Two-component sensor kinase signaling systems are widespread in bacteria, but gaining mechanistic insight into how kinase activity is controlled by ligand binding has proved challenging. Here, we discuss this problem in the context of our structural and functional studies of bacterial quorum sensing receptors. Specifically, this chapter focuses on the transmembrane sensor kinase complex LuxPQ, which serves as the receptor for the "universal" quorum sensing signal molecule autoinducer-2 (AI-2). Methods are presented for the overproduction, purification, crystallization, and functional characterization of LuxPQ's ligand-binding (periplasmic) domain.