The inositol 1, 4, 5-trisphosphate receptor (IP 3 R) is a major regulator of apoptotic signaling. Through interactions with members of the Bcl-2 family of proteins, it drives calcium (Ca 2+) transients from the endoplasmic reticulum (ER) to mitochondria, thereby establishing a functional and physical link between these organelles. Importantly, the IP 3 R also regulates autophagy, and in particular, its inhibition/depletion strongly induces macroautophagy. Here, we show that the IP 3 R antagonist xestospongin B induces autophagy by disrupting a molecular complex formed by the IP 3 R and Beclin 1, an interaction that is increased or inhibited by overexpression or knockdown of Bcl-2, respectively. An effect of Beclin 1 on Ca 2+ homeostasis was discarded as siRNA-mediated knockdown of Beclin 1 did not affect cytosolic or luminal ER Ca 2+ levels. Xestospongin B-or starvation-induced autophagy was inhibited by overexpression of the IP 3 R ligand-binding domain, which coimmunoprecipitated with Beclin 1. These results identify IP 3 R as a new regulator of the Beclin 1 complex that may bridge signals converging on the ER and initial phagophore formation.