No abstract

The SQD1 enzyme is believed to be involved in the biosynthesis of the sulfoquinovosyl headgroup of plant sulfolipids, catalyzing the transfer of SO 3 ؊ to UDP-glucose. We have determined the structure of the complex of SQD1 from Arabidopsis thaliana with NAD ؉ and the putative substrate UDP-glucose at 1.6-Å resolution. Both bound ligands are completely buried within the binding cleft, along with an internal solvent cavity which is the likely binding site for the, as yet, unidentified sulfur-donor substrate. SQD1 is a member of the short-chain dehydrogenase͞reductase (SDR) family of enzymes, and its structure shows a conservation of the SDR catalytic residues. Among several highly conserved catalytic residues, Thr-145 forms unusually short hydrogen bonds with both susceptible hydroxyls of UDP-glucose. A His side chain may also be catalytically important in the sulfonation.T he sulfolipid sulfoquinovosyldiacylglycerol (SQDG) is common to all plants and many photosynthetic bacteria (1) and is found exclusively in the photosynthetic (thylakoid) membranes. SQDG-deficient null mutants from different photosynthetic bacteria exhibit a conditionally lethal phenotype under phosphate-limiting growth conditions (2, 3), suggesting that SQDG can substitute for phosphatidylglycerol under these conditions (1). This hypothesis also seems to be valid for plants such as Arabidopsis thaliana, for which concomitant changes in thylakoid membrane lipid composition and in the regulation of sulfolipid gene expression have been observed in response to phosphate starvation (4). Beyond its role in photosynthetic membranes, SQDG has been found to inhibit retroviral infections of mammalian cells and specifically the viral reverse transcriptase (5, 6).Genes essential for the biosynthesis of the SQDG were isolated from photosynthetic bacteria by genetic complementation of mutants (7,8). The amino acid sequence of the product of one of these genes, sqdB, is highly conserved among different organisms and a plant ortholog, SQD1, has been recently isolated from A. thaliana and expressed in Escherichia coli (4). The SQDB͞SQD1 proteins show modest sequence similarity to sugar-nucleotide enzymes (4), and this similarity has been recently exploited by Essigmann and coworkers (9) to predict the structure of the A. thaliana SQD1 protein. As they also demonstrated that SQD1 binds NAD ϩ and contains characteristic conserved Y-XXX-K and glycine-rich (G-XX-G-XX-G) sequence patterns, SQD1 appears to be a member of the shortchain dehydrogenase͞reductase (SDR) family (10-12).The sugar-nucleotide UDP-sulfoquinovose is thought to be the headgroup donor for SQDG biosynthesis (1), a conclusion supported by in situ labeling experiments with isolated chloroplasts and using synthetic UDP-sulfoquinovose (13,14) and by the discovery of UDP-sulfoquinovose in bacterial sulfolipid mutants and other organisms (15, 16). Pugh et al. (17) proposed a metabolic pathway for SQDG biosynthesis in which UDPglucose is first converted to a UDP-4-ketoglucose-5-ene intermediate, with ...