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Vol. 20, Issue 2, 658-672, January 15, 2009

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Role of an Ancestral D-Bifunctional Protein Containing Two Sterol-Carrier Protein-2 Domains in Lipid Uptake and Trafficking in Toxoplasma

Bao Lige^*, Bamini Jayabalasingham^*, Hui Zhang^*, Marc Pypaert, and Isabelle Coppens^*

*Department of Molecular Microbiology and Immunology, Johns Hopkins University School of Public Health, Baltimore, MD 21205; and Department of Cell Biology, Yale University School of Medicine, New Haven, CT 06520

Submitted May 14, 2008; Revised November 3, 2008; Accepted November 4, 2008
Monitoring Editor: Howard Riezman

The inability to synthesize cholesterol is universal among protozoa. The intracellular pathogen Toxoplasma depends on host lipoprotein-derived cholesterol to replicate in mammalian cells. Mechanisms of cholesterol trafficking in this parasite must be important for delivery to proper organelles. We characterized a unique D-bifunctional protein variant expressed by Toxoplasma consisting of one N-terminal D-3-hydroxyacyl-CoA dehydrogenase domain fused to two tandem sterol carrier protein-2 (SCP-2) domains. This multidomain protein undergoes multiple cleavage steps to release free SCP-2. The most C-terminal SCP-2 carries a PTS1 that directs the protein to vesicles before processing. Abrogation of this signal results in SCP-2 accumulation in the cytoplasm. Cholesterol specifically binds to parasite SCP-2 but with 10-fold lower affinity than phosphatidylcholine. In mammalian cells and Toxoplasma, the two parasite SCP-2 domains promote the circulation of various lipids between organelles and to the surface. Compared with wild-type parasites, TgHAD-2SCP-2–transfected parasites replicate faster and show enhanced uptake of cholesterol and oleate, which are incorporated into neutral lipids that accumulate at the basal end of Toxoplasma. This work provides the first evidence that the lipid transfer capability of an ancestral eukaryotic SCP-2 domain can influence the lipid metabolism of an intracellular pathogen to promote its multiplication in mammalian cells.

Address correspondence to: Isabelle Coppens (icoppens{at}jhsph.edu).

Abbreviations used: DBP, D-bifunctional protein; HFF, human foreskin fibroblast; LDL, low-density lipoprotein; LPDS, lipoprotein-deficient serum; NHF, normal human fibroblast; PC, phosphatidylcholine; p.i., post-infection; PTS1, peroxisomal targeting signal 1; PV, parasitophorous vacuole; SCP-2, sterol carrier protein-2; TLC, thin layer chromatography; ZF, Zellweger syndrome fibroblast.