Appearance of Smaller Lipid Bodies and Protein Kinase Activation in the Lipid Body Fraction Are Induced by an Increase in the Nitrogen Source in the Mortierella Fungus

Plant lipid bodies and cell-cell signaling: a new role for an old organelle?

Plant lipid droplets are found in seeds and in post-embryonic tissues. Lipid droplets in seeds have been intensively studied, but those in post-embryonic tissues are less well characterised. Although known by a variety of names, here we will refer to all of them as lipid bodies (LBs). LBs are unique spherical organelles which bud off from the endoplasmic reticulum, and are composed of a single phospholipid (PL) layer enclosing a core of triacylglycerides. The PL monolayer is coated with oleosin, a structural protein that stabilizes the LB, restricts its size, and prevents fusion with adjacent LBs. Oleosin is uniquely present at LBs and is regarded as a LB marker. Although initially viewed as simple stores for energy and carbon, the emerging view is that LBs also function in cytoplasmic signalling, with the minor LB proteins caleosin and steroleosin in a prominent role. Apart from seeds, a variety of vegetative and floral structures contain LBs. Recently, it was found that numerous LBs emerge in the shoot apex of perennial plants during seasonal growth arrest and bud formation. They appear to function in dormancy release by reconstituting cell-cell signalling paths in the apex. As apices and orthodox seeds proceed through comparable cycles of dormancy and dehydration, the question arises to what degree LBs in apices share functions with those in seeds. We here review what is known about LBs, particularly in seeds, and speculate about possible unique functions of LBs in post-embryonic tissues in general and in apices in particular.

A novel protein kinase localized to lipid droplets is required for droplet biogenesis in trypanosomes

Ubiquitous among eukaryotes, lipid droplets are organelles that function to coordinate intracellular lipid homeostasis. Their morphology and abundance is affected by numerous genes, many of which are involved in lipid metabolism. In this report we identify a Trypanosoma brucei protein kinase, LDK, and demonstrate its localization to the periphery of lipid droplets. Association with lipid droplets was abrogated when the hydrophobic domain of LDK was deleted, supporting a model in which the hydrophobic domain is associated with or inserted into the membrane monolayer of the organelle. RNA interference knockdown of LDK modestly affected the growth of mammalian bloodstream-stage parasites but did not affect the growth of insect (procyclic)-stage parasites. However, the abundance of lipid droplets dramatically decreased in both cases. This loss was dominant over treatment with myriocin or growth in delipidated serum, both of which induce lipid body biogenesis. Growth in delipidated serum also increased LDK autophosphorylation activity. Thus, LDK is required for the biogenesis or maintenance of lipid droplets and is one of the few protein kinases specifically and predominantly associated with an intracellular organelle.

DGA1 (diacylglycerol acyltransferase gene) overexpression and leucine biosynthesis significantly increase lipid accumulation in the Deltasnf2 disruptant of Saccharomyces cerevisiae

