Purification and characterization of recombinant sugarcane sucrose phosphate synthase expressed in E. coli and insect Sf9 cells: an importance of the N-terminal domain for an allosteric regulatory property

Current perspectives on the regulatory mechanisms of sucrose accumulation in sugarcane

Sugars transported from leaves (source) to stems (sink) energize cell growth, elongation, and maintenance. which are regulated by a variety of genes. This review reflects progress and prospects in the regulatory mechanism for maximum sucrose accumulation, including the role of sucrose metabolizing enzymes, sugar transporters and the elucidation of post-transcriptional control of sucrose-induced regulation of translation (SIRT) in the accumulation of sucrose. The current review suggests that SIRT is emerging as a significant mechanism controlling Scbzip44 activities in response to endogenous sugar signals (via the negative feedback mechanism). Sucrose-controlled upstream open reading frame (SC-uORF) exists at the 5' leader region of Scbzip44's main ORF, which inhibits sucrose accumulation through post-transcriptional regulatory mechanisms. Sucrose transporters (SWEET1a/4a/4b/13c, TST, SUT1, SUT4 and SUT5) are crucial for sucrose translocation from source to sink. Particularly, SWEET13c was found to be a major contributor to the efflux in the transportation of stems. Tonoplast sugar transporters (TSTs), which import sucrose into the vacuole, suggest their tissue-specific role from source to sink. Sucrose cleavage has generally been linked with invertase isozymes, whereas sucrose synthase (SuSy)-catalyzed metabolism has been associated with biosynthetic processes such as UDP-Glc, cellulose, hemicellulose and other polymers. However, other two key sucrose-metabolizing enzymes, such as sucrose-6-phosphate phosphohydrolase (S6PP) and sucrose phosphate synthase (SPS) isoforms, have been linked with sucrose biosynthesis. These findings suggest that manipulation of genes, such as overexpression of SPS genes and sucrose transporter genes, silencing of the SC-uORF of Scbzip44 (removing the 5' leader region of the main ORF that is called SIRT-Insensitive) and downregulation of the invertase genes, may lead to maximum sucrose accumulation. This review provides an overview of sugarcane sucrose-regulating systems and baseline information for the development of cultivars with higher sucrose accumulation.

Mutation of UDP-glucose binding motif residues lead to increased affinity for ADP-glucose in sugarcane sucrose phosphate synthase

Plant sucrose-phosphate synthase (SPS) contains a glycosyltransferase domain, which specifically catalyzes reactions with the nucleotide sugar uridine diphosphate glucose (UDP-G) as a donor substrate. Unlike plant SPS, bacterial SPS is predicted to bind other nucleotide sugars, such as adenosine diphosphate glucose (ADP-G). This study aimed to identify the UDP-G binding site of sugarcane (Saccharum officinarum) SPS (SoSPS1) and to improve its affinity for ADP-G by site-directed mutagenesis. To achieve targeted mutagenesis, amino acid distribution and comparative modeling studies were performed, followed by site-directed mutagenesis of SoSPS1 in the putative UDP-G binding motif. The N-terminal deletion of SoSPS1 (∆N-SoSPS1) was used for enzymatic analysis. The results showed that mutations in the R-X₄-K, E-X₇-E, and H-X₅-V motifs significantly affect UDP-G and ADP-G binding. Mutations at R496 and K501 severely attenuate the affinity for UDP-G. Additionally, alanine substitutions at E591 and V570 decreased the UDP-G affinity but remarkably increased its ADP-G affinity. The R-X₄-K motif plays a crucial role in the UDP-G binding site and catalytic activity of plant SPS; thus, its alteration to other amino acids was not viable. The E-X₇-E and H-X₅-V motifs may bind to the nucleotide glucose substrate, indicating that these motifs are involved in substrate specificity. These results agree with substrate docking simulations at the mutated residue positions, supporting the experimental results. These results demonstrate that mutation of E591 and V570 severely attenuated the UDP-G affinity, while retaining its activity against ADP-G, offering strategic insights into increasing sucrose synthesis and plant growth.

The tertiary structure of the human Xkr8-Basigin complex that scrambles phospholipids at plasma membranes

Xkr8-Basigin is a plasma membrane phospholipid scramblase activated by kinases or caspases. We combined cryo-EM and X-ray crystallography to investigate its structure at an overall resolution of 3.8 Å. Its membrane-spanning region carrying 22 charged amino acids adopts a cuboid-like structure stabilized by salt bridges between hydrophilic residues in transmembrane helices. Phosphatidylcholine binding was observed in a hydrophobic cleft on the surface exposed to the outer leaflet of the plasma membrane. Six charged residues placed from top to bottom inside the molecule were essential for scrambling phospholipids in inward and outward directions, apparently providing a pathway for their translocation. A tryptophan residue was present between the head group of phosphatidylcholine and the extracellular end of the path. Its mutation to alanine made the Xkr8-Basigin complex constitutively active, indicating that it plays a vital role in regulating its scramblase activity. The structure of Xkr8-Basigin provides insights into the molecular mechanisms underlying phospholipid scrambling.

