Abstract

BackgroundThe oleaginous fungus, Mucor circinelloides, is attracting considerable interest as it produces oil rich in γ-linolenic acid. Nitrogen (N) deficiency is a common strategy to trigger the lipid accumulation in oleaginous microorganisms. Although a simple pathway from N depletion in the medium to lipid accumulation has been elucidated at the enzymatic level, global changes at protein levels upon N depletion have not been investigated. In this study, we have systematically analyzed the changes at the levels of protein expression in M. circinelloides WJ11, a high lipid-producing strain (36 %, lipid/cell dry weight), during lipid accumulation.ResultsProteomic analysis demonstrated that N depletion increased the expression of glutamine synthetase, involved in ammonia assimilation, for the supply of cellular nitrogen but decreased the metabolism of amino acids. Upon N deficiency, many proteins (e.g., fructose-bisphosphate aldolase, glyceraldehyde-3-phosphate dehydrogenase, enolase, pyruvate kinase) involved in glycolytic pathway were up-regulated while proteins involved in the tricarboxylic acid cycle (e.g., isocitrate dehydrogenase, succinyl-CoA ligase, succinate dehydrogenase, fumarate hydratase) were down-regulated, indicating this activity was retarded thereby leading to a greater flux of carbon into fatty acid biosynthesis. Moreover, glucose-6-phosphate dehydrogenase, transaldolase and transketolase, which participate in the pentose phosphate pathway, were up-regulated, leading to the increased production of NADPH, the reducing power for fatty acid biosynthesis. Furthermore, protein and nucleic acid metabolism were down-regulated and some proteins involved in energy metabolism, signal transduction, molecular chaperone and redox homeostasis were up-regulated upon N depletion, which may be the cellular response to the stress produced by the onset of N deficiency.ConclusionN limitation increased those expressions of the proteins involved in ammonia assimilation but decreased that involved in the biosynthesis of amino acids. Upon N deprivation, the glycolytic pathway was up-regulated, while the activity of the tricarboxylic acid cycle was retarded, thus, leading more carbon flux to fatty acid biosynthesis. Moreover, the pentose phosphate pathway was up-regulated, then this would increase the production of NADPH. Together, coordinated regulation of central carbon metabolism upon N limitation, provides more carbon flux to acetyl-CoA and NADPH for fatty acid biosynthesis.Electronic supplementary materialThe online version of this article (doi:10.1186/s12934-016-0428-4) contains supplementary material, which is available to authorized users.

Highlights

  • The oleaginous fungus, Mucor circinelloides, is attracting considerable interest as it produces oil rich in γ-linolenic acid

  • N limitation increased the expression of the proteins involved in ammonia assimilation for the supply of cellular nitrogen but decreased the expression of proteins involved in the biosynthesis of amino acids

  • Carbon metabolism is crucial for lipid accumulation, which was significantly affected upon N deficiency

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Summary

Introduction

The oleaginous fungus, Mucor circinelloides, is attracting considerable interest as it produces oil rich in γ-linolenic acid. Nitrogen (N) deficiency is a common strategy to trigger the lipid accumulation in oleaginous microorganisms. The metabolism of lipid accumulation in oleaginous microorganisms has been extensively studied as microbial oils can be used as commercial sources of several nutritionally-important polyunsaturated fatty acids (PUFAs) and as potential sources of biofuels [2, 3]. Mucor circinelloides has been considered as an important model organism for lipid accumulation studies due to its ability to produce an oil rich in γ-linolenic acid (GLA, 18:3; n-6), that may have beneficial effects for the treatment of premenstrual tension, atopic dermatitis and some other diseases [4] and due to the availability of genome data and genetic tools. The biochemistry of lipid accumulation in oleaginous microorganisms triggered by N deficiency has been widely investigated. ATP: citrate lyase (ACL), which generates acetyl-CoA as the precursor of fatty acids via the cleavage of citric acid, is an essential enzyme for fatty acid biosynthesis and possibly catalyzes the rate-limiting reaction for fatty acid biosynthesis in some oleaginous organism [7,8,9]

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