First, they made use of a photocaging method to control whether a substance could enter the cell at all. To do so, they attached a photosensitive chemical to the target enzyme rapamycin. The combined molecule is too large to pass through the cell membrane but upon UV irradiation the photosensitive cage is released and the now smaller rapamycin molecule can freely enter the cell. Among other functions, rapamycin is a 'dimerizer' (an enzyme that induces other proteins to pair up).
Next, they modified two other proteins (A and B) such that if they were to pair up, the combination would translocate from the center of the cell to the plasma membrane. One of these two proteins induces ‘membrane ruffle formation’, a key feature in cell migration. Thus, if the protein complex did move from the interior of the cell to the edge, that displacement could be detected in a microscope by observing the cell membrane structure.
When the researchers shined a UV light on a small region of the cell, they got membrane ruffles just in that region. The UV light was breaking up the photocage around the rapamycin, the rapamycin was entering the cell and dimerizing A and B, and these combined proteins were shuttling to the edge of the cell and causing visible membrane ruffles. The researchers were able to direct where on the cell membrane the A/B dimer appeared by controlling where they shone their light.
Ultimately, the scientists hope to attach a third, target protein to the A/B complex. In that case, the above method would allow the biologists to move that protein of interest to different locations in the cell for study. If different protein dimers that relocate to different specific cell regions could be used, that would make the technique even more useful.