Starch biosynthetic enzymes form multi-protein complexes consisting of starch synthase (SS) I, SSIIa, and starch branching enzyme (BE) IIb, which synthesize amylopectin clusters. This study analyzed the starch properties in two double mutant rice lines lacking SSIIa and BEIIb, one of which expressed an inactive BEIIb protein. The ss2a be2b lines showed similar or greater seed weight than the be2b lines, and plant growth was not affected. The ss2a line showed increased short amylopectin chains resulting in a lower gelatinization temperature. Starch granule morphology and A-type crystallinity were similar between the ss2a line and the wild type, except for a mild chalky seed phenotype in the ss2a line. However, the starch phenotype of the ss2a be2b lines, which was similar to that of be2b but not ss2a, was characterized by increased long amylopectin chains, abnormal starch granules, and B-type crystallinity. The similarity in phenotype between the ss2a be2b and be2b lines may be attributed to the inability of the be2b mutants to generate short amylopectin branches, which serve as primers for SSIIa. Therefore, the presence or absence of SSIIa hardly affected the amylopectin structure under the be2b background. The amylose content was significantly higher in the ss2a be2b lines than in the be2b lines. Starch crystallinity was greater in ss2a be2b lines than in be2b lines, despite the fact that starch crystallinity is generally negatively correlated with amylose content. This suggests that the formation of a double helix between long amylopectin chains and amylose affects starch crystallinity in the ss2a be2b mutants.
Glycogen is a highly branched storage polysaccharide found mainly in the liver and the muscles. Glycogen is also present in the skin, but its functional role is poorly understood. Recently, it has been reported that glycogen plays an important role in intracellular signal transduction. In the epidermis of the skin, keratinocytes are the predominant cells that produce ceramide. Ceramides are lipids composed of sphingosine, and prevent water loss, as well as protecting the skin against environmental stressors. In this study, we investigated the effects of glycogen on ceramide production in cultured keratinocytes. Thin-layer chromatography revealed that incubation of keratinocytes with 2 % glycogen enhanced the cellular amount of ceramide NS (ceramide 2) by 3.4-fold compared to the control. We also found that glycogen regulated the mRNA expression levels of signaling molecules of the sphingomyelin-ceramide pathway by quantitative real-time PCR. The activity of sphingomyelinase was also significantly enhanced by 2.5-fold in cultures with 1 % glycogen compared to the control. Moreover, glycogen increased the ATP production by 1.5-fold compared to the control, while glucose did not affect the production. Western blotting showed that phosphorylation of Akt, a cellular signaling molecule, was inhibited in the presence of glycogen in cultured keratinocytes. This study shows that glycogen upregulates the ceramide production pathway from sphingomyelin in epidermal keratinocytes, and provides new insights into the role of glycogen in cellular signal transduction.
We recently characterized a 3-O-α-D-galactosyl-α-L-arabinofuranosidase (GAfase) for the release of α-D-Gal-(1→3)-L-Ara from gum arabic arabinogalactan protein (AGP) in Bifidobacterium longum subsp. longum JCM7052. In the present study, we cloned and characterized a neighboring α-galactosidase gene (BLGA_00330; blAga3). It contained an Open Reading Frame of 2151-bp nucleotides encoding 716 amino acids with an estimated molecular mass of 79,587 Da. Recombinant BlAga3 released galactose from α-D-Gal-(1→3)-L-Ara, but not from intact gum arabic AGP, and a little from the related oligosaccharides. The enzyme also showed the activity toward blood group B liner trisaccharide. The specific activity for α-D-Gal-(1→3)-L-Ara was 4.27- and 2.10-fold higher than those for melibiose and raffinose, respectively. The optimal pH and temperature were 6.0 and 50 °C, respectively. BlAga3 is an intracellular α-galactosidase that cleaves α-D-Gal-(1→3)-L-Ara produced by GAfase; it is also responsible for a series of gum arabic AGP degradation in B. longum JCM7052.