Ca²⁺ binding to Esyt modulates membrane contact site density in Drosophila photoreceptors

Membrane contact sites (MCS) between the plasma membrane (PM) and endoplasmic reticulum (ER) regulate Ca²⁺ influx. However, the mechanisms by which cells modulate ER–PM MCS density are not understood, and the role of Ca²⁺, if any, in regulating these is unknown. We report that in Drosophila photoreceptors, MCS density is regulated by the Ca²⁺ channels, TRP and TRPL. Regulation of MCS density by Ca²⁺ is mediated by Drosophila extended synaptotagmin (dEsyt), a protein localized to ER–PM MCS and previously shown to regulate MCS density. We find that the Ca²⁺-binding activity of dEsyt is required for its function in vivo. dEsyt^CaBM, a Ca²⁺ non-binding mutant of dEsyt is unable to modulate MCS structure. Further, reconstitution of dEsyt null photoreceptors with dEsyt^CaBM is unable to rescue ER–PM MCS density and other key phenotypes. Thus, our data supports a role for Ca²⁺ binding to dEsyt in regulating ER–PM MCS density in photoreceptors thus tuning signal transduction during light-activated Ca²⁺ influx.