Figure 1.

Model of LD biogenesis in wild-type cells and cells lacking FIT proteins. (A) Stages of LD biogenesis in wild-type cells. Neutral lipids are synthesized in the ER (1). When the amount of neutral lipid in the ER bilayer exceeds a threshold, lens-like structures form (2). Neutral lipid lenses grow and emerge out of the ER toward the cytoplasm, resulting in LDs consisting of a core of neutral lipids surrounded by a phospholipid monolayer. Mature LDs remain connected to the ER via membrane-bridge or separate from the ER. (B) In cells lacking FIT proteins LDs fail to bud from the ER and remain embedded in the ER membrane. They often also become wrapped by the ER membrane.