to photoreactive amino acid Mapping membrane protein interactions on the molecular scale with unnatural amino acids -- From the Mehl Research Lab at Franklin & Marshal College If we could zoom into the tip of a cell we would see a myriad of complex proteins interacting designed help the cell communicate with the outside world. These interacting membrane proteins in bacteria communicate to the cell what chemicals are outside and where to swim in a process called chemotaxis. These systems in bacteria are the models for how all cells communicate in living organisms. Unfortunately because these proteins are stuck in the membrane they can not be removed to be studied and we don’t know how they are organized or move to perform their complex signaling tasks. To test some of the hypothetical structures shown here we go into the living cell - into the proteins with unnatural photoreactive amino acids. C’ C N Signal binds Periplasm Membrane Cytoplasm Change individual amino acids at the tip Change F373 to photoreactive amino acid shine light on cells and separate By placing these photoreactive amino acids at defined location within the protein in the living cell we can map out how the proteins and when the proteins are contacting each other. If two proteins are touching at the location of the photoreactive amino acid they form a permanent bond. The covalently attached proteins can be removed from the cell and investigated to ascertain the organization of proteins in the membrane. In this pictorial example we have shown that the bacterial chemotactic receptor, tsr, is organized as a trimer of dimers in its functional form. Using this technology we can study the interactions of any protein in side the living cell. Irradiation time (min) 0 45 0 15 45 Trimer crosslink Dimer crosslink Tsr monomer Native Tsr Tsr-373-pBpa Ryan A. Mehl, Department of Chemistry, Franklin &Marshall College