ATG6/BECLIN 1 COUPLES THE REPLICATION STRESS RESPONSE TO AMINO ACID METABOLISM

In the budding yeast Saccharomices Cerevisiae, Atg6 is a non-catalytic component of the phosphatidylinositol (PtdIns) kinase complex Vps34-Vps15-Atg6, which phosphorylates PtdIns to produce phosphatidylinositol 3-phosphate (PtdIns(3)P). Atg6 is conserved among species, including its mammalian ortologue, Beclin 1, which is partially inactivated in breast and ovarian cancers. Beclin 1 involvement in human cancer is not fully established, but it could act as a tumor suppressor by regulating autophagy. Genome instability is a hallmark of cancer cells. The replication stress checkpoint has evolved to prevent the occurrence of genome instability. Once activated, the checkpoint orchestrates series of protective responses, including the production of deoxyribonucleotides (dNTPs) and DNA repair. In this work we show that low levels of intracellular PtdIns(3)P, as caused by deletion of ATG6, protect cells during replication stress conditions. Moreover, sensitivity to replication stress is enhanced in the presence of amino acid imbalances in the extracellular medium. This effect depends on PtdIns(3)P involvement in endosomal vesicle trafficking, but is independent from both autophagy and the canonical replication stress checkpoint mediated by Rad53. PtdIns(3)P levels, as determined by the activity of the Vps34-Vps15-Atg6 on endosomal membranes, likely affect the internalization of amino acids that are crucial to survive during replication stress. Elucidating the molecular mechanisms that link the metabolism of specific amino acids with the response to replication stress will have profound impact in understanding the role of Beclin 1 in human cancer and also in cancer therapy.