Dictyostelium discoideum cells display a strong chemotactic response to cyclic adenosine 3',5'-monophosphate (cAMP), mediated by a cell surface receptor and G protein-linked signaling pathway. In October 26 Science, Masahiro Ueda and colleagues from Osaka University, Japan, show the first real-time images of how single fluorescent-labeled cAMP molecules bind to their receptors on living Dictyostelium amoebae.

Ueda et al. used an objective-type total internal reflection fluorescence microscope to record single-molecule imaging of Cy3-cAMP molecules on the surface of living amoebae. They found that binding sites were uniformly distributed on the membrane and, after coupling, the receptors move inside the cell. The probability of an individual association was greater for receptors at the anterior of the cell. In addition, they could visualize how G protein coupling influenced the binding kinetics (Science 2001, 294:864-867).

"People know that receptors bind and release molecules, but until now no one has seen...