The Function of Rice Starch Synthase I Expressed in Escherichia coli

Genetic diversity, functional properties and expression analysis of NnSBE genes involved in starch synthesis of lotus (Nelumbo nucifera Gaertn.)

Background

Starch branching enzyme (SBE) is one of the key enzymes in starch biosynthetic metabolism, determining amylopectin structure.

Methods

Full length coding sequences (CDS) of SBE genes were cloned using reverse transcription PCR (RT-PCR) technology, and neighbor-joining (NJ) tree was used for phylogenetic analysis. Single nucleotide polymorphisms (SNPs) were determined to assess the genetic polymorphisms and variation indexes between individuals and clusters. Quantitative real time PCR (qRT-PCR) was performed to analyze the spatial and temporal expression of NnSBE genes. The effect of NnSBE genes on amylopectin’s fine structures was explored using affinity and the enzyme activity analysis of two isoforms in amylopectin and amylose.

Results

In this study, two SBE family genes, NnSBEI and NnSBEIII, were identified in lotus (Nelumbo nucifera Gaertn.). Phylogenetic analysis sorted NnSBEI into SBE family B and NnSBEIII into SBE family A. UPGMA phylogenetic tree divided 45 individuals of lotus into three classes. The homozygous haplotype (A G G A G) of NnSBEIII was observed in seed lotus. During the seed embryo development stage, NnSBEIII reached the peak in the middle of the development stage, while NnSBEI increased in the mid-late developmental stage. The different affinity activity of the two isozymes binding amylopectin and amylose assay indicated NnSBEI has higher activity and wider affinity.

Discussion

Genetic diversity showed that NnSBE genes received artificial selection during the process of cultivation and domestication in lotus seeds. Furthermore, the expression pattern and affinity activity analysis indicated that NnSBE genes were related to the chain length of amylopectin.

Characterization of the functional interactions of plastidial starch phosphorylase and starch branching enzymes from rice endosperm during reserve starch biosynthesis

Functional interactions of plastidial phosphorylase (Pho1) and starch branching enzymes (BEs) from the developing rice endosperm are the focus of this study. In the presence of both Pho1 and BE, the same branched primer molecule is elongated and further branched almost simultaneously even at very low glucan concentrations present in the purified enzyme preparations. By contrast, in the absence of any BE, glucans are not, to any significant extent, elongated by Pho1. Based on our in vitro data, in the developing rice endosperm, Pho1 appears to be weakly associated with any of the BE isozymes. By using fluorophore-labeled malto-oligosaccharides, we identified maltose as the smallest possible primer for elongation by Pho1. Linear dextrins act as carbohydrate substrates for BEs. By functionally interacting with a BE, Pho1 performs two essential functions during the initiation of starch biosynthesis in the rice endosperm: First, it elongates maltodextrins up to a degree of polymerization of at least 60. Second, by closely interacting with BEs, Pho1 is able to elongate branched glucans efficiently and thereby synthesizes branched carbohydrates essential for the initiation of amylopectin biosynthesis.

Underlying Mechanisms of Zymographic Diversity in Starch Synthase I and Pullulanase in Rice-Developing Endosperm

Amylopectin is synthesized by the coordinated actions of many (iso)enzymes, including ADP-glucose pyrophosphorylase (AGPase), starch synthases (SSs), branching enzymes (BEs), and debranching enzymes (DBEs). Here, two polymorphic forms of starch synthase I (SSI) and pullulanase (PUL) in rice-developing seeds, designated as SSI-1/SSI-2 and PUL-1/PUL-2, were discovered for the first time by zymographic analysis. The SSI and PUL polymorphisms were strongly associated with the SSI microsatellite marker (p = 3.6 × 10(-37)) and PUL insertion/deletion (InDel) markers (p < 3.6 × 10(-51)). Western blotting and mass spectrometric analysis confirmed that the polymorphic bands were truly the SSI and PUL enzymes. Only one non-synonymous variation in SSI DNA sequence (the SNP A/G) causing the change of the amino acid K438 to E438 was observed, which coincided well with the polymorphic forms of SSI. Nine non-synonymous variations were found between PUL-1 and PUL-2. Two non-synonymous variations of PUL (F316L and D770E) were identified by mass spectrometric analysis, but all of the variations did not change the structure of PUL. The co-immunoprecipitation results revealed the differences in protein-protein interaction patterns, i.e., strong or weaker signals of SSI-BEI and SSI-BEIIb, between the two forms of SSI. The results will enhance our understanding of SSI and PUL properties and provide helpful information to understand their functions in starch biosynthesis in rice endosperm.

