While much is known about different allosteric regulation mechanisms, the nature of the "allosteric signal", and the timescale on which it propagates, remains elusive. The PDZ3 domain from postsynaptic density-95 protein is a small protein domain with a terminal third alpha helix -- the $\alpha$3-helix, which is known to be allosterically active. By cross-linking the allosteric helix with an azobenzene moiety, we obtained a photocontrollable PDZ3 variant. Photoswitching triggers its allosteric transition, resulting in a change in binding affnity of a peptide to the remote binding pocket. Using time-resolved infrared and UV/Vis spectroscopy, we follow the allosteric signal transduction and reconstruct the timeline in which the allosteric signal propagates through the protein within 200 ns.
