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J. Biol. Chem., Vol. 279, Issue 32, 33228-33236, August 6, 2004

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Natural Phosphatidylcholine Is Actively Translocated across the Plasma Membrane to the Surface of Mammalian Cells^*

Nanette Kälin, José Fernandes, Sigrún Hrafnsdóttir, and Gerrit van Meer

From the Department of Membrane Enzymology, CBLE, Institute of Biomembranes, Padualaan 8, 3584 CH Utrecht, The Netherlands

The cell surface of eukaryotic cells is enriched in choline phospholipids, whereas the aminophospholipids are concentrated at the cytosolic side of the plasma membrane by the activity of one or more P-type ATPases. Lipid translocation has been investigated mostly by using short chain lipid analogs because assays for endogenous lipids are inherently complicated. In the present paper, we optimized two independent assays for the translocation of natural phosphatidylcholine (PC) to the cell surface based on the hydrolysis of outer leaflet phosphoglycerolipids by exogenous phospholipase A₂ and the exchange of outer leaflet PC by a transfer protein. We report that PC reached the cell surface in the absence of vesicular traffic by a pathway that involved translocation across the plasma membrane. In erythrocytes, PC that was labeled at the inside of the plasma membrane was translocated to the cell surface with a half-time of 30 min. This translocation was probably mediated by an ATPase, because it required ATP and was vanadate-sensitive. The inhibition of PC translocation by glibenclamide, an inhibitor of various ATP binding cassette transporters, and its reduction in erythrocytes from both Abcb1a/1b and Abcb4 knockout mice, suggest the involvement of ATP binding cassette transporters in natural PC cell surface translocation. The relative importance of the outward translocation of PC as compared with the well characterized fast inward translocation of phosphatidylserine for the overall asymmetric phospholipid organization in plasma membranes remains to be established.

Received for publication, February 17, 2004 , and in revised form, May 7, 2004.

^* This work was supported by Swiss National Science Foundation Grant 83EU-054737, French CF Foundation "Vaincre la Mucoviscidose" Grant FC0115, and Grant 902-23-197 from ZonMW with financial support from the Dutch Organization for Scientific Research. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed. Tel.: 31-30-253-3966; Fax: 31-30-2522478; E-mail: nanette.kaelin{at}dlr.de.

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