Amylopectin biosynthetic enzymes from developing rice seed form enzymatically active protein complexes

Amylopectin is a highly branched, organized cluster of glucose polymers, and the major component of rice starch. Synthesis of amylopectin requires fine co-ordination between elongation of glucose polymers by soluble starch synthases (SSs), generation of branches by branching enzymes (BEs), and removal of misplaced branches by debranching enzymes (DBEs). Among the various isozymes having a role in amylopectin biosynthesis, limited numbers of SS and BE isozymes have been demonstrated to interact via protein-protein interactions in maize and wheat amyloplasts. This study investigated whether protein-protein interactions are also found in rice endosperm, as well as exploring differences between species. Gel permeation chromatography of developing rice endosperm extracts revealed that all 10 starch biosynthetic enzymes analysed were present at larger molecular weights than their respective monomeric sizes. SSIIa, SSIIIa, SSIVb, BEI, BEIIb, and PUL co-eluted at mass sizes >700kDa, and SSI, SSIIa, BEIIb, ISA1, PUL, and Pho1 co-eluted at 200-400kDa. Zymogram analyses showed that SSI, SSIIIa, BEI, BEIIa, BEIIb, ISA1, PUL, and Pho1 eluted in high molecular weight fractions were active. Comprehensive co-immunoprecipitation analyses revealed associations of SSs-BEs, and, among BE isozymes, BEIIa-Pho1, and pullulanase-type DBE-BEI interactions. Blue-native-PAGE zymogram analyses confirmed the glucan-synthesizing activity of protein complexes. These results suggest that some rice starch biosynthetic isozymes are physically associated with each other and form active protein complexes. Detailed analyses of these complexes will shed light on the mechanisms controlling the unique branch and cluster structure of amylopectin, and the physicochemical properties of starch.

Analysis of the functional interaction of Arabidopsis starch synthase and branching enzyme isoforms reveals that the cooperative action of SSI and BEs results in glucans with polymodal chain length distribution similar to amylopectin

Starch synthase (SS) and branching enzyme (BE) establish the two glycosidic linkages existing in starch. Both enzymes exist as several isoforms. Enzymes derived from several species were studied extensively both in vivo and in vitro over the last years, however, analyses of a functional interaction of SS and BE isoforms are missing so far. Here, we present data from in vitro studies including both interaction of leaf derived and heterologously expressed SS and BE isoforms. We found that SSI activity in native PAGE without addition of glucans was dependent on at least one of the two BE isoforms active in Arabidopsis leaves. This interaction is most likely not based on a physical association of the enzymes, as demonstrated by immunodetection and native PAGE mobility analysis of SSI, BE2, and BE3. The glucans formed by the action of SSI/BEs were analysed using leaf protein extracts from wild type and be single mutants (Atbe2 and Atbe3 mutant lines) and by different combinations of recombinant proteins. Chain length distribution (CLD) patterns of the formed glucans were irrespective of SSI and BE isoforms origin and still independent of assay conditions. Furthermore, we show that all SS isoforms (SSI-SSIV) were able to interact with BEs and form branched glucans. However, only SSI/BEs generated a polymodal distribution of glucans which was similar to CLD pattern detected in amylopectin of Arabidopsis leaf starch. We discuss the impact of the SSI/BEs interplay for the CLD pattern of amylopectin.

In vitro studies of enzymatic properties of starch synthases and interactions between starch synthase I and starch branching enzymes from rice

The present study was conducted to characterize the functions of the major starch synthase (SS) isozymes SSI, SSIIa, and SSIIIa in rice endosperm and their functional interaction with starch branching enzyme (BE), by using their purified recombinant proteins. All the SS isozymes had similarly significant activities toward branched glucans such as amylopecin and glycogen whereas they scarcely showed activities toward maltohexaose. In vitro studies indicate that SSI mainly attacked A and B chains with degree of polymerization (DP) of 6 and 7 in their external segments and elongated them to DP8. It is likely that SSIIa and SSIIIa produced wider ranges of intermediate chains and long chains, respectively. This study also revealed that without addition of exogenous primer, the glucan synthesis of SSI in the presence of ≧0.3 M citrate was accelerated by the addition of any of the rice BE isozymes- BEI, BEIIa, or BEIIb, whereas no such interaction occurred between SSIIa or SSIIIa with any of the BEs. The SSI-BE unprimed glucan synthesis absolutely required citrate. The interaction between SSI and BE was established by stimulation of SSI activity with BE and by activation of the BE activity by SSI.

Relationships between starch synthase I and branching enzyme isozymes determined using double mutant rice lines

BACKGROUND

Starch is the most important carbohydrate in plant storage tissues. Multiple isozymes in at least four enzyme classes are involved in starch biosynthesis. Some of these isozymes are thought to interact and form complexes for efficient starch biosynthesis. Of these enzyme classes, starch synthases (SSs) and branching enzymes (BEs) play particularly central roles.

