RASSF1A: Not a prototypical Ras effector

Abstract

The Ras association domain family (RASSF) of genes are commonly silenced by promoter specific methylation in human cancers. After the cloning of the first two family members in early 2000 (RASSF1 and RASSF5), eight other related genes have been identified (RASSF2, 3, 4 and 6-10). The unifying motif amongst all RASSF family members is the presence of the Ras association (RA) domain that could potentially associate with the Ras family of GTPases. Detailed analyses have determined that RASSF family members are tumor suppressor proteins, activators of cell death, cell cycle modulators, microtubule stabilizers and possibly inflammatory mediators linked to NFκB. As such, exploring the biological function of this gene family is needed and if indeed RASSF proteins could be the missing link between Ras signaling and apoptosis. Several RASSF family members have been demonstrated to associate with Ras. However, there is still controversy regarding the ability of RASSF1A to utilize Ras to promote cell death and of the importance of the RASSF1A RA domain. The focus of this review is to highlight the importance of Ras binding to the RASSF family of proteins and discuss what we currently know about the biology of RASSF1A.

Figure 1

A simplified model of Ras signaling. Ras signaling pathways originate from growth factor binding and activation of its receptor such as the epidermal growth factor [EGF] binding to its receptor, EGF-R. The activation of the EGF-R by EGF primes the autophosphorylation of the cytoplasmic domain of EGF-R (P) and the recruitment of adapter proteins, Grb2 and Sos. Sos is a primary Ras-GEF that will promote GTP loading of Ras and subsequent Ras activation. Ras can then continue the signaling cascade to Raf-MAPKK (MEK)-MAPK (ERK) and eventually to the activation of the transcription factor Elk-1. Once activated by phosphorylation, Elk-1 can then acquire DNA binding competency to activate gene transcription to drive proliferation. Elk-1 is just one of many transcription factors modulated by Ras signaling and is only shown here for simplicity. There is evidence for Ras activation of cell death and it is thought to proceed through the RA SSF family of proteins. In the bottom right corner of this figure is a limited list of diseases that are a direct result of abnormal Ras signaling (the RASopathies).

Figure 2

Schematic of RASSF1A. The SH3 binding domain of RASSF1A has the sequence PAGP that is conserved in mouse and human forms of RA SSF1A and found as PQDP in RA SSF5A. The ATM (Ataxia telangiectasia mutated) phosphorylation site is underlined with surrounding residues shown. ATM is a serine/threonine protein kinase that is a DNA damage sensor activated upon double strand breaks (DSBs), apoptosis or the addition of genotoxic stresses such as ultraviolet A light (UVA). The binding sites for several RA SSF1A effector proteins are shown. The Ras association domain (RA) may potentially associate with the Ras family of oncogenes. The SARAH domain modulates heterotypic associations with the sterile-20 like kinases, MST1 and MST2. Approximate positions of exons and amino acids are also indicated as well as the identification of several polymorphisms to RASSF1A. Adapted from El-Kalla et al. (2010). See text for details on the other functional domains.

Figure 3

Model for RASSF1A modulated apoptosis. Death receptor-induced cell death (TNFα is used as an example) can result in the recruitment of protein complexes to activate Bax and promote apoptosis. Basally, RASSF1A is kept complexed with 14-3-3 by GSK-3β phosphorylation in order to prevent unwanted recruitment of RASSF1A to death receptor and uncontrolled stimulation of Bax and apoptosis. Once a death receptor stimuli has been received (TNFα as shown above), the TNFR1/MOAP-1/RASSF1A complex promotes the “open” form of MOAP-1 to associate with Bax. This in turn results in Bax conformational change and recruitment to the mitochondria to initiate cell death. Following release from TNF-R1/MOAP-1 complex, RASSF1A may re-associate with 14-3-3 to prevent continued stimulation of this cell death pathway (unpublished observations). Please see text for further details.