Sac1 is found in ER–PM contact sites in superior cervical ganglia and hippocampal neurons. (A, left) Confocal image of a living SCG neuron expressing GFP-Sac1. (right) TIRF footprint of a SCG neuron, fixed and stained for anti-Sac1. (B) Confocal images of a single, living SCG neuron overexpressing GFP-Sac1 (left) and RFP-LDR (middle), and the resulting FRET image (right). (C) Summary (mean ± SEM, n = 5; t test; *, P < 0.05) of the KCNQ2/3 current densities from control neurons and SCG neurons overexpressing GFP-Sac1. (D, left) Representative inverted TIRF footprint from an SCG neuron overexpressing GFP-Sac1. (middle) Representative normalized time course of GFP-Sac1 from the footprint of a SCG neuron during the application of 10 µM Oxo-M (purple bar). (right) Same SCG neuron. Normalized time course of GFP-Sac1 from the distal end of an SCG neurite after addition of 10 µM Oxo-M. (E, left) Inverted, maximum-intensity projection of a hippocampal neuron. Inset is 2.5× magnification. (middle) TIRF footprint from a single, living hippocampal neuron expressing GFP-Sac1. (right) Normalized, mean time courses of GFP-Sac1 within the TIRF footprint of hippocampal neuron soma (n = 9). (F) Model illustrating the role of Sac1 in regulating “steady-state” PM phosphoinositide homeostasis and its change in distribution after hydrolysis of PM PI(4,5)P₂. 1 and 2 represent Sac1 dephosphorylating PM PI(4)P in trans and cis, respectively.