A Chloroplastic UDP-Glucose Pyrophosphorylase from Arabidopsis Is the Committed Enzyme for the First Step of Sulfolipid Biosynthesis

Yozo Okazaki,Yuji Sawada,Kazuki Saito,Mie Shimojima,Masami Yokota Hirai,Keiichi Mochida,Tomoko Narisawa,Hiroyuki Ohta,Fumio Matsuda,Kiminori Toyooka,Hironori Tanaka,Akiko Hirai

doi:10.1105/tpc.108.063925

Abstract

Plants synthesize a sulfur-containing lipid, sulfoquinovosyldiacylglycerol, which is one of three nonphosphorus glycerolipids that provide the bulk of the structural lipids in photosynthetic membranes. Here, the identification of a novel gene, UDP-glucose pyrophosphorylase3 (UGP3), required for sulfolipid biosynthesis is described. Transcriptome coexpression analysis demonstrated highly correlated expression of UGP3 with known genes for sulfolipid biosynthesis in Arabidopsis thaliana. Liquid chromatography-mass spectrometry analysis of leaf lipids in two Arabidopsis ugp3 mutants revealed that no sulfolipid was accumulated in these mutants, indicating the participation of UGP3 in sulfolipid biosynthesis. From the deduced amino acid sequence, UGP3 was presumed to be a UDP-glucose pyrophosphorylase (UGPase) involved in the generation of UDP-glucose, serving as the precursor of the polar head of sulfolipid. Recombinant UGP3 was able to catalyze the formation of UDP-glucose from glucose-1-phosphate and UTP. A transient assay using fluorescence fusion proteins and UGPase activity in isolated chloroplasts indicated chloroplastic localization of UGP3. The transcription level of UGP3 was increased by phosphate starvation. A comparative genomics study on UGP3 homologs across different plant species suggested the structural and functional conservation of the proteins and, thus, a committing role for UGP3 in sulfolipid synthesis.

Highlights

The photosynthetic membranes of plants are rich in nonphosphorus glycolipids, including galactolipids, mono- and digalactosyldiacylglycerol (MGDG and DGDG), and sulfolipid, sulfoquinovosyldiacylglycerol (SQDG)
Genes that coexpressed with at least two of these four genes were selected based on their correlation coefficient (>0.50 with all conditions), considering annotations for gene functions. These analyses revealed that an unknown gene, At3g56040, and two genes coding putative lipases (At1g08310 and At2g42690) were coexpressed with SQD1, SQD2, MGD2, and MGD3 (Figure 2), suggesting the involvement of these genes in the biosynthesis of SQDG or galactolipid
The prediction that UDP-glucose pyrophosphorylase3 (UGP3) was involved in SQDG biosynthesis was reinforced by examining the tissue-specific expression patterns of transcripts of UGP3, SQD1, and SQD2; Arabidopsis eFP-Browser (Winter et al, 2007) showed that these three genes are coordinately expressed in leaves, stems, and flowers, supporting the close relationship among these genes

Summary

Introduction

The photosynthetic membranes of plants are rich in nonphosphorus glycolipids, including galactolipids, mono- and digalactosyldiacylglycerol (MGDG and DGDG), and sulfolipid, sulfoquinovosyldiacylglycerol (SQDG). In SQDG-deficient mutants, the proportion of PG does not decrease under phosphate limitation, and these mutants become phosphate starved sooner than their respective wild types (Benning et al, 1993; Guler et al, 1996; Yu et al, 2002; Riekhof et al, 2003). SQDG is one of the primary sulfur-containing organic compounds in higher plants, but it remains unclear whether it plays the same role in higher plants as observed in C. reinhardtii

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