Rabac1 has been shown to interact with BHRF1, a viral homologue of Bcl-2. Interestingly the anti-apoptotic activity of BHRF1 was reduced upon interaction with Rabac1. Furthermore it has been shown that Praf2 expression Chloroquine Phosphate correlated with cerulenin-induced apoptosis and Arl6IP5 is a key mediator of the apoptosis induced by C/EBPalpha. We speculated therefore that the interaction of Bcl-xL with Praf2 might constitute a link between the secretory and the apoptotic pathways. In this study, we have confirmed the ability of Praf2 to interact with Bcl-xL and defined that the interaction depends on Bcl-xL’s C-terminal transmembrane region. The dependency on the TM region might be interpreted in different ways: either the TM region is directly involved in the interaction or is a pre-requisite for the interaction because it is the anchor that localises Bcl-xL on the membrane were it can find its partner Praf2. Given that secondary structure prediction algorithms suggest that Praf2 is almost entirely embedded in the membrane bilayer, it is reasonable to think that the interaction could be directly mediated by a hydrophobic amino-acid stretch like the TM domain. Unlike Bcl-2, that is predominantly located to the ER, Bcl-xL has been suggested to be mainly a mitochondrial protein located at the Mitochondrial Outer Membrane. Praf2, instead, has been shown to be mainly located at the ER. How can two proteins located at different intracellular compartments interact? In our cellular fractionation experiments we can always observe a small fraction of Bcl-xL located to the microsomal light membrane fraction. One possibility is that the interaction is Catharanthine sulfate limited to the small fraction of ER-based Bcl-xL. We do observe, in fact, a partial colocalization between 
Bcl-xL and Praf2 particularly pronounced in the cellular perinuclear region. Another possibility is that the localisation of one or both proteins is dynamic and the interaction is limited to a particular physiological stage. It is known for instance that Bcl-xL translocates from the cytosol to the mitochondria following an apoptotic stimulus. There could be a yet undefined stimulus that induces Bcl-xL relocalisation mainly to the ER. One can even hypothesize that Praf2 would function as the receptor of Bcl-xL under such a condition. A final possibility is that the most relevant physiological interaction occurs between Praf2 and Bcl-2, given that Bcl-2 localises mainly to the ER. We have indeed demonstrated that Praf2 is able to interact with both Bcl-2 and Bcl-xL. Independently to which is the relevant physiological partner, our study shows that modulation of Praf2 expression could affect cellular viability. We found that overexpression of Praf2 induces translocation and aggregation of Bax at the mitochondria and, ultimately, increases cellular caspase activity and cell death. On the same track, reducing Praf2 expression by RNAi in the U2OS osteosarcoma cell line, we observed an increased resistance to cytotoxic effect of the chemotherapeutic drug etoposide. What could be the mechanism explaining Praf2’s proapoptotic activity? And how to reconcile these data with the observation that the expression of Praf2 in human tumor tissues is higher than in normal tissues of the same patient? Another group of ER based proteins found to interact with Bcl-xL and/or Bcl-2 and to modulate cell death is composed of members of the Reticulon family of proteins: RTN1-C, RTN4A and RTN3. Interestingly the yeast homologue of Praf2, Yip3p, has been found interacting with the reticulon Rtn1p, and our preliminary results suggest that Praf2 is also able to interact with RTN4. The mechanism by which RTN proteins modulate sensitivity to apoptosis is also not entirely clear but is supposed to be based on the ability of RTN proteins to influence Bcl-xL/Bcl-2 subcellular localisation. RTN4 has been suggested to reduce the antiapoptotic activity of Bcl-xL and Bcl-2 by forcing their localisation onto the ER and reducing their presence on the mitochondria. RTN3, instead, was shown to be able to increase the accumulation of Bcl-2 on mitochondria.