The endoplasmic reticulum membrane protein complex localizes to the mitochondrial - endoplasmic reticulum interface and its subunits modulate phospholipid biosynthesis in Trypanosoma brucei

Abstract

The endoplasmic reticulum membrane complex (EMC) is a versatile complex that plays a key role in membrane protein biogenesis in the ER. Deletion of the complex has wide-ranging consequences including ER stress, disturbance in lipid transport and organelle tethering, among others. Here we report the function and organization of the evolutionarily conserved EMC (TbEMC) in the highly diverged eukaryote, Trypanosoma brucei. We show that the TbEMC is composed of 9 subunits that are present in a high molecular mass complex localizing to the mitochondrial-endoplasmic reticulum interface. Knocking out or knocking down of single TbEMC subunits led to growth defects of T. brucei procyclic forms in culture. Interestingly, we found that depletion of individual TbEMC subunits lead to disruption of de novo synthesis of phosphatidylcholine (PC) or phosphatidylethanolamine (PE), the two most abundant phospholipid classes in T. brucei . Downregulation of TbEMC1 or TbEMC3 inhibited formation of PC while depletion of TbEMC8 inhibited PE synthesis, re-emphasizing the role of the EMC in phospholipid synthesis. In addition, we found that in TbEMC7 knock-out parasites, TbEMC3 is released from the complex, implying that TbEMC7 is essential for the formation or the maintenance of the TbEMC. The organization of the TbEMC and its role in phospholipid metabolism in T. brucei may represent ancestral functions.

Protein control of membrane and organelle dynamics: Insights from the divergent eukaryote Toxoplasma gondii

“…Like VDAC, most EMC subunits are found in most eukaryotic organisms [ 37 ], raising the possibility of a putative common function across eukaryotes. This notion was supported by the report of EMC found in the ER–mitochondrial interface of Trypanosomes , where it plays a role in the control of phospholipid synthesis [ 38 ].…”

“…In yeast, EMC genes could be individually deleted without a major effect on viability, but deletion of two (or more) subunits results in lipid synthesis defect [ 35 ]. Interestingly, in Trypanosomes , individual gene deletion or depletion affects viability, and some have a drastic effect on lipid synthesis [ 38 ]. Single-subunit essentiality seems to be the case also in Toxoplasma, where seven of the eight predicted EMC encoding genes are critical for growth, as predicted by a genome wide CRISPR screen ( Table 1 ) and confirmed in our unpublished work (Ovciarikova et al., in prep).…”