Abstract
Sphingolipids such as ceramide are important constituents of cell membranes. The ceramide transfer protein (CERT) moves ceramide from the endoplasmic reticulum to the Golgi apparatus in a nonvesicular manner. Hyperphosphorylation of the serine-repeat motif (SRM) adjacent to the pleckstrin homology (PH) domain of CERT down-regulates the inter-organelle ceramide transport function of CERT. However, the mechanistic details of this down-regulation remain elusive. Using solution NMR and binding assays, we herein show that a hyperphosphorylation-mimetic CERT variant in which 10 serine/threonine residues of SRM had been replaced with glutamate residues (the 10E variant) displays an intramolecular interaction between SRM and positively charged regions of the PH domain, which are involved in the binding of this domain to phosphatidylinositol 4-monophosphate (PI4P). Of note, the binding of the PH domain to PI4P-embedded membranes was attenuated by the SRM 10E substitutions in cell-free assays. Moreover, the 10E substitutions reduced the Golgi-targeting activity of the PH-SRM construct in living cells. These results indicate that hyperphosphorylated SRM directly interacts with the surface of the PH domain in an intramolecular manner, thereby decreasing the PI4P-binding activity of the PH domain. In light of these findings, we propose that the hyperphosphorylation of SRM may trigger the dissociation of CERT from the Golgi apparatus, resulting in a functionally less active conformation of CERT.