Calcium flux through ER-TGN contact sites facilitates cargo export

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 a 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 elusive, as the cytosol has a very low steady-state Ca2+ concentration. The TGN forms membrane contact sites (MCS) with the Endoplasmic Reticulum (ER), whereby the close apposition of the two organelles allows protein-mediated exchange of molecular species such as lipids. Here we show that TGN export of Cab45 clients requires the integrity of ER-TGN MCS and IP3R-dependent Ca2+ fluxes in the MCS, suggesting Ca2+ transfer between these organelles. Using a MCS targeted Ca2+ FRET sensor module, we measure the Ca2+ flow in these sites in real-time. These data show for the first time that ER-TGN MCS facilitate Ca2+ transfer required for SPCA1-dependent cargo sorting and export from the TGN and thus solves a fundamental question in cell biology. Summary The current study demonstrates the trafficking of Cab45 clients relies on Ca2+ flux between the endoplasmic reticulum (ER) and trans-Golgi Network (TGN). This process requires the activity of IP3 receptors, present in ER membranes, and depends on the integrity of the membrane contact site between these two organelles.