An INF2-dependent actin-mediated step in inositol 1,4,5-trisphosphate receptor cluster formation and activity.

Intracellular calcium signaling plays a vital role in regulating various cellular processes including gene regulation, motility, metabolism, and cell death. Inositol 1,4,5-trisphosphate receptors (IP3Rs) on the endoplasmic reticulum (ER) are a major cation channel that regulates stimulus-induced calcium release from the ER. While several molecular players regulate the activity of IP3R, its regulation by actin filaments was uncharacterized. Here, we show that actin filaments polymerized by the specific actin nucleator INF2 facilitate agonist-induced IP3R activity. Our results demonstrate that INF2-mediated actin filaments regulate the formation and/or stability of IP3R clusters on the ER that have been previously shown to be hotspots of ER calcium release. Using cell-biological and biochemical techniques, we further show that INF2 physically interacts with IP3R isoforms, often at IP3R clusters. While INF2-IP3R interaction is independent of INF2 activity, the ability of INF2 to mediate IP3R clusters is dependent on its actin polymerization activity. Finally, we demonstrate that in addition to its calcium mobilization activity, INF2 on the ER specifically regulates IP3R cluster positioning to mediate ER-mitochondrial contacts and facilitate ER-to-mitochondrial calcium transfer. Overall, these results reveal an actin-dependent step in the regulation of IP3R activity both in terms of ER calcium release and modulation of ER-mitochondrial contacts.