Serine palmitoyltransferase assembles at ER-mitochondria contact sites.

Mari J Aaltonen,Eric A Shoubridge,Irina Alecu,Tim König,Steffany Al Bennett

doi:10.26508/lsa.202101278

Abstract

The accumulation of sphingolipid species in the cell contributes to the development of obesity and neurological disease. However, the subcellular localization of sphingolipid-synthesizing enzymes is unclear, limiting the understanding of where and how these lipids accumulate inside the cell and why they are toxic. Here, we show that SPTLC2, a subunit of the serine palmitoyltransferase (SPT) complex, catalyzing the first step in de novo sphingolipid synthesis, localizes dually to the ER and the outer mitochondrial membrane. We demonstrate that mitochondrial SPTLC2 interacts and forms a complex in trans with the ER-localized SPT subunit SPTLC1. Loss of SPTLC2 prevents the synthesis of mitochondrial sphingolipids and protects from palmitate-induced mitochondrial toxicity, a process dependent on mitochondrial ceramides. Our results reveal the in trans assembly of an enzymatic complex at an organellar membrane contact site, providing novel insight into the localization of sphingolipid synthesis and the composition and function of ER-mitochondria contact sites.

Highlights

The lipid composition of cellular membranes is determined by the specific localization of proteins mediating lipid synthesis and transport
Serine palmitoyltransferase (SPT) formed by ubiquitously expressed SPTLC1 and SPTLC2 preferably synthesizes the palmitoyl-containing sphingoid base (Lone et al, 2020), which is the most abundant sphingoid base in mammalian cells (Pruett et al, 2008), whereas SPTLC3 expression is restricted to specific tissues and leads to the production of short, long, and branched-chain sphingolipid species (Hornemann et al, 2006, 2009; Lone et al, 2020)
SPTLC2-GFP co-localized with the ER-marker and with the mitochondrial marker MRPL12 (Fig 1C). Consistent with these results, subcellular fractionation of mouse liver showed that endogenous SPTLC1 and SPTLC2 were present in the microsomal light membrane fraction, and in the heavy membrane fraction containing mitochondria and mitochondria-associated ER membrane (MAM) (Fig 1D)

Summary

Introduction

The lipid composition of cellular membranes is determined by the specific localization of proteins mediating lipid synthesis and transport. Most lipids are synthesized in the ER and need to be transported to other organelles via vesicles or by lipid transfer proteins at membrane contact sites. SPT formed by ubiquitously expressed SPTLC1 and SPTLC2 preferably synthesizes the palmitoyl-containing sphingoid base (Lone et al, 2020), which is the most abundant sphingoid base in mammalian cells (Pruett et al, 2008), whereas SPTLC3 expression is restricted to specific tissues and leads to the production of short-, long-, and branched-chain sphingolipid species (Hornemann et al, 2006, 2009; Lone et al, 2020)

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Results