FIT2 organizes lipid droplet biogenesis with ER tubule-forming proteins and septins

Abstract

Lipid droplets (LDs) are critical for lipid storage and energy metabolism. LDs form in the endoplasmic reticulum (ER). However, the molecular basis for LD biogenesis remains elusive. Here, we show that fat storage–inducing transmembrane protein 2 (FIT2) interacts with ER tubule-forming proteins Rtn4 and REEP5. The association is mainly transmembrane domain based and stimulated by oleic acid. Depletion of ER tubule-forming proteins decreases the number and size of LDs in cells and Caenorhabditis elegans, mimicking loss of FIT2. Through cytosolic loops, FIT2 binds to cytoskeletal protein septin 7, an interaction that is also required for normal LD biogenesis. Depletion of ER tubule-forming proteins or septins delays nascent LD formation. In addition, FIT2-interacting proteins are up-regulated during adipocyte differentiation, and ER tubule-forming proteins, septin 7, and FIT2 are transiently enriched at LD formation sites. Thus, FIT2-mediated nascent LD biogenesis is facilitated by ER tubule-forming proteins and septins.

Intertwined and Finely Balanced: Endoplasmic Reticulum Morphology, Dynamics, Function, and Diseases

“…Indeed, it was found that the rate of lipid transfer from the ER to the plasma membrane does not appreciably decrease when vesicular pathways are blocked [ 148 , 149 , 150 , 151 ], indicating that non-vesicular transport alone can sustain the required lipid transfer to the plasma membrane. As MCSs between the ER and other organelles, particularly the plasma membrane, mitochondria, and endosomes [ 73 , 74 , 75 , 76 , 77 , 78 ], preferentially form in the tubular ER network, these findings suggest that lipid transfer occurs primarily in the tubular ER.…”

“…In support of this idea, cells responsible for steroid synthesis, such as adrenal cortical cells, have proportionally more ribosome-free smooth ER [72]. It has also been suggested that membrane contact sites (MCSs) between the ER and other organelles may be more common in the ribosome-free tubular region of the network, particularly those MCSs formed with the plasma membrane, endosomes, lipid droplets and mitochondria [73][74][75][76][77][78]. The MCSs and relevant proteins discussed in this review are shown in Figure 2.…”

Membrane shaping proteins, lipids, and cytoskeleton: Recipe for nascent lipid droplet formation

“…Taken together, these data indicate that the high membrane curvature of ER tubules catalyzes LD biogenesis. [ 15,32–34 ]…”

“…Proteins such as seipin, LDAF1, fat‐inducing transcript (FIT) proteins, and ER shaping proteins such as Pex30 in yeast, MCTP1, MCTP2, and REEP5, in mammalian cells are localized at ER subdomains. [ 14–17 ] These proteins are at ER subdomains where nascent LD markers such as LiveDrop or perilipin 3 are recruited suggesting that LD formation sites could be pre‐determined in the ER membrane. [ 14,16,18 ] Interestingly, extended incubation of cells with oleic acid, an LD inducing growth condition, results in increased size of LDs but not the number, potentially due to limited sites for LD biogenesis.…”

“…We will summarize latest contributions from the field to elucidate the role of membrane curvature generated by ER shaping proteins, and membrane lipids on the LD biogenesis with a special focus on recently discovered role of cytoskeleton protein septin in the formation of LDs. [ 15 ]…”