We previously found that SNF2, a gene encoding a transcription factor forming part of the SWI/SNF (switching/sucrose non-fermenting) chromatin-remodelling complex, is involved in lipid accumulation, because the Deltasnf2 disruptant of Saccharomyces cerevisiae has a higher lipid content. The present study was conducted to identify other factors that might further increase lipid accumulation in the Deltasnf2 disruptant. First, expression of LEU2 (a gene encoding beta-isopropylmalate dehydrogenase), which was used to select transformed strains by complementation of the leucine axotroph, unexpectedly increased both growth and lipid accumulation, especially in the Deltasnf2 disruptant. The effect of LEU2 expression on growth and lipid accumulation could be reproduced by adding large amounts of leucine to the culture medium, indicating that the effect was not due to Leu2p (beta-isopropylmalate dehydrogenase) itself, but rather to leucine biosynthesis. To increase lipid accumulation further, genes encoding the triacylglycerol biosynthetic enzymes diacylglycerol acyltransferase (DGA1) and phospholipid:diacylglycerol acyltransferase (LRO1) were overexpressed in the Deltasnf2 disruptant. Overexpression of DGA1 significantly increased lipid accumulation, especially in the Deltasnf2 disruptant, whereas LRO1 overexpression decreased lipid accumulation in the Deltasnf2 disruptant. Furthermore, the effect of overexpression of acyl-CoA synthase genes (FAA1, FAA2, FAA3 and FAA4), which each supply a substrate for Dga1p (diacylglycerol acyltransferase), was investigated. Overexpression of FAA3, together with that of DGA1, did not further increase lipid accumulation in the Deltasnf2 disruptant, but did enhance lipid accumulation in the presence of exogenous fatty acids. Lastly, the total lipid content in the Deltasnf2 disruptant transformed with DGA1 and FAA3 overexpression vectors reached approx. 30%, of which triacylglycerol was the most abundant lipid. Diacylglycerol acyltransferase activity was significantly increased in the Deltasnf2 disruptant strain overexpressing DGA1 as compared with the wild-type strain overexpressing DGA1; this higher activity may account for the prominent increase in lipid accumulation in the Deltasnf2 disruptant with DGA1 overexpression. The strains obtained have a lipid content that is high enough to act as a model of oleaginous yeast and they may be useful for the metabolic engineering of lipid production in yeast.

Functional analysis of the Alternaria brassicicola non-ribosomal peptide synthetase gene AbNPS2 reveals a role in conidial cell wall construction

SUMMARY Alternaria brassicicola is a necrotrophic pathogen causing black spot disease on virtually all cultivated Brassica crops worldwide. In many plant pathosystems fungal secondary metabolites derived from non-ribosomal peptide synthetases (NPSs) are phytotoxic virulence factors or are antibiotics thought to be important for niche competition with other micro-organisms. However, many of the functions of NPS genes and their products are largely unknown. In this study, we investigated the function of one of the A. brassicicola NPS genes, AbNPS2. The predicted amino acid sequence of AbNPS2 showed high sequence similarity with A. brassicae, AbrePsy1, Cochliobolus heterostrophus, NPS4 and a Stagonospora nodorum NPS. The AbNPS2 open reading frame was predicted to be 22 kb in length and encodes a large protein (7195 amino acids) showing typical NPS modular organization. Gene expression analysis of AbNPS2 in wild-type fungus indicated that it is expressed almost exclusively in conidia and conidiophores, broadly in the reproductive developmental phase. AbNPS2 gene disruption mutants showed abnormal spore cell wall morphology and a decreased hydrophobicity phenotype. Conidia of abnps2 mutants displayed an aberrantly inflated cell wall and an increase in lipid bodies compared with wild-type. Further phenotypic analyses of abnps2 mutants showed decreased spore germination rates both in vitro and in vivo, and a marked reduction in sporulation in vivo compared with wild-type fungus. Moreover, virulence tests on Brassicas with abnps2 mutants revealed a significant reduction in lesion size compared with wild-type but only when aged spores were used in experiments. Collectively, these results indicate that AbNPS2 plays an important role in development and virulence.

Inhibition of lipid accumulation and lipid body formation in oleaginous yeast by effective components in spices, carvacrol, eugenol, thymol, and piperine

We screened natural organic compounds, which affected the lipid accumulation and the lipid body formation in oleaginous yeast, Lipomyces starkeyi, generating large lipid bodies. We found that four natural components in spices, carvacrol, thymol, eugenol, and piperine, inhibited the lipid accumulation at concentrations of 20-50 mg/L with a slight growth inhibition. The inhibitory effects were quantitatively represented by the total lipid accumulation amount, the triacylglycerol accumulation amount, and the average lipid body size. At 50 mg/L, the effects of these compounds were not identical and exhibited 11-37% decrease in lipid amount and 15-21% decrease in lipid body size with 13-39% decrease in cell growth. The inhibitory effect of these compounds lead to 30-69% decrease in triacylglycerol accumulation without any additional accumulation of its intermediates, suggesting that they will suppress the total carbon inflow into the triacylglycerol biosynthesis.