ALG-2 And Peflin Stimulate or Inhibit COPII Targeting And Secretion In Response To Calcium Signaling

Abstract

ER-to-Golgi transport is the first step in the constitutive secretory pathway which, unlike regulated secretion, is believed to proceed non-stop regardless of Ca2+ flux. Rowever, here we demonstrate that penta-EF hand (PEF) proteins ALG-2 and peflin constitute a hetero-bifunctional COPII regulator that responds to Ca2+ signaling by adjusting the ER export rate of COPII-sorted cargos up or down by ~50%. At steady-state Ca2+, ALG-2/peflin hetero-complexes bind to ER exit sites (ERES) through the ALG-2 subunit to confer a low, buffered secretion rate, while peflin-lacking ALG-2 complexes markedly stimulate secretion. During Ca2+ signaling, ALG-2 complexes lacking peflin can either increase or decrease the secretion rate depending on signaling intensity and duration-phenomena that could contribute to cellular growth and intercellular communication, following secretory increases, or protection from excitotoxicity and infection following decreases. In epithelial normal rat kidney (NRK) cells, the Ca2+-mobilizing agonist ATP causes ALG-2 to depress ER export, while in neuroendocrine PC12 cells, Ca2+ mobilization by ATP results in ALG-2-dependent enhancement of secretion. Within the NRK cell model, distinct Ca2+ signaling patterns can produce opposing ALG-2-dependent effects on secretion. Mechanistically, ALG-2-dependent depression of secretion involves decreased COPTT outer shell and increased peflin targeting to ERES, while ALG-2-dependent enhancement of secretion involves increased COPTT outer shell and decreased peflin at ERES. PEF protein complexes represent a true regulator of transport as they are dispensable for secretion yet adjust the secretion rate to physiological conditions. Their dynamics affects secretion of important physiological cargoes such as collagen T and significantly impacts ER stress.