Department of Chemical Biology, Department of Biology, School of Life Sciences, Southern University of Science and Technology, Shenzhen, Guangdong 518055, China.
Department of Biochemistry and Molecular Biology, Uniformed Services University of the Health Sciences, Bethesda, MD 20814, USA.
Sci Adv. 2023 Mar 29;9(13):eadg0728. doi: 10.1126/sciadv.adg0728.
The serine palmitoyltransferase (SPT) complex catalyzes the first and rate-limiting step in sphingolipid biosynthesis in all eukaryotes. ORM/ORMDL proteins are negative regulators of SPT that respond to cellular sphingolipid levels. However, the molecular basis underlying ORM/ORMDL-dependent homeostatic regulation of SPT is not well understood. We determined the cryo-electron microscopy structure of SPT-ORM1 complex, composed of LCB1, LCB2a, SPTssa, and ORM1, in an inhibited state. A ceramide molecule is sandwiched between ORM1 and LCB2a in the cytosolic membrane leaflet. Ceramide binding is critical for the ORM1-dependent SPT repression, and dihydroceramides and phytoceramides differentially affect this repression. A hybrid β sheet, formed by the amino termini of ORM1 and LCB2a and induced by ceramide binding, stabilizes the amino terminus of ORM1 in an inhibitory conformation. Our findings provide mechanistic insights into sphingolipid homeostatic regulation via the binding of ceramide to the SPT-ORM/ORMDL complex that may have implications for plant-specific processes such as the hypersensitive response for microbial pathogen resistance.
丝氨酸棕榈酰转移酶（SPT）复合物在所有真核生物中催化鞘脂生物合成的第一步和限速步骤。ORM/ORMDL 蛋白是 SPT 的负调节剂，可响应细胞鞘脂水平。然而，ORM/ORMDL 依赖的 SPT 稳态调节的分子基础尚不清楚。我们确定了 LCB1、LCB2a、SPTssa 和 ORM1 组成的 SPT-ORM1 复合物在抑制状态下的低温电子显微镜结构。在细胞质膜小叶中，神经酰胺分子夹在 ORM1 和 LCB2a 之间。神经酰胺结合对于 ORM1 依赖性 SPT 抑制至关重要，二氢神经酰胺和植物神经酰胺对这种抑制作用有不同的影响。由 ORM1 和 LCB2a 的氨基末端形成的混合β 片层，由神经酰胺结合诱导，稳定 ORM1 的氨基末端处于抑制构象。我们的发现为通过神经酰胺与 SPT-ORM/ORMDL 复合物的结合对鞘脂进行稳态调节提供了机制见解，这可能对植物特有的过程（例如对微生物病原体抗性的过敏反应）具有重要意义。
