I'm currently working on in vivo and in vitro cross-linking experiments.  The basis for chemical cross-linking experiments is to determine whether two proteins interact with each other or not.  Cross-linking reagents have specific reactive groups and target specific functional groups on the amino acids of proteins.  The logic behind these types of experiments is that if two proteins get close enough to each other to interact, the chemical cross-linking compound can interact with the functional groups of each of the two interacting proteins and "link" them together via the bridge created by the chemical cross-linking compound.  Everything is in the preliminary stages, so I don't have anything more specfic to say at this point.  All I can say is that no one has ever done these experiments for the histidine permease, so it's all up to me figure out how to get things to work.  Well, this is science and this is what I'm here for.