Size control of lipid droplets in budding yeast requires a collaboration of Fld1 and Ldb16

Abstract

The human congenital generalized lipodystrophy type 2 protein seipin (Fld1 in budding yeast) controls lipid droplet (LD) size through an unknown mechanism. Here, we report that deletion of yeast LDB16/YCL005W, similar to deletion of FLD1, causes supersized and small clustered LDs, altered phospholipid metabolism and impaired distribution of a subset of LD proteins. Ldb16 is a transmembrane protein in the endoplasmic reticulum (ER) that assembles together with Fld1 at ER-LD contact sites, a region that probably links neutral lipid synthesis with LD assembly. The formation of the Fld1-Ldb16 complex involves putative transmembrane segments of both proteins, thus, directly contributing to the maintenance of LD morphology. The stability of Ldb16 requires Fld1, as Ldb16 is subjected to ER-associated degradation (ERAD) in the absence of Fld1 but is stabilized when Fld1 is present. Strikingly, human seipin, but not yeast Fld1, complements the defects in LDs in ldb16D yeast, implying that seipin can substitute for the function of the Fld1-Ldb16 complex. We propose that human seipin might adopt the architecture of the yeast Fld1-Ldb16 complex in order to properly maintain the size of LDs.

Lipid synthesis and membrane contact sites: a crossroads for cellular physiology

“…BSCL2 encodes seipin, an integral protein of the ER that assembles into oligomeric complexes at ER-LD contact sites. It was noted that loss-of-function of Fld1 results in a LD phenotype very similar to that observed in lbd16 cells (59). Consistent with this, Fld1 and Lbd16 form a protein complex where Fld1 interacts with itself and with Ldb16 at ER-LD contact sites, referred to as the seipin complex (59,61).…”

“…It was noted that loss-of-function of Fld1 results in a LD phenotype very similar to that observed in lbd16 cells (59). Consistent with this, Fld1 and Lbd16 form a protein complex where Fld1 interacts with itself and with Ldb16 at ER-LD contact sites, referred to as the seipin complex (59,61). In the absence of Fld1, Ldb16 becomes unstable and is targeted to the ER-associated degradation machinery.…”

“…In spite of the similar domain architecture and the common function, there is no homology between these two proteins. Interestingly, the expression of human seipin complements the simultaneous absence of both proteins, as well as the absence of each one individually, suggesting that the function of the yeast seipin complex constituted by Fld1 and Ldb16 is performed by human seipin (59). The absence of Fld1 or Lbd16 elicits similar LD phenotypes, as well as similar perturbations on lipid metabolism.…”

“…Inositol supplementation did not affect the expression of a reporter gene driven by the PAH1 promoter for cells growing exponentially; whereas for cells these genes resulted in a few supersized LDs (SLDs) (diameter >1 m) when cells were grown in minimal media (SC media). This phenotype is conditional because supplementation of the growth media with choline or inositol, which drive PL synthesis, or growth on YPD-rich media, which contain both choline and inositol, restored LD morphology to a wild-type appearance (51,59). Under limiting PL synthesis, it is thought that SLD formation is favored, as the surface/volume ratio of LDs is minimized, while fusogenic PLs such as PE and PA accumulate simultaneously.…”

“…The absence of Fld1 or Lbd16 elicits similar LD phenotypes, as well as similar perturbations on lipid metabolism. Typically, fld1 and ldb16 cells display a heterogeneous LD phenotype that depends on growth condition (18,20,25,51,59,60). When grown in minimal media, most of the fld1 and ldb16 mutant cells present one or a few SLDs.…”