Scientists Discover Essential Step in Recharging the Eye’s Light-sensing Retina

Researchers have uncovered a mechanism by which a protein region shape-shifts in order to transform vitamin A into a form that can be utilized by the photoreceptor cells in the eye, which detect light. A previously uncharacterized area of the protein known as RPE65 spontaneously turns spiral-shaped when it encounters intracellular membranes, or thin structures that surround different parts of a cell.

Thanks to this shape-shifting, RPE65 performs the vital function of converting vitamin A in the endoplasmic reticulum, a network of sac-like structures and tubes.

The revelation, according to the researchers, improves our knowledge of RPE65's role and will help us develop treatments for vision impairments caused by RPE65 gene mutations. The study was carried out by researchers at the National Eye Institute, a division of the National Institutes of Health, and was published in Life Science Alliance.

When light strikes opsin-containing photoreceptor pigments, it produces signals for the brain through a sequence of chemical reactions. Opsins are recharged by the retinal pigment epithelium (RPE), a tissue that serves as a support for the photoreceptors, to restore their sensitivity to light.

RPE65 is crucial for converting all-trans retinol, a spent vitamin A product, back into photosensitive 11-cis retinal during a process known as the visual cycle. Early-onset severe blinding diseases are linked to RPE65 gene mutations.

Making 11-cis retinal requires RPE65 to interact with the endoplasmic reticulum of the RPE cell, but the exact way that RPE65 binds to this membrane has remained a mystery.

In a recent study, T. Michael Redmond, Ph.D., and researchers from the NEI's Laboratory of Retinal Cell and Molecular Biology demonstrate how RPE65 enters the endoplasmic reticulum membrane of the RPE cells, where the conversion process involving RPE65 takes place.

“Methods such as crystallography, which we use to visualize the atoms of a protein in crystal form, failed to give us a complete picture of RPE’s structure with this crucial region missing” said Sheetal Uppal, Ph.D., a research fellow at NEI and the study’s first author. “We had to think of a new strategy to characterize this aspect of RPE65’s structure, so we turned to biochemistry.”

The NEI researchers found that a particular region of RPE65 lacks structure in aqueous solution, but when it comes into contact with membranes, it spontaneously forms an amphipathic alpha-helix, a unique type of spiral shape in proteins. This change makes it possible for RPE65 to bind to the endoplasmic reticulum membrane of the RPE cells, which is where 11-cis retinal is made from all-trans retinol.

More than that, when a single specific amino acid within the previously uncharacterized region of RPE65 was modified by a specific lipid, it greatly expedited the formation of the alpha-helix, “locking” it into place, and facilitating its insertion into the cell membrane. This was something never seen before in a protein, according to Uppal.

The results of the molecular dynamic simulations supported by computer modeling. “Our findings solve a longstanding puzzle in RPE65’s structure, clarifying its function, while expanding our knowledge of membrane-binding in a way that we hope will inform disease models in more accurate ways.”

References

Sheetal Uppal, Tingting Liu, Emily Galvan, Fatima Gomez, Tishina Tittley, Eugenia Poliakov, Susan Gentleman, and T. Michael Redmond. An Inducible Amphipathic α-Helix Mediates Subcellular Targeting and Membrane Binding of RPE65.DOI: 10.26508/Isa.202201546