Coupled intra- and interdomain dynamics support domain cross-talk in Pin1.

The functional mechanisms of multi-domain proteins often exploit interdomain interactions, or "cross-talk." An example is human Pin1, an essential mitotic regulator consisting of a Trp-Trp (WW) domain flexibly tethered to a peptidyl-prolyl isomerase (PPIase) domain, resulting in interdomain interactions important for Pin 1 function. Substrate binding to the WW domain alters its transient contacts with the PPIase domain via means only partially understood. Accordingly, we have investigated Pin1 interdomain interactions using NMR paramagnetic relaxation enhancement (PRE) and molecular dynamics (MD) simulations. The PREs show that apo Pin1 samples interdomain contacts beyond the range suggested by previous structural studies. They further show that substrate binding to the WW domain simultaneously alters interdomain separation and the internal conformation of the WW domain. A 4.5 microsecond all-atom MD simulation of apo Pin1 suggests that the fluctuations of interdomain distances are correlated with fluctuations of WW domain inter-residue contacts involved in substrate binding. Thus, the interdomain/WW domain conformations sampled by apo Pin1 may already include a range of conformations appropriate for binding Pin1's numerous substrates. The proposed coupling between intra/interdomain conformational fluctuations is a consequence of the dynamic modular architecture of Pin1. Such modular architecture is common among cell-cycle proteins; thus, the WW-PPIase domain cross-talk mechanisms of Pin1 may be relevant for their mechanisms as well.