"Because reflectin works to control osmotic pressure, I can imagine applications for new means of energy storage and conversion, pharmaceutical and industrial applications involving viscosity and other liquid properties, and medical applications," he added.
Remarkably, some of the processes that work in this reflectin protein are shared by proteins that accumulate pathologically in Alzheimer's disease and other degenerative conditions, Morse observed. He plans to investigate why this mechanism is reversible, recyclable, harmless and beneficial in the case of reflectin, but cannot be recovered and is pathological to other proteins. Maybe the differences are well structured in their order to explain the differences, and even show new pathways for disease prevention and treatment.
Marine biologists explain how special cells in squid skin are able to control the color of animals
Robert Levenson et al. Calibration between trigger and color: Neutralization of genetically encoded coulomb switches and dynamic capture precisely accomplishes reflective assembly, Journal of Chemical Biology (2019). DOI: 10.1074 / jbc.RA119.010339
Outstanding molecular machine (2019, 15 November)
taken November 16 2019
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