Specific serine proteases selectively damage KCNH2 (hERG1) potassium channels and IKr

Departments of 1 Medicine (Cardiology) and 2 Physiology, University of Wisconsin, Madison, Wisconsin Submitted 22 July 2005 ; accepted in final form 8 October 2005 KCNH2 ( hERG1 ) encodes the -subunit proteins for the rapidly activating delayed rectifier K + current ( I Kr ), a major K + current for cardiac myocyte repolarization. In isolated myocytes I Kr frequently is small in amplitude or absent, yet KCNH2 channels and I Kr are targets for drug block or mutations to cause long QT syndrome. We hypothesized that KCNH2 channels and I Kr are uniquely sensitive to enzymatic damage. To test this hypothesis, we studied heterologously expressed K + , Na + , and L-type Ca 2+ channels, and in ventricular myoctyes I Kr , slowly activating delayed rectifier K + current ( I Ks ), and inward rectifier K + current ( I K1 ), by using electrophysiological and biochemical methods. 1 ) Specific exogenous serine proteases (protease XIV, XXIV, or proteinase K) selectively degraded KCNH2 current ( I KCNH2 ) and its mature channel protein without damaging cell integrity and with minimal effects on the other channel currents; 2 ) immature KCNH2 channel protein remained intact; 3 ) smaller molecular mass KCNH2 degradation products appeared; 4 ) protease XXIV selectively abolished I Kr ; and 5 ) reculturing HEK-293 cells after protease exposure resulted in the gradual recovery of I KCNH2 and its mature channel protein over several hours. Thus the channel protein for I KCNH2 and I Kr is uniquely sensitive to proteolysis. Analysis of the degradation products suggests selective proteolysis within the S5-pore extracellular linker, which is structurally unique among Kv channels. These data provide 1 ) a new mechanism to account for low I Kr density in some isolated myocytes, 2 ) evidence that most complexly glycosylated KCNH2 channel protein is in the plasma membrane, and 3 ) new insight into the rate of biogenesis of KCNH2 channel protein within cells. rapidly activating delayed rectifier potassium current; enzymes; myocyte isolation Address for reprint requests and other correspondence: C. T. January, Section of Cardiology, Rm. H6/354, Univ. of Wisconsin Hospital, 600 Highland Ave., Madison, WI 53792 (e-mail: ctj{at}medicine.wisc.edu )