Calcium flow at ER-TGN contact sites facilitates secretory cargo export

Abstract

Ca²⁺ influx into the trans-Golgi Network (TGN) promotes secretory cargo sorting by the Ca²⁺-ATPase SPCA1 and the luminal Ca²⁺ binding protein Cab45. Cab45 oligomerizes upon local Ca²⁺ influx, and Cab45 oligomers sequester and separate soluble secretory cargo from the bulk flow of proteins in the TGN. However, how this Ca²⁺ flux into the lumen of the TGN is achieved remains mysterious, as the cytosol has a nanomolar steady-state Ca²⁺ concentration. The TGN forms membrane contact sites (MCS) with the Endoplasmic Reticulum (ER), allowing protein-mediated exchange of molecular species such as lipids. Here, we show that the TGN export of secretory proteins requires the integrity of ER-TGN MCS and inositol 3 phosphate receptor (IP3R)-dependent Ca²⁺ fluxes in the MCS, suggesting Ca²⁺ transfer between these organelles. Using an MCS-targeted Ca²⁺ FRET sensor module, we measure the Ca²⁺ flow in these sites in real time. These data show that ER-TGN MCS facilitates the Ca²⁺ transfer required for Ca²⁺-dependent cargo sorting and export from the TGN, thus solving a fundamental question in cell biology.