Biochemical consequences of heritable mutations in the α-tocopherol transfer protein

Tocopherol transfer protein (TTP) regulates vitamin E status by facilitating the secretion of tocopherol from liver to circulating lipoproteins. Heritable mutations in the ttpA gene, encoding for TTP, result in ataxia with vitamin E deficiency (AVED) syndrome, typified by low vitamin E levels and a plethora of neurological disorders. The molecular mechanisms by which TTP facilitates tocopherol secretion are presently unknown. We recently showed that vitamin E is taken up by hepatocytes through an endocytic process and that, shortly following uptake, the vitamin is found primarily in lysosomes. We showed further that TTP is localized to late endocytic vesicles and that it facilitates the intracellular trafficking of tocopherol from lysosomes to the plasma membrane. To gain insight into the molecular mechanisms that underlie TTP actions, we studied the physiological impact of three naturally occurring heritable mutations in the ttpA gene (the R59W, R221W, and A120T substitutions). We found that these mutations impair the ability of TTP to facilitate the secretion of vitamin E from cells. Furthermore, the degree of impairment corresponded to the severity of the AVED pathology associated with each mutation. In cells that express mutated TTP proteins, vitamin E did not traffic to the plasma membrane and remained trapped in lysosomes. In addition, we observed that substitution mutations that cause the AVED syndrome impart a marked instability on the TTP protein. These observations suggest that the physiological role of TTP is anchored in its ability to direct vitamin E trafficking from the endocytic compartment to transport vesicles that deliver the vitamin to the site of secretion at the plasma membrane.