Structure of a Highly Active Cephalopod S-crystallin Mutant: New Molecular Evidence for Evolution from an Active Enzyme into Lens-Refractive Protein

Abstract

Crystallins are found widely in animal lenses and have important functions due to their refractive properties. In the coleoid cephalopods, a lens with a graded refractive index provides good vision and is required for survival. Cephalopod S-crystallin is thought to have evolved from glutathione S-transferase (GST) with various homologs differentially expressed in the lens. However, there is no direct structural information that helps to delineate the mechanisms by which S-crystallin could have evolved. Here we report the structural and biochemical characterization of novel S-crystallin-glutathione complex. The 2.35-Å crystal structure of a S-crystallin mutant from Octopus vulgaris reveals an active-site architecture that is different from that of GST. S-crystallin has a preference for glutathione binding, although almost lost its GST enzymatic activity. We’ve also identified four historical mutations that are able to produce a “GST-like” S-crystallin that has regained activity. This protein recapitulates the evolution of S-crystallin from GST. Protein stability studies suggest that S-crystallin is stabilized by glutathione binding to prevent its aggregation; this contrasts with GST-σ, which do not possess this protection. We suggest that a tradeoff between enzyme activity and the stability of the lens protein might have been one of the major driving force behind lens evolution.

Structure of HIV TAR in complex with a Lab-Evolved RRM provides insight into duplex RNA recognition and synthesis of a constrained peptide that impairs transcription

“…Eye crystallins represent a different case in which housekeeping genes were co-opted to promote transparency and optical clarity of the lens. Remarkably, only four point mutations were required to restore the glutathione S-transferase activity of cephalopod S-crystallin, which was exapted from an ancestral GST ( 94 ). Minute numbers of base changes are also effective in reassignment of non-coding RNA functions.…”

Dietary Effect on the Proteome of the Common Octopus (Octopus vulgaris) Paralarvae

“…In fact, as pointed out by Tomarev et al ( 1991 ) the use of detoxification stress proteins like GST and aldehyde dehydrogenase as cephalopod crystallins, is indicative of a common strategy for recruitment of enzyme-crystallins during the convergent evolution of vertebrate and invertebrate lenses. A recent study on O. vulgaris has revealed that the loss of GST enzyme activity in lens tissue is linked to the enhanced protein stability of S-crystallin via glutathione binding (Tan et al, 2016 ). In the light of these data, and although protein identification in our study was not carried out at the cellular location level, it is tempting to suggest that the decrease of GST might also be indicative of possible alterations in the lens structure of starved paralarvae.…”

Dynamic pigmentary and structural coloration within cephalopod chromatophore organs

“…It is also the predominant isoform of crystallin found in the bioluminescent light organ of the Hawaiian bobtail squid Euprymna scolopes 25 . Although the S- and Ω-isoforms are compositionally dissimilar, they share (i) high-refractive indices, (ii) optical transparency, and (iii) solubility in water, suggesting key functional roles in both the eyes and skin of the animals 21,24,26 . The structural protein reflectin is cephalopod-specific and aggregates into nanoparticles or self-assembles into ribbons 27–29 , contributing to its ability to scatter and reflect light that produces colorful iridescence or diffuse whiteness 30,31 .…”