2 The abbreviations used are: MUC2, mucin-2; CF, cystic fibrosis; MUC5AC, mucin-5AC; NCX, sodium-calcium exchanger; NHBE, normal human bronchial epithelial cells; PMA, phorbol 12-myristate 13-acetate; TRPM4, transient receptor potential cation channel subfamily M member 4; TRPM5, transient receptor potential cation channel subfamily M member 5; MARCKS, myristoylated alanine-rich protein kinase C substrate; PM, plasma membrane; SOCE, store-operated calcium entry; TG, thapsigargin; ALI, air-liquid interface; IL-13, interleukin-13; ER, endoplasmic reticulum; DAPI, 4Ј,6-diamidino-2-phenylindole; AUC, area under the curve; PKC, protein kinase C, SNARE, soluble NSF attachment protein receptor(s); IP 3 , inositol 1,4,5-trisphosphate; shRNA, short hairpin RNA; qPCR, quantitative PCR.

Regulated mucin secretion from specialized goblet cells by exogenous agonist-dependent (stimulated) and -independent (baseline) manner is essential for the function of the epithelial lining. Over extended periods, baseline release of mucin can exceed quantities released by stimulated secretion, yet its regulation remains poorly characterized. We have discovered that ryanodine receptor-dependent intracellular Ca2+ oscillations effect the dissociation of the Ca2+-binding protein, KChIP3, encoded by KCNIP3 gene, from mature mucin-filled secretory granules, allowing for their exocytosis. Increased Ca2+ oscillations, or depleting KChIP3, lead to mucin hypersecretion in a human differentiated colonic cell line, an effect reproduced in the colon of Kcnip3-/- mice. Conversely, overexpressing KChIP3 or abrogating its Ca2+-sensing ability, increases KChIP3 association with granules, and inhibits baseline secretion. KChIP3 therefore emerges as the high-affinity Ca2+ sensor that negatively regulates baseline mucin secretion. We suggest KChIP3 marks mature, primed mucin granules, and functions as a Ca2+ oscillation-dependent brake to control baseline secretion.Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

In this study, Bassaganyas et al. develop a powerful pooled genome-wide CRISPRi screen and uncover new factors for protein transport and secretion. This work opens new and stimulating perspectives for a deeper understanding of secretory pathway processes in health and disease.