Low pH-driven folding of WW45-SARAH domain leads to stabilization of the WW45-Mst2 complex.

The scaffolding protein Salvador (Sav) plays a key role in the Hippo (Hpo) signalling pathway, which controls tissue growth by inhibiting cell proliferation and promoting apoptosis. Dysregulation of the Hippo pathway contributes to cancer development. Since the identification of the first Sav gene in 2002, very little is known regarding the molecular basis of Sav-SARAH mediating interactions due to its insolubility. In this study, refolding of the first Sav (known as WW45)-SARAH provided insight into the biochemical and biophysical properties, indicating that WW45-SARAH exhibits properties of a disordered protein, when the domain was refolded at a neutral pH. Interestingly, WW45-SARAH shows folded and rigid conformations relative to the decrease in pH. Further, diffracting crystals were obtained from protein refolded under acidic pH, suggesting that the refolded WW45 protein at low pH has a homogeneous and stable conformation. A comparative analysis of molecular properties found that the acidic-stable fold of WW45-SARAH enhances a heterotypic interaction with Mst2-SARAH. In addition, using an Mst2 mutation that disrupts homotypic dimerization, we showed that the monomeric Mst2-SARAH domain could form a stable complex of 1:1 stoichiometric ratio with WW45 refolded under acidic pH.