IKs Channels Regulation by Phosphatidylinositol 4,5-bisphosphate (PIP2) and Adenosine 5'-triphosphate (ATP)

Ion channels are integral proteins found in the membranes of every cell in our body that underlie electrical impulses in a variety of tissues and organs, including the heart and the brain. The IKs channel, formed by the KCNQ1 and KCNE1 subunits, plays an important role in shortening the action potential duration in cardiac myocytes. The IKs channel opens in response to depolarization to conduct potassium ions out of the cell, which contributes to repolarization of the membrane, terminating the cardiac action potential and thereby the heart contraction. Abnormal IKs channels can lead to lethal arrhythmias. Type 1 and type 5 Long QT Syndrome are caused by mutations of KCNQ1 and KCNE1, respectively, and both compromise IKs function significantly. Phosphatidylinositol 4,5-bisphosphate (PIP2) is a minor acidic membrane lipid found primarily in the inner leaflet of the plasma membrane. PIP2 has been shown to be a necessary cofactor for a wide variety of ion channels. Adenosine-5'-triphosphate (ATP) also serves as both an energy source and a signaling molecule that modulates ion channel and transporter functions. IKs has been shown to require PIP2 and ATP for channel activity [1]. Since PIP2 and ATP regulation of IKs was discovered, no further study of this regulation has been reported. The goal of my study is to determine the nature of PIP2 and ATP modulation of IKs channel. The first study shows that the auxiliary subunit of IKs, KCNE1, increases PIP2 sensitivity 100-fold over channels formed by the pore-forming KCNQ1 subunits alone, which effectively amplifies current because native PIP2 levels in the membrane are insufficient to activate all KCNQ1 channels. A juxtamembranous site in the KCNE1 C-terminus is a key structural determinant of PIP2 sensitivity. The second study shows that intracellular ATP modulates IKs in cardiac myocytes by directly binding to the C-terminus of KCNQ1 to allow the pore to open. The channel is most sensitive to ATP near its physiological concentration, and lowering [ATP] in cardiac myocytes results in IKs reduction and action potential prolongation. Multiple mutations that suppress IKs by decreasing the ATP sensitivity of the channel are associated with the long QT syndrome that predisposes afflicted individuals to cardiac arrhythmia. These results demonstrate for the first time the activation of an ion channel by intracellular ATP binding; and ATP dependent gating allows IKs to couple myocyte energy state to its electrophysiology.