α-Tocopherol (α-toc) is the most biologically active form of vitamin E; it is secreted from mammalian hepatocytes into the plasma. This process is promoted by the α-tocopherol transfer protein (α-TTP), a lipid-binding protein thought to transfer endocytosed α-toc to the plasma membrane; a transporter would then secrete it into circulation. Mutations in α-TTP can result in ataxia with vitamin E deficiency (AVED), a genetic disorder characterized by low levels of circulating vitamin E and neurodegeneration. Crystal structures of α-TTP have been available for a decade, but its precise function in α-toc transport, as well as why some of the AVED mutations are deleterious, remained unclear. Now Arai and colleagues have used those mutants to obtain further insights into α-TTP function. First, they found that the disease-related R59W mutant could bind α-toc and mediate its intermembrane transfer between liposomes, but it could not facilitate α-toc secretion by hepatoma cells. In addition, the wild-type protein could bind two forms of phosphatidylinositol bisphosphate (PI(3,4)P2 and PI(4,5)P2, or PIPs) in vitro, both of which are concentrated in the plasma membrane, whereas the R59W mutant could not. The authors then solved the crystal structures of α-TTP in complex with α-toc and either one of the PIPs. The structures showed that the negatively charged head of the PIPs was bound to a positively charged cleft in α-TTP. Arg59 and other arginine residues mutated in AVED line the cleft and mediate contacts with PIPs. The new α-TTP structures also revealed that PIPs are bound close to the α-toc binding site, a hydrophobic groove with a lid. Interestingly, this lid appears in a more open conformation in the new structures, compared to a previous α-TTP–α-toc structure. This change in lid conformation suggests that PIP binding to α-TTP could induce the opening of the α-toc pocket and hence promote its transfer. In fact, the authors saw an increase in α-toc transfer by wild-type α-TTP when the acceptor liposome membrane contained PIPs; conversely, the presence of PIPs in the donor membrane inhibited α-toc transfer. In contrast, addition of PIPs had no effect on intermembrane transfer of α-toc by the R59W mutant. The authors propose that α-TTP loaded with α-toc interacts with PIPs in the plasma membrane; this would result in the opening of the α-toc binding-pocket lid and α-toc transfer to the plasma membrane. Thus, binding to PIPs would have a dual role in α-TTP function: targeting the protein to the plasma membrane and stimulating α-toc release. The new crystal structures would represent a transient intermediate form, in which both α-toc and PIP are bound. (Science http://dx.doi.org/10.1126/science.1233508, published online 18 April 2013)