C. elegans Rassf homolog, rasf-1, is functionally associated with rab-39 Rab GTPase in oxidative stress response

The Ras association domain family (Rassf) is one of the Ras effectors, which can bind to several GTP-charged Ras-like GTPases. The Rassf proteins are widely conserved beyond species from nematode to human. To explore the novel functions of Rassf proteins, we took advantage of nematode C. elegans as a model animal with only one Rassf homolog, T24F1.3 (rasf-1). The rasf-1-mutant as well as rasf-1-knockdown animals were found to be more sensitive to oxidative stress of arsenite than in wild type, indicating that rasf-1 is involved in oxidative stress response. We next screened for proteins that interact with RASF-1 by the yeast two-hybrid system and identified RAB-39 Rab GTPase as an interacting partner of RASF-1. We not only confirmed specific binding between these molecules but also demonstrated that RASF-1 binds to GTP-bound form but not GDP-bound form of RAB-39. Importantly, rab-39 mutant animals were also sensitive to oxidative stress, which was dependent on rasf-1 according to the epistasis analysis. Moreover, Rassf1 and Rab39, mammalian homologs of rasf-1 and rab-39, respectively, were shown to interact with each other in vitro. These results indicate that the RASF-1 functionally interacts with RAB-39 and that the interaction between their homologs is conserved in mammals.