Abstract

Lipid droplets are ubiquitous cellular compartments that store neutral lipids and specific proteins localize on their surface. These proteins work as a scaffold in maintaining the lipid droplet structure or as regulators of lipogenesis or lipolysis. Previously, the most abundant lipid droplet protein, namely stramenopile-type lipid droplet protein (StLDP), was identified in the marine diatom Phaeodactylum tricornutum; however, its function remains unclear because StLDP does not reveal homology with known lipid droplet proteins and lacks a predictable domain. In this study, P. tricornutum was transformed to express a homologous StLDP gene under an fcpA promoter in order to determine its function. StLDP expression was strongly enhanced in the mutant (H8), especially in nitrogen-sufficient conditions; however, it was attenuated in nitrogen-deficient conditions. Despite the strong expression, no significant difference was observed in the lipid composition between the wild type (WT) and H8 under nitrogen-sufficient conditions. After cultivation in nitrogen-free medium for 6 days, neutral lipid content significantly increased in H8 than in WT. After 2 days of cultivation in nitrogen-free medium, 97.0% of single cells in WT formed one or two lipid droplets, whereas in H8, this proportion decreased to 78.8%, and the proportion of cells forming three or four lipid droplets increased. Thus, the function of StLDP was speculated to sequester triacylglycerol on the initial lipid droplet formation.

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