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Journal of Lipid Research, Vol. 44, 1605-1613, September 2003
Copyright © 2003 by American Society for Biochemistry and Molecular Biology

Localization of the PE methylation pathway and SR-BI to the canalicular membrane

Ephraim Sehayek^1,*, Rong Wang, Jennie G. Ono^*, Vadim S. Zinchuk, Elizabeth M. Duncan^*, Sarah Shefer^**, Dennis E. Vance, Meenakshisundaram Ananthanarayanan, Brian T. Chait and Jan L. Breslow^*

^* Laboratory of Biochemical Genetics and Metabolism, The Rockefeller University, 1230 York Avenue, New York, NY 10021
Laboratory of Mass Spectrometry and Gaseous Ion Chemistry, The Rockefeller University, 1230 York Avenue, New York, NY 10021
Department of Anatomy and Cell Biology, Kochi Medical School, Kochi, Japan
^** Department of Medicine and Liver Center, University of Medicine and Dentistry of New Jersey, Newark, NJ 07103
CIHR Group on Molecular and Cell Biology of Lipids and Department of Biochemistry, University of Alberta, Edmonton, Alberta, T6G 2S2 Canada
Department of Pediatrics, Laboratory of Developmental and Molecular Hepatology, The Mount Sinai Medical Center, New York, NY 10029

¹ To whom correspondence should be addressed. e-mail: sehayee{at}rockefeller.edu

To better understand the regulation of biliary phospholipid and cholesterol excretion, canalicular membranes were isolated from the livers of C57BL/6J mice and abundant proteins separated by SDS-PAGE and identified by matrix-assisted laser desorption/ionization mass spectrometry. A prominent protein revealed by this analysis was betaine homocysteine methyltransferase (BHMT). This enzyme catalyzes the first step in a three-enzyme pathway that promotes the methylation of phosphatidylethanolamine (PE) to phosphatidylcholine (PC). Immunoblotting confirmed the presence of BHMT on the canalicular membrane, failed to reveal the presence of the second enzyme in this pathway, methionine adenosyltransferase, and localized the third enzyme of the pathway, PE N-methyltransferase (PEMT). Furthermore, immunfluorescence microscopy unambiguously confirmed the localization of PEMT to the canalicular membrane. These findings indicate that a local mechanism exists in or around hepatocyte canalicular membranes to promote phosphatidylethnolamine methylation and PC biosynthesis. Finally, immunoblotting revealed the presence and immunofluorescence microscopy unambiguously localized the scavenger receptor class B type I (SR-BI) to the canalicular membrane. Therefore, SR-BI, which is known to play a role in cholesterol uptake at the hepatocyte basolateral membrane, may also be involved in biliary cholesterol excretion.

Based on these findings, a model is proposed in which local canalicular membrane PC biosynthesis in concert with the phospholipid transporter mdr2 and SR-BI, promotes the excretion of phospholipid and cholesterol into the bile.

Abbreviations: BHMT, betaine homocysteine methyltransferase; MAT, methionine adenosyltransferase; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PEMT, phosphatidylethanolamine N-methyl transferase; SR-BI, scavenger receptor class B type I

Supplementary key words phosphatidylethanolamine • phosphatidylcholine • methionine adenosyltransferase • scavenger receptor class B type I

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