A pan-vertebrate signaling motif controls the molecular function of intracellular AQP12

Unorthodox AQP12-type channels are poorly understood intracellular aquaporins localized in the endoplasmic reticulum and zymogen granules (ZGs) of pancreatic acinar cells. Despite connections to major diseases, their biophysical properties and intracellular trafficking regulation remain largely unknown. Here, we show that heterologously expressed plant and metazoan AQP12-related channels specifically localize to the intracellular yolk platelet (YP) membrane of frog oocytes, a feature that is recapitulated in vivo for invertebrate and vertebrate orthologs. Using native YP membranes, we show that the vertebrate channels are mercury-sensitive polytransporters with intracellular trafficking regulated by Ca2+ and cAMP signaling pathways. We identify a novel pan-vertebrate C-terminal YP-targeting domain (YPD) in AQP12, which also drives orthodox aquaporin chimeras and truncated channels to YPs and ZGs. In cultured pancreatic cells, the YPD and Ca2+-induced AQP12 N-terminal phosphorylation coregulate secretagogue-triggered channel transport to the ZGs for enzyme secretion. These findings uncover conserved signaling mechanisms for AQP12 trafficking to intracellular protein storage vesicles, and open unexpected avenues for targeted delivery systems.