Credit: AAAS

In contrast to human eyes, the eyes of fish and squid tend to have fixed spherical lenses with a radial refractive index profile that is parabolic. This design is considered optimal for maximizing sensitivity and visual acuity in the light-limited ocean and, as predicted by the famous nineteenth century Scottish physicist James Clerk Maxwell, allows an aberration-free image to be formed. However, until now, it has been unclear how the graded-index lenses found in such creatures are formed from homogeneous materials such as protein solutions. Now, Jing Cai and co-workers from the University of Pennsylvania, USA have investigated the spherical lens of a decapod squid (Doryteuthis pealeii) (pictured) and have found the answer (Science 357, 564–569; 2017).

The refractive-index material of the squid’s lens is made up of proteins called S-crystallins that are composed of a glutathione S-transferase enzyme and a variable-length peptide loop.

The research team used gel electrophoresis to characterize the protein molecular weight distribution as a function of lens radius. The experimental results showed that proteins with very short loops were highly abundant in the core of the lens, whereas those with medium-sized loops were more abundant at the periphery. S-crystallins with long loops were found throughout the lens.

The spatial structure of S-crystallins was investigated by small-angle X-ray scattering. A structural fitting analysis confirmed that the protein solutions contained pairwise-linked chains of S-crystallin and complex, multiparticle nodes within the protein network.

When tissue taken from different radial positions of the lens was diluted to a volume fraction of 0.01 and then centrifuged, a protein fluid and a pellet were observed. Both the relative amount of protein and the density of the pellet were found to increase towards the centre of the lens, thus explaining its graded refractive-index profile. In samples taken from the periphery, the pellet was a soft, volume-spanning translucent gel, whereas samples taken from the core of the lens formed a white powder.