The dynamic mechanism of RASSF5 and MST kinase activation by Ras.

As a tumor suppressor, RASSF5 (NORE1A) activates MST1/2 thereby modulating the Hippo pathway. Structurally, activation involves RASSF5 and MST1/2 swapping their SARAH domains to form a SARAH heterodimer. This exposes the MST1/2 kinase domain which homodimerizes, leading to trans-autophosphorylation. The SARAH-SARAH interaction shifts RASSF5 away from its autoinhibited state and relieves MST1/2 autoinhibition. Separate crystal structures are available for the RA (Ras association) domain and SARAH dimer, where SARAH is a long straight α-helix. Using all-atom molecular dynamics simulations, we modeled the RASSF5 RA with a covalently connected SARAH to elucidate the dynamic mechanism of how SARAH mediates between autoinhibition and Ras triggered-activation. Our results show that in inactive RASSF5 the RA domain retains SARAH, yielding a self-associated conformation in which SARAH is in a kinked α-helical motif that increases the binding interface. When RASSF5 binds K-Ras4B-GTP, the equilibrium shifts toward SARAH's interacting with MST. Since the RA/SARAH affinity is relatively low, whereas that of the SARAH heterodimer is in the nM range, we suggest that RASSF5 exerts its tumor suppressor action through competition with other Ras effectors for Ras effector binding site, as well as coincidentally its recruitment to the membrane to help MST activation. Thus, SARAH plays a key role in RASSF5's tumor suppression action by linking the two major pathways in tumor cell proliferation: Ras and the MAPK (tumor cell proliferation-promoting) pathway, and the Hippo (tumor cell proliferation-suppressing) pathway.