Various kinds of organisms, including microalgae, accumulate neutral lipids in distinct intracellular compartments called lipid droplets. Generally, lipid droplets are generated from the endoplasmic reticulum, and particular proteins localize on their surface. Some of these proteins function as structural proteins to prevent fusion between the lipid droplets, and the others could have an enzymatic role or might be involved in intracellular membrane trafficking. However, information about lipid droplet proteins in microalgae is scarce as compared with that in animals and land plants. We focused on the oil-producing, marine, pennate diatom Phaeodactylum tricornutum that forms lipid droplets during nitrogen deprivation and we investigated the proteins located on the lipid droplets. After 6 d of cultivation in a nitrate-deficient medium, the mature lipid droplets were isolated by sucrose density gradient centrifugation. Proteomic analyses revealed five proteins, with Stramenopile-type lipid droplet protein (StLDP) being the most abundant protein in the lipid droplet fraction. Although the primary sequence of StLDP did not have homology to any known lipid droplet proteins, StLDP had a central hydrophobic domain. This structural feature is also detected in oleosin of land plants and in lipid droplet surface protein (LDSP) of Nannochloropsis species. As a proline knot motif of oleosin, conservative proline residues existed in the hydrophobic domain. StLDP was up-regulated during nitrate deprivation, and fluctuations of StLDP expression levels corresponded to the size of the lipid droplets.

No abstract

22Lipid droplets are ubiquitous cellular compartments that store neutral lipids and 23 specific proteins localize on their surface. These proteins work as a scaffold in 24 maintaining the lipid droplet structure or as regulators of lipogenesis or lipolysis. 25Previously, our group was the first to identify the most abundant lipid droplet protein, 26 namely stramenopile-type lipid droplet protein (StLDP), in the marine diatom 27 Phaeodactylum tricornutum; however, its function remains unclear because StLDP 28 does not reveal homology with known lipid droplet proteins and lacks a predictable 29 domain. In this study, we transformed P. tricornutum to express a homologous StLDP 30 gene under an fcpA promoter in order to determine its function. StLDP expression 31was strongly enhanced in the mutant (H8), especially in nitrogen-sufficient conditions; 32 however, it was attenuated in nitrogen-deficient conditions. Despite the strong 33 expression, no significant difference was observed in the lipid composition between 34 the wild type (WT) and H8 under nitrogen-sufficient conditions. After cultivation in 35 nitrogen-free medium for 6 days, neutral lipid content significantly increased in H8 36 than in WT. After 2 days of cultivation in nitrogen-free medium, 97.0% of single cells in 37 WT formed one or two lipid droplets, whereas in H8, this proportion decreased to 38 78.8%, and the proportion of cells forming three or four lipid droplets increased. Thus, 39 we speculate that StLDP functions to sequester triacylglycerol on the initial lipid 40 droplet formation. 41 42 Keywords 43 Nitrogen starvation, Overexpression, StLDP, Triacylglycerol 44 45 48 protect the global environment and to develop a sustainable society (Huang et al.49

No abstract

No abstract

No abstract

Proteins immobilized on biosilica which have superior reactivity and specificity and are innocuous to natural environments could be useful biological materials in industrial processes. Living diatom silica immobilization (LiDSI) is a recently developed technique for in vivo protein immobilization on the diatom frustule.

No abstract