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. 2020 May 29;8:343. doi: 10.3389/fcell.2020.00343

FIGURE 2.

FIGURE 2

The ER implication in de novo membrane-bound structures and organelles. The ER is the specific site of vesicles implied in ER-to-Golgi transport, which notably requires COPII coatomers to deform the ER membrane. Peroxisome biogenesis initiates at the ER membrane via the recruitment of Pex family proteins and the maturation of pre-peroxisome is finalized in the cytoplasm. Accumulation of neutral lipids [triglycerides (TGs)] in the bilayer of the ER leads to ER membrane deformation and release of lipid droplets (LDs) in the cytoplasm, via the coordinated action of FIT and seipin proteins. In specialized cells such as hepatocytes and enterocytes, the ER is also responsible for lipoproteins [chylomicrons in enterocytes and very low density lipoprotein (VLDL) in hepatocytes]. In parallel to LD biogenesis, the TG accumulation in the bilayer leads as well to ER intraluminal budding of neutral lipid structures, via the microsomal triglyceride transfer protein (MTTP), and this structure will be stabilized by fusion with lipidated apoliprotein B (apoB48 in enterocytes and apoB100 in hepatocytes), which leads to pre-lipoprotein synthesis inside the ER lumen. This particle is then transported via the prechylomicron transport vesicle (PCTV) along the secretory pathway via the Golgi apparatus.