Utilizing a nanobody recruitment approach for assessing serine palmitoyltransferase activity in ER sub-compartments of yeast.

Sphingolipids (SP) are one of the three major lipid classes in eukaryotic cells and serve as structural components of the plasma membrane. The rate-limiting step in SP biosynthesis is catalyzed by serine palmitoyltransferase (SPT). In yeast, SPT consists of two catalytic subunits (Lcb1 and Lcb2), a regulatory subunit (Tsc3), negative regulators (Orm1 and Orm2), and the phosphatidylinositol-4-phosphate (PI4P) phosphatase Sac1, collectively known as the SPOTS complex. Regulating SPT activity enables cells to adapt SP metabolism to changing environmental conditions. Therefore, the Orm proteins are phosphorylated by two signaling pathways originating from either the plasma membrane localized target of rapamycin (TOR) complex 2 or the lysosomal/vacuolar TOR complex 1. Moreover, uptake of exogenous serine is necessary for the regulation of SP biosynthesis, which suggests the existence of differentially regulated SPT pools based on their intracellular localization. However, tools for measuring lipid metabolic enzyme activity in different cellular compartments are currently not available. We have developed a nanobody recruitment system that enables the re-localization of the SPOTS complex to the nuclear or peripheral ER. By combining this system with sphingolipid flux analysis, we have identified two distinct active SPT pools in cells. Our method thus serves as a new and versatile tool to measure lipid metabolism with sub-cellular resolution.