This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We have an ongoing program to use crosslinking to probe for proteins that interact with a target protein in vivo and ultimately to determine the sites of interaction between these proteins. We have written software that will help us to deduce this information by mass spectrometric analysis of the crosslinked proteins after digestion by various proteases.