Glycolipid transfer protein modulates vesicular trafficking from the endoplasmic reticulum in HeLa cells: GLTP as a key player in COPII-coated vesicle budding

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 membrane protein vesicle-associated membrane protein-associated protein A (VAPA), GLTP may also be involved in regulating intracellular vesicular transport. Here we show that GLTP knockout (GLTP KO) leads to the sequestration of coat protein complex II coat proteins Sec23 A and Sec31 A at the ER exit sites. The intracellular localization of the small GTPase Sar1A is altered in GLTP KO 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 KO 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 glycosphingolipid levels associated with GLTP KO. We propose a role for GLTP in modulating the coat protein complex II vesicle trafficking pathway, thereby indirectly regulating the cellular glycosphingolipid homeostasis by controlling the vesicular ceramide transport from the endoplasmic reticulum 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 GLTP’s in vivo function.