Steady-state Regulation of Secretory Cargo Export by Inositol Trisphosphate Receptors and Penta EF Hand Proteins

ER Ca regulates ER-to-Golgi transport machinery. Sustained Ca signaling by inositol trisphosphate receptors (IP3Rs) leads to depression of cargo export through activation of penta EF hand protein (PEF) ALG-2 which reduces outer COPII coat at ER exit sites (ERES). However, we do not know whether tonic Ca signals during steady-state conditions affect ER export rates and if so by what mechanisms. Here we report that partial depletion of IP3 receptors from NRK epithelial cells causes a marked increase of basal ER export of the transmembrane glycoprotein cargo VSV-G. The increased ER-to-Golgi transport required ALG-2 and was actuated by decreased peflin and increased ALG-2 at ER exit sites (ERES) – a condition previously demonstrated to stimulate COPII-dependent ER export. Upon IP3R depletion the amount of outer coat at ERES increased, the opposite to what occurs during ALG-2-dependent inhibition of secretion during agonist-driven Ca signaling. The increased ER export correlated with reduced spontaneous cytosolic Ca oscillations caused by the reduced number of Ca release channels. IP3R depletion also unexpectedly resulted in partial depletion of ER luminal Ca stores. The low Ca conditions appeared to decrease both ALG-2 and peflin expression levels somewhat, but these were the only detectable expression changes in COPII trafficking machinery and the Ca decrease had no detectable impact on ER stress. We conclude that at steady state, IP3Rs produce tonic Ca signals that suppress the basal rate of ER export by maintaining lower outer coat targeting to ERES.