title: Structural basis for specific lipid recognition by CERT responsible for nonvesicular trafficking of ceramide.
abstract: In mammalian cells, ceramide is synthesized in the endoplasmic reticulum and transferred to the Golgi apparatus for conversion to sphingomyelin. Ceramide transport occurs in a nonvesicular manner and is mediated by CERT, a cytosolic 68-kDa protein with a C-terminal steroidogenic acute regulatory protein-related lipid transfer (START) domain. The CERT START domain efficiently transfers natural D-erythro-C16-ceramide, but not lipids with longer (C20) amide-acyl chains. The molecular mechanisms of ceramide specificity, both stereo-specific recognition and length limit, are not well understood. Here we report the crystal structures of the CERT START domain in its apo-form and in complex with ceramides having different acyl chain lengths. In these complex structures, one ceramide molecule is buried in a long amphiphilic cavity. At the far end of the cavity, the amide and hydroxyl groups of ceramide form a hydrogen bond network with specific amino acid residues that play key roles in stereo-specific ceramide recognition. At the head of the ceramide molecule, there is no extra space to accommodate additional bulky groups. The two aliphatic chains of ceramide are surrounded by the hydrophobic wall of the cavity, whose size and shape dictate the length limit for cognate ceramides. Furthermore, local high-crystallographic B-factors suggest that the alpha-3 and the Omega1 loop might work as a gate to incorporate the ceramide into the cavity. Thus, the structures demonstrate the structural basis for the mechanism by which CERT can distinguish ceramide from other lipid types yet still recognize multiple species of ceramides.
authors.affiliation: Structural Biology Research Center, Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan.
authors.forename: Norio
authors.initials: N
authors.lastname: Kudo
authors.forename: Keigo
authors.initials: K
authors.lastname: Kumagai
authors.forename: Nario
authors.initials: N
authors.lastname: Tomishige
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Yamaji
authors.forename: Soichi
authors.initials: S
authors.lastname: Wakatsuki
authors.forename: Masahiro
authors.initials: M
authors.lastname: Nishijima
authors.forename: Kentaro
authors.initials: K
authors.lastname: Hanada
authors.forename: Ryuichi
authors.initials: R
authors.lastname: Kato
mesh_terms: D000596:Amino Acids / Q000737:chemistry; D002518:Ceramides / Q000378:metabolism*; D018360:Crystallography, X-Ray / Q000379:methods; D004075:Diglycerides / Q000737:chemistry; D004926:Escherichia coli / Q000378:metabolism; D006801:Humans; D006860:Hydrogen Bonding; D008055:Lipids / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D010957:Plasmids / Q000378:metabolism; D011487:Protein Conformation; D017346:Protein Serine-Threonine Kinases / Q000737:chemistry*; D017434:Protein Structure, Tertiary; D013109:Sphingomyelins / Q000737:chemistry
chemical_list: D000596:Amino Acids; D002518:Ceramides; D004075:Diglycerides; D008055:Lipids; D013109:Sphingomyelins; C119090:CERT1 protein, human; D017346:Protein Serine-Threonine Kinases