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Cholesterol Sensitivity and Lipid Raft Targeting of Kir2.1 Channels

Victor G. Romanenko ^*, Yun Fang ^*, Fitzroy Byfield ^*, Alexander J. Travis , Carol A. Vandenberg , George H. Rothblat ^¶ and Irena Levitan ^*

^* Institute for Medicine and Engineering, Department of Pathology and Laboratory Medicine, University of Pennsylvania, Philadelphia, Pennsylvania; Baker Institute for Animal Health, College of Veterinary Medicine, Cornell University, Ithaca, New York; Department of Molecular, Cellular, and Developmental Biology, and Neuroscience Research Institute, University of California, Santa Barbara, California; and ^¶ Division of Gastroenterology and Nutrition, Department of Pediatrics, The Children's Hospital of Philadelphia, Philadelphia, Pennsylvania

Correspondence: Address reprint requests to Dr. Irena Levitan, University of Pennsylvania, IME 1160 Vagelos Research Laboratories, Philadelphia, PA 19104. Tel.: 215-573-8161; Fax: 215-573-7227; E-mail: ilevitan{at}mail.med.upenn.edu.

This study investigates how changes in the level of cellular cholesterol affect inwardly rectifying K⁺ channels belonging to a family of strong rectifiers (Kir2). In an earlier study we showed that an increase in cellular cholesterol suppresses endogenous K⁺ current in vascular endothelial cells, presumably due to effects on underlying Kir2.1 channels. Here we show that, indeed, cholesterol increase strongly suppressed whole-cell Kir2.1 current when the channels were expressed in a null cell line. However, cholesterol level had no effect on the unitary conductance and only little effect on the open probability of the channels. Moreover, no cholesterol effect was observed either on the total level of Kir2.1 protein or on its surface expression. We suggest, therefore, that cholesterol modulates not the total number of Kir2.1 channels in the plasma membrane but rather the transition of the channels between active and silent states. Comparing the effects of cholesterol on members of the Kir2.x family shows that Kir2.1 and Kir2.2 have similar high sensitivity to cholesterol, Kir2.3 is much less sensitive, and Kir2.4 has an intermediate sensitivity. Finally, we show that Kir2.x channels partition virtually exclusively into Triton-insoluble membrane fractions indicating that the channels are targeted into cholesterol-rich lipid rafts.

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