The glycolipid transfer protein GLTP as a mediator of vesicular transport of ceramide

The glycolipid transfer protein (GLTP) has been proposed to function as a sensor and regulator of glycosphingolipid homeostasis in the cell, as levels of GLTP directly influence the quantities of many glycosphingolipids. Furthermore, through its interaction with the endoplasmic reticulum (ER) membrane protein VAPA (vesicle-associated membrane protein-associated protein A), GLTP may also be involved in regulating intracellular vesicular transport. Here we show that GLTP knockout leads to the sequestration of COPII coat proteins Sec23A and Sec31A at the ERES. The intracellular localization of the small GTPase Sar1A is altered in GLTP knockout cells and in cells expressing a GLTP mutant incapable of VAPA binding, indicating a correlation between the inhibition of the GLTP/VAPA interaction and the altered localization of Sar1A. We also observed alterations in the intracellular localization of the Sar1A-activating GEF Sec12 in GLTP knockout cells, implying a Sec12-activating role of the GLTP/VAPA interaction. Knockout of GLTP does not alter the amounts of other lipid transfer proteins or glucosylceramide synthase, indicating that a decrease in the levels of these proteins is not responsible for the decreased GSL levels associated with GLTP knockout. We propose a role for GLTP in modulating the COPII vesicle trafficking pathway, thereby indirectly regulating the cellular glycosphingolipid homeostasis by controlling the vesicular ceramide transport from the ER to the Golgi. Moreover, the discovery that GLTP localizes to the nucleus at the onset of the DNA-replicating S-phase of the cell cycle introduces an entirely new and unexpected dimension to GLTPs in vivo function.