Abstract
RASSF2 is a tumour suppressor that in common with the rest of the RASSF family contains Ras association and SARAH domains. We identified the proapoptotic kinases MST1 and MST2 as the most significant binding partners of RASSF2, confirmed the interactions at endogenous levels and demonstrated that RASSF2 immunoprecipitates active MST1/2. We then demonstrated that RASSF2 can be phosphorylated by a co-immunoprecipitating kinase which is likely to be MST1/2. Furthermore, we demonstrated that RASSF2 and MST2 do indeed colocalise, but whilst RASSF2 alone is nuclear, the presence of MST1 or MST2 results in colocalisation in the cytoplasm. Expression of RASSF2 (stably in MCF7 or transiently in HEK-293) increases MST2 levels and knockdown of RASSF2 in HEK-293 cells reduces MST2 levels, additionally colorectal tumour cell lines and primary tumours with low RASSF2 levels show decreased MST2 protein levels. This is likely to be mediated by RASSF2-dependent protection of MST2 against proteolytic degradation. Our findings suggest that MST2 and RASSF2 form an active complex in vivo where RASSF2 is maintained in a phosphorylated state and protects MST2 from degradation and turnover. Thus we propose that the frequent loss of RASSF2 in tumours results in destabilisation of MST2 and thus decreased apoptotic potential.