Comparative analysis of sucrose phosphate synthase (SPS) gene family between Saccharum officinarum and Saccharum spontaneum

BACKGROUND

Sucrose phosphate synthase (SPS) genes play vital roles in sucrose production across various plant species. Modern sugarcane cultivar is derived from the hybridization between the high sugar content species Saccharum officinarum and the high stress tolerance species Saccharum spontaneum, generating one of the most complex genomes among all crops. The genomics of sugarcane SPS remains under-studied despite its profound impact on sugar yield.

RESULTS

In the present study, 8 and 6 gene sequences for SPS were identified from the BAC libraries of S. officinarum and S. spontaneum, respectively. Phylogenetic analysis showed that SPSD was newly evolved in the lineage of Poaceae species with recently duplicated genes emerging from the SPSA clade. Molecular evolution analysis based on Ka/Ks ratios suggested that polyploidy reduced the selection pressure of SPS genes in Saccharum species. To explore the potential gene functions, the SPS expression patterns were analyzed based on RNA-seq and proteome dataset, and the sugar content was detected using metabolomics analysis. All the SPS members presented the trend of increasing expression in the sink-source transition along the developmental gradient of leaves, suggesting that the SPSs are involved in the photosynthesis in both Saccharum species as their function in dicots. Moreover, SPSs showed the higher expression in S. spontaneum and presented expressional preference between stem (SPSA) and leaf (SPSB) tissue, speculating they might be involved in the differentia of carbohydrate metabolism in these two Saccharum species, which required further verification from experiments.

CONCLUSIONS

SPSA and SPSB genes presented relatively high expression and differential expression patterns between the two Saccharum species, indicating these two SPSs are important in the formation of regulatory networks and sucrose traits in the two Saccharum species. SPSB was suggested to be a major contributor to the sugar accumulation because it presented the highest expressional level and its expression positively correlated with sugar content. The recently duplicated SPSD2 presented divergent expression levels between the two Saccharum species and the relative protein content levels were highest in stem, supporting the neofunctionalization of the SPSD subfamily in Saccharum.

Overexpression of Sucrose Phosphate Synthase Enhanced Sucrose Content and Biomass Production in Transgenic Sugarcane

Sucrose phosphate synthase (SPS) is a key enzyme in sucrose synthesis, which controls sucrose content in plants. This study was designed to examine the efficacy of the overexpression of SoSPS1 gene on sucrose accumulation and carbon partitioning in transgenic sugarcane. The overexpression of SoSPS1 gene increased SPS activity and sucrose content in transgenic sugarcane leaves. More importantly, the overexpression enhanced soluble acid invertase (SAI) activity concomitant with the increase of glucose and fructose levels in the leaves, whereas sucrose synthase activity exhibited almost no change. In the stalk, a similar correlation was observed, but a higher correlation was noted between SPS activity and sugar content. These results suggest that SPS overexpression has both direct and indirect effects on sugar concentration and SAI activity in sugarcane. In addition, SPS overexpression resulted in a significant increase in plant height and stalk number in some transgenic lines compared to those in non-transgenic control. Taken together, these results strongly suggest that enhancing SPS activity is a useful strategy for improving sugarcane yield.

Identification of UDP-glucose binding site in glycosyltransferase domain of sucrose phosphate synthase from sugarcane (Saccharum officinarum) by structure-based site-directed mutagenesis

Sucrose phosphate synthase (SPS) is believed to be the key enzyme for controlling the biosynthesis of sucrose. SPSs consist of a functional glycosyltransferase domain that shares conserved residues with the glycosyltransferase domain of sucrose biosynthesis-related protein. The formation of sucrose-6-phosphate is catalyzed by SPS with the transfer of a glycosyl group of uridine diphosphate glucose (UDP-G) as an activated donor sugar to a fructose-6-phosphate as a sugar acceptor. However, understanding of the mechanism of catalytic and substrate binding in SPS is very limited. Based on amino acid sequence alignments with several enzymes that belong to the glycosyltransferase family, the UDP-G binding sites that might be critical for catalytic mechanism were identified. Here, we report that single point mutation of R496, D498, and V570 located in the proposed UDP-G binding site led to less active or complete loss of enzyme activity. Through structure-based site-directed mutagenesis and biochemical studies, the results indicated that these residues contribute to the catalytic activity of plant SPS. Moreover, understanding of the UDP-G binding site provides an insight into new strategies for enzyme engineering and redesigning a catalytic mechanism for UDP.