Their finding adds to growing evidence that some creatures -- and probably people -- adapt to light not only by mechanically shrinking the pupil to physically limit how much light enters the eye, but also by a chemical response.
Building on their previous work showing that specific proteins in eye cells are redistributed in response to bright light, the Johns Hopkins team now reports how a key protein called arrestin is shuttled from a "holding area" where it binds and calms a light-detecting protein. Writing in the July 7 issue of Neuron, the team says arrestin is moved around by a tiny molecular motor, called myosin, which travels along the "train tracks" of the cell's internal skeleton.
Arrestin's swift relocation, the researchers proposed, helps prevent temporary blindness that would otherwise be caused by a sudden increase in light intensity, such as occurs when stepping from a dark movie theater into the bright afternoon sunshine.
"We knew that arrestin was transported, but we didn't know how this occurred," says Craig Montell, Ph.D., professor of biological chemistry. "Fly and mammalian eyes have similar light detector cells and proteins, and it takes about the same amount of time for our eyes to adapt to light, so we suspect that comparable mechanisms exist in humans."
The light-detecting cells in fruit flies are similar to the rod and cone cells found in the human retina. One end of each cell contains the protein that directly responds to light, but other proteins critical for the light response are shifted back and forth into different parts of the cell in a light-dependent manner. Scientists didn't know how these molecules might be moved from one end of the cell to the other, until now.