From: Paul Scherrer Institute (PSI)
January 3, 2022 -- PSI scientists have
shed light on an important component of the eye: a protein in the rod cells of
the retina which helps us see in dim light. Acting as an ion channel in the
cell membrane, the protein is responsible for relaying the optical signal from the
eye to the brain. If a genetic disorder disrupts the molecular function in a
person, they will go blind. Scientists have deciphered the protein's
three-dimensional structure, preparing the way for innovative medical
treatments. The study is published in the scientific journal Nature
Structural & Molecular Biology.
"It's thanks to the rod cells in
our eye that we can observe the stars in the night sky," explains Jacopo
Marino, a biologist with PSI's Laboratory of Biomolecular Research. "These
photo cells are so sensitive to light that they can detect even a single photon
reaching us from a very remote part of the universe -- a truly incredible
feat." The ability of our brain to eventually translate these light beams
into a visual impression is partly down to the cyclic nucleotide-gated (CNG)
ion channels whose three-dimensional structure has now been illuminated by a
PSI research group led by Jacopo Marino.
The ion channel acts as a gatekeeper
controlling whether specific particles are allowed through to the interior of
the receptor cell. It is embedded in the protein-rich shell -- the cell
membrane -- of the rod cells. In darkness, the ion channel, and thus the gate
to the cell, is completely open. But when light hits the eye, it triggers a
cascade of processes in the rod cells. This ultimately causes the gate to
close, with the result that positively charged particles, such as calcium ions,
can no longer enter into the cell.
This electrochemical signal continues
via the nerve cells into the brain's visual cortex, where a visual impression
-- such as a flash of light -- is created. "The idea of solving the
structure of this channel dates back to nearly 20 years ago, when Gebhard
Schertler and Benjamin Kaupp already collaborated on this topic," says
Jacopo Marino. Both are co-authors of the new study.
Endurance paid off
PhD student Diane Barret first had to
extract the channel protein from cows' eyes supplied by an abattoir -- a
complicated and arduous process. "This was a very challenging task, as the
protein is extremely sensitive and decomposes very quickly. In addition, it is
only available in tiny quantities in the source material," Barret
explains. It took a whole two years to obtain enough protein to work with.
"We were both too stubborn to simply give up," says Jacopo Marino,
laughing. "But in the end that stubbornness paid off."
The scientists then used cryo-electron
microscopy to reveal the three-dimensional structure of the ion channel.
"In contrast to previous studies on the structure of the ion channel, we
investigated the native protein as it exists in the eye. We are therefore much
closer to the real conditions that exist in living creatures," Diane
Barret says.
One of the reasons why a clearer
understanding of the channel protein's natural structure is important is to
advance the development of treatments for genetic disorders for which there is
no known cure, such as retinitis pigmentosa. With this disease, photoreceptors
gradually die off, leaving people blind. One possible cause is that the body is
unable to correctly produce the CNG channel protein due to a genetic defect. As
a result, the ion channel does not close completely when light hits the eye,
disrupting the cell's electrochemical balance and causing the cells to die.
"If we could find molecules that
affect the protein in such a way that the channel would completely close, we
could prevent the cells from dying -- and thus stop people going blind,"
explains Jacopo Marino. Now that researchers have identified the precise
structure of the protein they are able to search specifically for such
molecules.
Additional barrier
The protein comprises four parts: three
lots of subunit A, and one lot of subunit B. A correctly functioning ion
channel is only possible in this combination. In their study, PSI scientists
show why the B subunit seems to play such an important role: a side arm of the
protein -- a single amino acid
-- protrudes from the rest of the
protein, like a barrier across a gateway. This narrows the passage in the
channel to the point where no ions can pass through.
"No one expected that -- it came as
a total surprise," says Diane Barret. Other narrow places already exist in
the A subunit -- like main gateways -- which were previously thought to be the
only ones. It is interesting to note that the additional barrier is found not
only in the protein from the cow's eye, but seems to apply to all types of
animal, as the scientists showed. Whether crocodiles, eagles or humans -- all
living creatures with an ion channel in their eye have the same protruding
amino acid at this position in the protein. As it has been preserved so
consistently during evolution, it must be essential for the functioning of the
channel.
https://www.sciencedaily.com/releases/2022/01/220103121742.htm
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