Imaging protein–protein interactions in plants and single cells

As in any society, the many different proteins found within cells must often work together to carry out their intricate business. The protein complexes that form can be stable, as seen with multisubunit enzymes or structures such as the proteasome, or transient, such as those seen during the assembly of transcription factors or the intermediates in a signaling cascade. Wherever they are found, the identification of the interacting protein partners can be a difficult and sometimes daunting task. Interestingly, when two proteins are in close proximity, light energy absorbed by one can be passed to the other by a process called resonance energy transfer (RET). During fluorescence resonance energy transfer (FRET), the input light energy is obtained by illuminating the sample, whereas in bioluminescence resonance energy transfer (BRET), the input light energy is derived from an in situ bioluminescence reaction (1). Light derived from bioluminescence is generally weak and does not photobleach the sample or cause autofluorescence, a problem particularly acute in plant cells because of chlorophyll. BRET is therefore an excellent choice for measuring protein interactions in plant cells but in the past has seen limited application precisely because the bioluminescence emission is so weak. The work of Xu et al. (2) in this issue of PNAS marries an exquisitely sensitive CCD camera with a beam splitter to solve the light intensity problem, and the ensemble allows BRET to be visualized not only in plant seedlings but also in single cells. There are a number of techniques available that have been developed to address the thorny issue of protein–protein interactions inside cells. Perhaps the most frequently found in molecular biology …