[excerpt] Proteins are major players in the biological world—among other things, they make biochemical reactions go and serve as building blocks for much of the structural elements of organisms. But before any protein can fulfill its biological role, it must fold itself into its designated shape, or conformation. Incorrectly folded proteins don’t work and diseases such as Parkinson’s, Huntington’s and mad cow are all associated with misfolded proteins that clump together.
While it is generally understood that proper protein folding happens spontaneously, scientists are still trying to understand what causes a particular protein to fold properly or go awry. The protein’s local environment plays a role, and other molecules in the neighborhood can assist or hinder the process, says Gierasch.
“It’s a delicate balance—proteins can tip to the dark side very readily,” she says.
Recently researchers have developed techniques that allow them to follow the fate of individual proteins within cells. Using an experimental set-up that they designed to do just that, Gierasch and Ignatova decided to see how the small molecule proline influenced protein folding. Proline is often taken up by cells in response to water loss, and other molecules with similar cellular roles have been shown to inhibit improper folding.
“There have been conflicting results,” says Gierasch, “In some cases these molecules inhibit misfolding, in some cases they promote it.”
To investigate, Gierasch and Ignatova used a strain of E. coli that makes a protein with a fluorescent tag attached. When the protein unfolds from its proper conformation and starts to clump, its fluorescence increases and can be tracked in real time.
When the researchers treated the cells with salt—causing proline to accumulate inside—they found that the proline prevented the protein from misfolding into the insoluble aggregates that are associated with several diseases. Increasing proline inside the cells after clumping was underway did slow aggregation down, but couldn’t stop it. Test tube experiments recapitulated these results.
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