RAS small GTPase signalling to the enigmatic RASSF death effectors

RAS small GTPases function as molecular switches to transduce signals from cell surface receptors to various downstream signalling networks. The RAS protein has roles in multiple biological functions, including cell proliferation, survival, and even apoptosis. Mutations in RAS genes are present in up to 30% of all human tumors. The three major RAS homologs HRAS, KRAS, and NRAS are each found mutated, predominantly at codons 12, 13 or 61. Gain-of-function mutations at these codons render these RAS proteins constitutively active and thereby produce hyperproliferative signals. Since the discovery of RAS, numerous downstream ‘RAS effector proteins’ have been identified. The biological role of most identified RAS effectors relates to cell proliferation and survival, however, in the past two decades a new family of RAS effectors, RASSF proteins, were discovered to have a pro-apoptotic function.   The RASSF family of tumor suppressors proteins are frequently downregulated in human cancer cells. There are 10 RASSF homologs (RASSF1-10) in humans, each comprising a RAS association (RA) domain presumed to bind RAS GTPases. RASSF also encode SARAH (Salvador-RASSF-Hippo) domain, known to interact with SARAH-containing Hippo kinases. It is not clear whether all the RASSF proteins are true RAS effectors. Only a single family member, RASSF5, has been shown to directly associate with HRAS and this interaction has been completely validated by X-Ray crystallographic studies. In the first part of this thesis, I demonstrate that no other RASSF family members directly interact with KRAS. I used homology modelling of the highly related RASSF1 RA domain to identify amino acids crucial to GTPase binding. I show that a single amino acid substitution in RASSF1 enables interaction with KRAS, and hypothesize that this evolutionarily diverged residue has altered RASSF1 specificity for small GTPases. Using an informatics approach, we predicted six candidate GTPases that could interact with RASSF1: GEM, REM1, REM2, RASL12, ERAS and DIRAS3. I validated interactions with several RGK GTPases (GEM, REM1, REM2) and show that co-expression of RGK GTPases with RASSF1 activates the Hippo pathway. Thus, I show a novel link between these unstudied GTPases to Hippo pathway regulation. In the second part of my thesis I attempt to rewire KRAS signalling from cell proliferation pathways (RAF/MAPK) to pro-apoptotic effectors (RASSF-Hippo). To achieve this, I designed mutations in KRAS that augmented its affinity for RASSF5 and weakened interaction with BRAF. As a second strategy, I reverted evolutionarily diverged residues in the RASSF1 effector to corresponding residues in RASSF5 and demonstrate that this variant now binds KRAS. Various biophysical and biochemical approaches were used to validate both the KRAS and RASSF1 rewired mutants, and co-localization studies show that these mutants interact with their new binding partners as predicted. Further, I demonstrate that our rewired KRAS no longer binds BRAF in cells but interacts strongly with RASSF5 and rewired RASSF1, and that the rewiring mutants activate the Hippo pathway. Thus, I have developed two rewiring approaches that will help us to study KRAS signalling towards pro-apoptotic RASSF pathway in the absence of strong MAP kinase activation. The data presented here provide several novel insights into how RASSF proteins diverged through evolution and are not all direct effectors of RAS. In addition, I present an innovative rewiring strategy to couple RAS signals towards RASSF effectors which can be a clinical tactic to target RAS oncogenesis.

Keyphrases: RAS GTPase, RGK GTPase, RASSF1, RASSF5, RAF, Apoptosis, Hippo signalling