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

Ca influx into the -Golgi Network (TGN) promotes secretory cargo sorting by the Ca-ATPase SPCA1 and the luminal Ca binding protein Cab45. Cab45 oligomerizes upon local Ca influx, and Cab45 oligomers sequester and separate soluble secretory cargo from the bulk flow of proteins in the TGN. However, how this Ca flux into the lumen of the TGN is achieved remains mysterious, as the cytosol has a nanomolar steady-state Ca 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 Ca fluxes in the MCS, suggesting Ca transfer between these organelles. Using an MCS-targeted Ca FRET sensor module, we measure the Ca flow in these sites in real time. These data show that ER-TGN MCS facilitates the Ca transfer required for Ca-dependent cargo sorting and export from the TGN, thus solving a fundamental question in cell biology.