Palmitoylation of KChIP3 controls baseline mucin secretion

Abstract

Baseline mucin secretion (BMS) is independent of external agonists and controlled by a small calcium binding protein named KChIP3. KChIP3-hosting mucin granules are not released until intracellular cytosolic calcium oscillations reach a threshold, KChIP3 binds calcium and detaches from granules, allowing their fusion to plasma membrane. Loss of KChIP3 or blocking its membrane attachment causes mucin hypersecretion. How is KChIP3 recruited to mucin granules? We show here that zDHHC (aspartate-histidine-histidine-cysteine motif in a cysteine-rich, zinc finger-like domain) S-acyl-transferase dependent palmitoylation modulates binding of KChIP3 to mucin granules thereby affecting mucin secretion. We have found that inhibiting zDHHC-mediated palmitoylation in differentiated HT29-18N2, which express the Golgi-localized zDHHC3 and zDHHC4, releases KChIP3 from mucin granules and increases baseline mucin secretion. Mutation of the palmitoylation sites in KChIP3 (Cysteines 122 and 123 to Alanine) quantitatively reduces its attachment to mucin granules. Expression of KChIP3-WT in HT29-18N2 cell lines stably depleted of KChIP3 inhibits mucin secretion, whereas expression of non-palmitoylated KChIP3 (KChIP3-AA) only partially rescues the effect of KChIP3 depletion and the cells maintain higher levels of baseline secretion compared to KChIP3-WT cells. Altogether, our data suggest that zDHHC3 or zDHHC4-dependent palmitoylation is involved in KChIP3 recruitment to mucin granules to control the baseline mucin secretion.