Photocrosslinkers illuminate interactions in living cells.

Transient and low-affinity interactions among macromolecules underlie many physiological events. Often, these interactions are difficult to study because they are not maintained when the participating molecules are removed from their cellular context. To circumvent this challenge, crosslinking reagents can be used to introduce covalent bonds between interacting macromolecules. Photoactivatable crosslinkers are particularly attractive because they allow crosslinking to proceed in time- and location-specific ways. Once the interacting partners have been crosslinked, they can be isolated and then analyzed by mass spectrometry or other analytical techniques to determine the identity of the interacting molecules and to pinpoint the interacting regions. This review highlights recent methodological developments that make it possible to introduce photocrosslinking groups into polypeptides or glycans as they are synthesized in cells. We also describe how these methods offer a non-invasive way to study macromolecular interactions in a native context.