A new regulation mechanism for KCNN4, the Ca2+-dependent K+ channel, by molecular interactions with the Ca2+pump PMCA4b

KCNN4, a Ca²⁺-activated K⁺ channel, is involved in various physiological and pathological processes. It is essential for epithelial transport, immune system, and other physiological mechanisms, but its activation is also involved in cancer pathophysiology as well as red blood cell (RBC) disorders. The activation of KCNN4 in RBC leads to loss of KCl and water, a mechanism known as the "Gardos effect" described 70 years ago. This Ca²⁺-induced dehydration is irreversible in human RBC and must be tightly controlled to prevent not only hemolysis but also alterations in RBC rheological properties. In this study, we have investigated the regulation of KCNN4 activity after changes in RBC Ca²⁺ concentration. Using electrophysiology, immunoprecipitation, and proximity ligation assay in human embryonic kidney 293-transfected cells, K562 cells, or RBCs, we have found that KCNN4 and the Ca²⁺ pump PMCA4b (plasma membrane calcium-transporting ATPase 4b) interact tightly with each other, such that the C-terminal domain of PMCA4b regulates KCNN4 activity, independently of the Ca²⁺ extrusion activity of the pump. This regulation was not restricted to KCNN4: the small-conductance Ca²⁺-activated K⁺ channel KCNN2 was similarly regulated by the calcium pump. We propose a new mechanism that could control KCNN4 activity by a molecular inhibitory interaction with PMCA4b. It is suggested that this mechanism could attenuate erythrocyte dehydration in response to an increase in intracellular Ca²⁺.