RESULTS

We generated double mutant lines (ss1/be1 and ss1L/be2b) between SSI (the largest component of total soluble SS activity) and BEI or BEIIb (major BEs in developing rice endosperm) to explore the relationships among these isozymes. The seed weight of ss1/be1 was comparable to that of wild type, although most ss1/be2b seeds were sterile and no double recessive plants were obtained. The seed weight of the double recessive mutant line ss1L/be2b, derived from the leaky ss1 mutant (ss1L) and be2b, was higher than that of the single be2b mutant. Analyses of the chain-length distribution of amylopectin in ss1/be1 endosperm revealed additive effects of SSI and BEI on amylopectin structure. Chain-length analysis indicated that the BEIIb deficiency significantly reduced the ratio of short chains in amylopectin of ss1L/be2b. The amylose content of endosperm starch of ss1/be1 and ss1L/be2b was almost the same as that of wild type, whereas the endosperm starch of be2b contained more amylose than did that of wild type. SSI, BEI, and BEIIb deficiency also affected the extent of binding of other isozymes to starch granules.

CONCLUSIONS

Analysis of the chain-length distribution in amylopectin of the double mutant lines showed that SSI and BEI or BEIIb primarily function independently, and branching by BEIIb is followed by SSI chain elongation. The increased amylose content in be2b was because of reduced amylopectin biosynthesis; however, the lower SSI activity in this background may have enhanced amylopectin biosynthesis as a result of a correction of imbalance between the branching and elongation found in the single mutant. The fact that a deficiency of SSI, BEI, or BEIIb affected the affinity of other starch biosynthetic isozymes for the starch granule implies that there is a close interaction among SSI, BEI and BEIIb during amylopectin biosynthesis in rice endosperm.

Elongated phytoglycogen chain length in transgenic rice endosperm expressing active starch synthase IIa affects the altered solubility and crystallinity of the storage α-glucan

The relationship between the solubility, crystallinity, and length of the unit chains of plant storage α-glucan was investigated by manipulating the chain length of α-glucans accumulated in a rice mutant. Transgenic lines were produced by introducing a cDNA for starch synthase IIa (SSIIa) from an indica cultivar (SSIIa (I), coding for active SSIIa) into an isoamylase1 (ISA1)-deficient mutant (isa1) that was derived from a japonica cultivar (bearing inactive SSIIa proteins). The water-soluble fraction accounted for >95% of the total α-glucan in the isa1 mutant, whereas it was only 35-70% in the transgenic SSIIa (I)/isa1 lines. Thus, the α-glucans from the SSIIa (I)/isa1 lines were fractionated into soluble and insoluble fractions prior to the following characterizations. X-ray diffraction analysis revealed a weak B-type crystallinity for the α-glucans of the insoluble fraction, while no crystallinity was confirmed for α-glucans in isa1. Concerning the degree of polymerization (DP) ≤30, the chain lengths of these α-glucans differed significantly in the order of SSIIa (I)/isa1 insoluble > SSIIa (I)/isa1 soluble > α-glucans in isa1. The amount of long chains with DP ≥33 was higher in the insoluble fraction α-glucans than in the other two α-glucans. No difference was observed in the chain length distributions of the β-amylase limit dextrins among these α-glucans. These results suggest that in the SSIIa (I)/isa1 transgenic lines, the unit chains of α-glucans were elongated by SSIIa(I), whereas the expression of SSIIa(I) did not affect the branch positions. Thus, the observed insolubility and crystallinity of the insoluble fraction can be attributed to the elongated length of the outer chains due to SSIIa(I).

Starch biosynthesis in rice endosperm requires the presence of either starch synthase I or IIIa

Starch synthase (SS) I and IIIa are the first and second largest components of total soluble SS activity, respectively, in developing japonica rice (Oryza sativa L.) endosperm. To elucidate the distinct and overlapping functions of these enzymes, double mutants were created by crossing the ss1 null mutant with the ss3a null mutant. In the F(2) generation, two opaque seed types were found to have either the ss1ss1/SS3ass3a or the SS1ss1/ss3ass3a genotype. Phenotypic analyses revealed lower SS activity in the endosperm of these lines than in those of the parent mutant lines since these seeds had different copies of SSI and SSIIIa genes in a heterozygous state. The endosperm of the two types of opaque seeds contained the unique starch with modified fine structure, round-shaped starch granules, high amylose content, and specific physicochemical properties. The seed weight was ∼90% of that of the wild type. The amount of granule-bound starch synthase I (GBSSI) and the activity of ADP-glucose pyrophosphorylase (AGPase) were higher than in the wild type and parent mutant lines. The double-recessive homozygous mutant prepared from both ss1 and ss3a null mutants was considered sterile, while the mutant produced by the leaky ss1 mutant×ss3a null mutant cross was fertile. This present study strongly suggests that at least SSI or SSIIIa is required for starch biosynthesis in rice endosperm.