  While allostery is of paramount importance for protein signaling and
regulation, the underlying dynamical process of allosteric communication is not
well understood. PDZ3 domain represents a prime example of an allosteric
single-domain protein, as it features a well-established long-range coupling
between the C-terminal $\alpha_3$-helix and ligand binding. In an intriguing
experiment, Hamm and coworkers employed photoswitching of the $\alpha_3$-helix
to initiate a conformational change of PDZ3 that propagates from the C-terminus
to the bound ligand within 200 ns. Performing extensive nonequilibrium
molecular dynamics simulations, the modeling of the experiment reproduces the
measured timescales and reveals a detailed picture of the allosteric
communication in PDZ3. In particular, a correlation analysis identifies a
network of contacts connecting the $\alpha_3$-helix and the core of the
protein, which move in a concerted manner. Representing a one-step process and
involving direct $\alpha_3$-ligand contacts, this cooperative transition is
considered as elementary step in the propagation of conformational change.
