We applied single-molecule epi-fluorescence microscopy to study the mobility of both the Gβγ and the Gα2 subunits of the G protein heterotrimer in comparison with the cAMP receptor responsible for chemotactic signaling in Dictyostelium discoideum. Our experimental results suggest that ~30% of the G protein heterotrimers exist in receptor-precoupled complexes. Upon stimulation in a chemotactic gradient, the complexes dissociate, subsequently leading to receptor diffusion and amplification of the external signal by a diffusion-reaction process. We further found that Gβγ was partially immobilized and confined in an agonist-, F-actin- and Gα2-dependent fashion. This led to the hypothesis that functional nanometric domains exist in the plasma membrane, which locally restrict the activation signal, and in turn, lead to faithful and efficient chemotactic signaling.

Thomas Schmidt received a PhD in physics on the topic of “Spectroscopic Investigations of Energy- Transfer in NaNO ₂ :KNO ₂ ” from the University of Düsseldorf, Germany, in 1988. Subsequently he was postdoc at Leiden University, The Netherlands, where he further pursued high-resolution spectroscopy to learn about the glassy state of inorganic and biological material at low temperature. In 1993 he moved to the University of Linz, Austria, where he built up a group that developed single-molecule microscopy at room temperature. In 1999 he was appointed as full professor in Physics of Life Processes at the Leiden Institute of Physics, Leiden University, The Netherlands. Since, his group further developed single-molecule techniques for research in cell biology. His current research interests range from biomimetics to initial steps in cellular signaling, and cell mechanics. He mostly utilizes high-resolution, high-sensitivity optical techniques that permit to follow cellular processes one molecule at a time.