How to get chloride ions into the cell
A molecular movie reveals the mechanism of a light-driven chloride pump


Date:
February 3, 2022
Source:
Paul Scherrer Institute
Summary:
A molecular movie has captured in detail the process of an anion
transported across the cell membrane by a light-fuelled protein
pump.

Researchers have unravelled the mystery of how light energy
initiates the pumping process -- and how nature made sure there
is no anion leakage back outside.



FULL STORY ==========================================================================
For the first time, a molecular movie has captured in detail the process
of an anion transported across the cell membrane by a light-fuelled
protein pump.

Publishing in Science,the researchers have unravelled the mystery of
how light energy initiates the pumping process - and how nature made
sure there is no anion leakage back outside.


==========================================================================
Many bacteria and unicellular algae have light-driven pumps in their
cell membranes: proteins that change shape when exposed to photons such
that they can transport charged atoms in or out of the cell. Thanks to
these pumps, their unicellular owners can adjust to the environment's
pH value or salinity.

One such bacteria is Nonlabens marinus, first discovered in 2012 in the
Pacific Ocean. Among others, it possesses a rhodopsin protein in its
cell membrane which transports chloride anions from outside the cell to
its inside. Just like in the human eye, a retinal molecule bound to the
protein isomerizes when exposed to light. This isomerization starts the
pumping process. Researchers now gained detailed insight into how the
chloride pump in Nonlabens marinus works.

The study was led by Przemyslaw Nogly, once a postdoc at PSI and now
an Ambizione Fellow and Group Leader at ETH Zu"rich. With his team, he
combined experiments at two of PSI's large-scale research facilities, the
Swiss Light Source SLS and the X-ray free-electron laser SwissFEL. Slower dynamics in the millisecond-range were investigated via time-resolved
serial crystallography at SLS while faster, up to picosecond, events
were captured at SwissFEL -- then both sets of data were put together.

"In one paper, we exploit the advantages of two state-of-the-art
facilities to tell the full story of this chloride pump," Nogly
says. Jo"rg Standfuss, co- author of the study who built up a PSI team dedicated to creating such molecular movies, adds: "This combination
enables first-class biological research as would only be possible at
very few other places in the world beside PSI." No backflow As the
study has revealed, the chloride anion is attracted by a positively
charged patch of the rhodopsin protein in Nonlabens marinus'cell membrane.

Here, the anion enters the protein and finally binds to a positive
charge at the retinal molecule inside. When retinal isomerizes due to
light exposure and flips over, it drags the chloride anion along and
thus transports it a bit further inside the protein. "This is how light
energy is directly converted into kinetic energy, triggering the very
first step of the ion transport," Sandra Mous says, a PhD student in
Nogly's group and first author of the paper.

Being on the other side of the retinal molecule now, the chloride ion
has reached a point of no return. From here, it goes only further inside
the cell.

An amino acid helix also relaxes when chloride moves along, additionally obstructing the passage back outside. "During the transport, two molecular gates thus make sure that chloride only moves in one direction: inside,"
Nogly says. One pumping process in total takes about 100 milliseconds.

Two years ago, Jo"rg Standfuss, Przemyslaw Nogly and their team unravelled
the mechanism of another light-driven bacterial pump: the sodium pump ofKrokinobacter eikastus. Researchers are eager to discover the details of light-driven pumps because these proteins are valuable optogenetic tools: genetically engineered into mammalian neurons, they make it possible to
control the neurons activities by light and thus research their function.

========================================================================== Story Source: Materials provided by Paul_Scherrer_Institute. Original
written by Brigitte Osterath. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Sandra Mous, Guillaume Gotthard, David Ehrenberg, Saumik Sen, Tobias
Weinert, Philip J. M. Johnson, Daniel James, Karol Nass, Antonia
Furrer, Demet Kekilli, Pikyee Ma, Steffen Bru"nle, Cecilia Maria
Casadei, Isabelle Martiel, Florian Dworkowski, Dardan Gashi, Petr
Skopintsev, Maximilian Wranik, Gregor Knopp, Ezequiel Panepucci,
Valerie Panneels, Claudio Cirelli, Dmitry Ozerov, Gebhard Schertler,
Meitian Wang, Chris Milne, Joerg Standfuss, Igor Schapiro, Joachim
Heberle, Przemyslaw Nogly.

Dynamics and mechanism of a light-driven chloride pump. Science,
Feb. 3, 2022; DOI: 10.1126/science.abj6663 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220203161138.htm

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