Endomembrane Cation Chloride Cotransporters (CCC1s) modulate endo- and exocytosis

Abstract The secretary and endocytic pathways intersect at the trans -Golgi network/Early Endosome (TGN/EE). TGN/EE function depends on a careful balance of ions within this compartment, and the electrochemical potential across its membrane. The identity of the proton pump required for acidification and the transporters that catalyse cation and anion import into endosome compartments are known. However, the protein needed to complete the transport circuit, and that mediates cation and anion efflux from the TGN/EE has not been identified. Here, we characterize Cation Chloride Cotransporters (CCC1s) from Arabidopsis and rice. We find that the AtCCC1 is localized to the TGN/EE, where it modulates important TGN functions such as endocytosis, exocytosis, and cell wall synthesis/secretion. Loss of CCC1 results in severe, widespread phenotypes in both rice and Arabidopsis, including plant growth and developmental perturbations, defects in root hair elongation and altered osmoregulation, consistent with CCC performing a core cellular function. Complementation of the root hair elongation phenotype of Atccc1 with root hair specific expression of GFP-AtCCC1 demonstrates CCC1 functions within the TGN/EE. Collectively, our results imply that CCC1 is a strong candidate for the missing component of the TGN/EE ion transportcircuit. One sentence summary Cation Chloride Cotransporters function in the TGN/EE and modulate endo- and exocystosis, impacting root cell identity, root hair elongation, osmoregulation and cell wall formation.