Specialized squid tissues contain refractive proteins called reflectins, which help animals evade detection and have potential applications in bioengineering.
Beneath the turquoise waters surrounding Hawaii's coastline live walnut-sized creatures that have mastered the art of camouflage. These animals, a species of squid called Euprymna scolopes, use their camouflage abilities to avoid becoming prey for predators such as lizardfish and seals.
During the day, squid burrow into the sand. When they come out at night to search for food, their undersides glow, making shadows disappear in the moonlight and hiding them from predators below. How do squid achieve this camouflage?
In the 1990s, Margaret McFall-Ngai, an animal physiologist and protein biochemist at the University of Southern California, and a colleague discovered that Euprymna scolopes forms a symbiotic relationship with luminous bacteria that live in a special reflective organ in the squid's head. This structure modulates light, preventing it from passing through the squid's upper torso and directing it beneath the cephalopod.
Just over 10 years later, researchers determined that this property was due to a type of protein abundant in the reflective tissue. Now, detailed characterization of the protein has shed light on its reflective properties, attracting the interest of bioengineers, including the military, who want to use it to develop a new camouflage material.
Reflectin proteins in reflective tissues help squid hide themselves
The path to the discovery of these reflective proteins was by no means easy. "It was kind of a happy coincidence," says McFall-Ngai, who now works at the California Institute of Technology. While studying various squid proteins, McFall-Ngai and her team found a prominent band of polypeptide on the gel. "I thought at the time, 'What is this?'," she recalls.
The researchers isolated the polypeptides from the gel and sequenced them, discovering an unusual amino acid composition: they were mostly composed of relatively rare amino acid residues such as tyrosine and tryptophan, but completely devoid of common ones such as alanine and isoleucine.
Antibodies generated to bind the protein were localized in the reflective organ, indicating a role for the proteins in masking the squid, so McFall-Ngai and her team named them "reflectins."
Prior to this discovery, scientists studying reflective tissues in fish and insect scales or the luminous eyes of animals found that they contained flat and insoluble structures with a high refractive index.
Living things place these structures between materials with a lower refractive index, causing light waves to reflect and interact with each other to create colorful patterns.
Photo: the-scientist.com
The researchers found that the reflective plates in aquatic living things are composed of purine crystals, specifically guanine and hypoxanthine. "Nobody has ever shown that proteins are reflectors," says McFall-Ngai. "These are amazing proteins, and they have a lot of properties that can be used in engineering."
Characterization of the biochemistry of reflexins and their applications
In the years since the discovery of reflectins, researchers have studied the biochemistry underlying the reflective properties of proteins and how these properties can be applied to bioengineering and materials science.
"The properties of the unusual amino acid composition of reflectins give them a high refractive index," explains Alon Gorodetsky, a biomolecular engineer at the University of California, Irvine, who is developing materials inspired by the optical properties of reflectins.
The refractive index of reflectins exceeds 0.2, while the refractive index of crystallins, which determine the refractive properties of the lens of the eye, is 0.19, meaning that reflectins can refract more incident light than other proteins with the same function.
Gorodetsky and other scientists have found that reflexins are partially disordered, meaning that under physiological conditions they do not have a fixed three-dimensional structure. External stimuli, such as changes in hydrogen - pH, can alter the configuration of the protein, which affects the outflow of water from the reflex structures by changing the refractive index. This property allows the squid to change its appearance when needed.
Given all these amazing properties, Gorodetsky and his team engineered human cells to express reflexins and observed that they could tune the optical properties of the cells by adjusting the salt content of the culture (nutrient) medium.
Photo: hvylya.net
The squid's color-changing system also prompted Gorodetsky's team to develop materials with adjustable infrared reflectivity. Gorodetsky noted that this material could coat objects and hide them from infrared cameras, which could find military applications.
Although researchers are increasingly appreciating the importance of these squid proteins in bioengineering and materials science, Horodetsky praises the basic research of McFall-Ngai and her team that led to these innovations. "Her discovery and isolation of the proteins has opened up this field to everyone," he says.