Making the complex less complicated: Scientists create a simple, functional ion transporter
December 18, 2014 § Leave a comment
Scientists have designed a stripped-down version of a protein that carries certain ions across a membrane. In doing so, the scientists have shown that the large, complex transporters found in all living things actually operate on simple principles. They’ve also demonstrated that we now know enough about these molecular machines to design them from scratch. The work is described in a paper just out in Science.
Natural transporters “are large and complex machines. And yet, transmembrane transport is an absolutely essential feature of cellular life so it must have evolved very early on,” says one of the scientists involved in the work, Gevorg Grigoryan at Dartmouth College. “How did such complex machines come into being?”
Grigoryan, William Degrado at the University of California, San Francisco, and others designed a bare-bones transporter, one that inserts into the lipid membrane to carry zinc or cobalt ions in one direction and protons in the opposite direction. They named their protein Rocker, after a feature engineered into the molecule that made it rock back and forth between different conformations to enable transport—similar to how natural transporters are thought to work. In Rocker’s case, it oscillated as it bound zinc ions at one end of the molecule and released them at the other
The researchers developed Rocker in two steps. First, they designed the minimalistic protein with the help of computer programs. “Because of the complicated design goals, having to balance membrane insertion, formation of the desired topology, ion binding in the membrane, and specific dynamic features of the molecule, we had to develop a novel computational design approach,” says Grigoryan, who led the computational work.
Next up was making the actual 25-amino acid protein in the lab and getting it to work as designed. The scientists showed that Rocker formed four-helix bundles in membranes, in agreement with the computational model. The amino acids within Rocker had to be precisely positioned so that the protein transported only zinc and cobalt, but not calcium, ions through it.
Every time Rocker transported a zinc or cobalt ion, it pushed three or four hydrogen ions in the opposite direction. The investigators used methods like X-ray crystallography, analytical ultracentrifugation, and nuclear magnetic resonance to study Rocker in action as it transported ions in and out of microscopic sacks made of lipids.
Although tiny in comparison to native transporters, “Rocker is essentially a reductive deconstruction of the transport process,” says Grigoryan. By designing a minimalistic transporter protein from scratch, Grigoryan says, he and his colleagues showed that “selective transport itself does not necessarily require complex structure and demonstrated a plausible evolutionary mechanism by which transport could have originated.”
Down the road, Rocker can be used as a model system to understand the structural and thermodynamic factors for ion transport. It also can be used as a mold to design other types of transporters.
Andrei Lupas at the Max Planck Institute for Developmental Biology, who wrote the Perspective article accompanying the paper by DeGrado’s and Grigoryan’s teams, used a quote of Richard Feynman to drive home the importance of the work: “What I cannot create, I do not understand.”