WE MAY be a step closer towards understanding how the first cells emerged on
the primordial stage, thanks to researchers in California. They have shown that
proteins can spontaneously form the ion pumps that help power cells.
Protein pumps are crucial in modern cells. They use the energy from light or
food to move ions across a membrane, creating an electrical gradient. These
gradients act as a kind of battery, driving cellular processes.
Unlike the sophisticated proteins that make up pumps in modern cells, the
pumps of 鈥減rotocells鈥� must have formed from simple proteins present on ancient
Earth. What鈥檚 more, these pumps must have assembled themselves.
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A team from NASA鈥檚 Center for Computational Astrobiology in California, led
by Andrew Pohorille, has now simulated the formation of such pumps, using a
protein called M2 from the human flu virus. Though it鈥檚 highly unlikely that M2
was used by protocells, similar proteins may have been involved. 鈥淲e really
don鈥檛 care all that much for the specific identity of the actors,鈥� says
Pohorille, 鈥渨e want to know if we can understand the play.鈥�
He and his colleagues simulated what happens to M2 molecules when they
interact with simple membranes made of a mixture of organic substances such as
carboxylic acids. Such membranes can spontaneously form vesicles reminiscent of
cells
(快猫短视频, 12 September 1998, p 30).
The M2 protein has a water-loving backbone and oily, water-hating side
chains. In water, the protein is held open. When it interacts with a membrane,
however, the protein folds into an alpha helix. According to the computer
simulation, four of these helices then bond to each other, forming a channel on
the inside, and the whole package inserts itself into the membrane to escape the
water.
Within the channel, parts of the proteins bond to form a gate that blocks
most ions. However, hydrogen ions captured at the outside of the gate are
rapidly conducted through the proteins by a series of chain reactions that
eventually spews hydrogen ions into the interior. This process causes protons to
accumulate inside the vesicle, creating an electrical gradient.
David Deamer, a biophysicist at the University of California at Santa Cruz,
says this type of modelling could help researchers create lab versions of
protocells in the near future. 鈥淔or the first time we are in a position that we
can do it,鈥� agrees Pohorille, whose team last week presented its findings to an
American Physical Society meeting in Minneapolis.
