CIP1 is an activator of the K+–Cl− cotransporter KCC2

In most neurons, efficient setting of the intracellular Cl−-concentration requires the coordinated regulation of the Cl−-inward transporter NKCC1 and the Cl−-outward transporter KCC2. Previously, the cation–chloride cotransporter interacting protein 1 (CIP1) was shown to inactivate NKCC1. Here, we investigated its role for KCC2 activity. After co-expression of CIP1 and KCC2 in HEK-293 cells, a physical and functional interaction was observed. CIP1 co-purified with KCC2 in pull-down experiments and significantly increased KCC2 transport activity, as determined by 86Rb+ flux measurements. RT-PCR analysis demonstrated a ubiquitous expression during postnatal development of the rat. Real-time PCR revealed a two-fold down-regulation of CIP1 during maturation of the rat brain. Taken together, our data identify CIP1 as a potent activator of KCC2. Furthermore, the results support previous data of heteromer formation among members of the cation–chloride cotransporter gene family.