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MBC in Press, published online ahead of print December 15, 2004
Mol. Biol. Cell 10.1091/mbc.E04-03-0224

A more recent version of this article appeared on February 1, 2005
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Submitted on March 16, 2004
Revised on November 29, 2004
Accepted on November 30, 2004

Triacylglycerol Hydrolase Is Localized to the ER by an Unusual Retrieval Sequence where It Participates in VLDL Assembly without Utilizing VLDL Lipids as Substrates

Dean Gilham,*{dagger} Mustafa Alam,{dagger}{ddagger}{sect} Wenhui Gao,{dagger}{sect} Dennis E. Vance,{dagger}{ddagger} and Richard Lehner*{dagger}{sect}||

Departments of *Cell Biology, {ddagger}Biochemistry, and {sect}Pediatrics, and the {dagger}CIHR Group on the Molecular and Cell Biology of Lipids, University of Alberta, Edmonton, Alberta T6G 2E1, Canada

Monitoring Editor: Jennifer Lippincott-Schwartz

The majority of hepatic intracellular triacylglycerol (TG) is mobilized by lipolysis followed by reesterification to reassemble TG before incorporation into a VLDL particle. Triacylglycerol hydrolase (TGH) is a lipase that hydrolyzes TG within hepatocytes. Immunogold electron microscopy in transfected cells revealed a disparate distribution of this enzyme within the endoplasmic reticulum (ER), with particularly intense localization in regions surrounding mitochondria. TGH is localized to the lumen of the ER by the C-terminal tetrapeptide sequence HIEL functioning as an ER retention signal. Deletion of HIEL resulted in secretion of catalytically active TGH. Mutation of HIEL to KDEL, which is the consensus ER retrieval sequence in animal cells, also resulted in ER retention and conservation of lipolytic activity. However, KDEL-TGH was not as efficient at mobilizing lipids for VLDL secretion and exhibited an altered distribution within the ER. TGH is a glycoprotein, but glycosylation is not required for catalytic activity. TGH does not hydrolyze apolipoprotein B-associated lipids. This suggests a mechanism for vectored movement of TGs onto developing VLDL in the ER as TGH may mobilize TG for VLDL assembly, but will not access this lipid once it is associated with VLDL.

||Corresponding author. E-mail: richard.lehner{at}ualberta.ca




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