Synthesis of beta-glucans in Prototheca zopfii. Isolation and characterization of the glycoprotein primer

Species- and strain-specific differences in the phagocytosis of Prototheca: insights from live-cell imaging

The genus Prototheca is an extremely unusual group of achlorophyllic, obligately heterotrophic algae. Six species have been identified as pathogens of vertebrates, including cattle and humans. In cattle, P. bovis is the main infectious pathogen and is associated with bovine mastitis. In contrast, human infections typically involve P. wickerhamii and are associated with a spectrum of varying clinical presentations. Prototheca spp. enter the host from the environment and are therefore likely to be initially recognized by cells of the innate immune system. However, little is known about the nature of the interaction between Prototheca spp. and host phagocytes. In the present study, we adopt a live-cell imaging approach to investigate these interactions over time. Using environmental and clinical strains, we show that P. bovis cells are readily internalized and processed by macrophages, whereas these immune cells struggle to internalize P. wickerhamii. Serum opsonization of P. wickerhamii only marginally improves phagocytosis, suggesting that this species (but not P. bovis) may have evolved mechanisms to evade phagocytosis. Furthermore, we show that inhibition of the kinases Syk or PI3K, which are both critical for innate immune signaling, drastically reduces the uptake of P. bovis. Finally, we show that genetic ablation of MyD88, a signaling adaptor critical for Toll-like receptor signaling, has little impact on uptake but significantly prolongs phagosome maturation once P. bovis is internalized. Together, our data suggest that these two pathogenic Prototheca spp. have very different host-pathogen interactions which have potential therapeutic implications for the treatment of human and animal disease.

Galactomannan enzymatic immunoassay cross-reactivity caused by Prototheca species

We report a reactive Aspergillus galactomannan enzymatic immunoassay against the serum of a patient with invasive Prototheca zopfii infection. Analysis of the supernatants of suspensions of P. zopfii and other Prototheca isolates revealed positive results as well. These data suggest cross-reactivity with the serum Aspergillus galactomannan assay in invasive protothecosis.

1H, 15N and 13C NMR assignments, secondary structure and overall topology of the Escherichia coli GlgS protein

GlgS is a 7892-Da protein which is involved in glycogen biosynthesis in bacteria. We report the 1H, 15N and 13C NMR assignments of the backbone and side-chain resonances at 25 degrees C and pH 6.7 from two-dimensional homonuclear and three-dimensional heteronuclear NMR experiments. The secondary structure of the protein was determined using sequential and medium-range NOE correlations, vicinal 3J(NH-H alpha) coupling values and amide proton exchange rates. The secondary structure obtained is consistent with the secondary chemical shifts of 1H alpha, 13C alpha and 13C = O. It was found that the secondary structure of GlgS comprises two amphipathic helices (Asn10-Met21 and Glu39-Arg60), one short highly hydrophobic helix (Ile30-Val33), a short extended beta-strand-like fragment (Arg26-Asp29) and two type I beta-turns (His22-Gly25 and Thr34-Met37). An overall topology of GlgS is suggested based on long-range NOEs. The elements of secondary structure form a sandwich in which the beta-strand and the short hydrophobic helix are positioned between the two amphipathic helices.

Requirement of the self-glucosylating initiator proteins Glg1p and Glg2p for glycogen accumulation in Saccharomyces cerevisiae

Glycogen, a branched polymer of glucose, is a storage molecule whose accumulation is under rigorous nutritional control in many cells. We report the identification of two Saccharomyces cerevisiae genes, GLG1 and GLG2, whose products are implicated in the biogenesis of glycogen. These genes encode self-glucosylating proteins that in vitro can act as primers for the elongation reaction catalyzed by glycogen synthase. Over a region of 258 residues, the Glg proteins have 55% sequence identify to each other and approximately 33% identity to glycogenin, a mammalian protein postulated to have a role in the initiation of glycogen biosynthesis. Yeast cells defective in either GLG1 or GLG2 are similar to the wild type in their ability to accumulate glycogen. Disruption of both genes results in the inability of the cells to synthesize glycogen despite normal levels of glycogen synthase. These results suggest that a self-glucosylating protein is required for glycogen biosynthesis in a eukaryotic cell. The activation state of glycogen synthase in glg1 glg2 cells is suppressed, suggesting that the Glg proteins may additionally influence the phosphorylation state of glycogen synthase.

The discovery of glycogenin and the priming mechanism for glycogen biogenesis

The biogenesis of glycogen in skeletal muscle requires a priming mechanism that has recently been elucidated. The first step is catalysed by a protein tyrosine glucosyltransferase and involves the formation of a novel glycosidic linkage, namely the covalent attachment of glucose to a single tyrosine residue (Tyr194) on a priming protein, termed glycogenin. The next stage is the extension of the glucan chain from Tyr194 and involves the sequential addition of up to seven further glucosyl residues. This reaction is brought about autocatalytically by glycogenin itself, which is a Mn2+/Mg(2+)-dependent UDP-Glc-requiring glucosyltransferase. The glucan primer is elongated by glycogen synthase, but only when glycogenin and glycogen synthase are complexed together. Glycogen synthase dissociates from glycogenin during the synthesis of a glycogen molecule, enabling glycogen molecules to reach their maximum theoretical size. Each mature glycogen beta particle in muscle contains one molecule of glycogenin attached covalently, and an average one glycogen synthase catalytic subunit bound non-covalently. As evidence accumulates that a priming protein may be a fundamental property of polysaccharide synthesis in general, the molecular details of mammalian glycogen biogenesis may serve as a useful model for other systems.

Synthesis of beta-glucans in Prototheca zopfii. Evidence for the existence of a glycoprotein primer

Membrane preparations from the non-photosynthetic alga Prototheca zopfii incorporate glucose from UDP-[3H]glucose into the trichloroacetic-acid-insoluble fraction and the polysaccharides insoluble in hot alkali. Time course and pulse-chase experiments indicate that the acid-insoluble fraction was a precursor of the alkali-insoluble fraction. Isolation of 3H-labeled membrane or soluble fraction showed that only membrane fractions were able to transfer radioactivity into polysaccharides. Treatment of glucosylated membranes with trypsin or cellulase only partially affect their transfer ability, indicating that the precursor was internalized in vesicles. Analysis of the in vitro synthesized polysaccharides by enzymatic and acid hydrolysis showed that glucose and cellobiose were present as radioactive sugars. Permethylation of the polysaccharide indicates that 80% of the glucose was beta-1,4-bonded with 20% in beta-1,3-linkages. This polysaccharide was found to be identical with the cell-wall beta-glucan obtained in vivo [Rivas, L.A. & Pont Lezica, R. (1978) Planta (Berl.) 165, 348-353].