Abstract
In certain maize genotypes, called "null," beta-glucosidase does not enter gels and therefore cannot be detected on zymograms after electrophoresis. Such genotypes were originally thought to be homozygous for a null allele at the glu1 gene and thus devoid of enzyme. We have shown that a beta-glucosidase-aggregating factor (BGAF) is responsible for the "null" phenotype. BGAF is a chimeric protein consisting of two distinct domains: the disease response or "dirigent" domain and the jacalin-related lectin (JRL) domain. First, it was not known whether the lectin domain in BGAF is functional. Second, it was not known which of the two BGAF domains is involved in beta-glucosidase binding and aggregation. To this end, we purified BGAF to homogeneity from a maize null inbred line called H95. The purified protein gave a single band on SDS-PAGE, and the native protein was a homodimer of 32-kDa monomers. Native and recombinant BGAF (produced in Escherichia coli) agglutinated rabbit erythrocytes, and various carbohydrates and glycoproteins inhibited their hemagglutination activity. Sugars did not have any effect on the binding of BGAF to the beta-glucosidase isozyme 1 (Glu1), and the BGAF-Glu1 complex could still bind lactosyl-agarose, indicating that the sugar-binding site of BGAF is distinct from the beta-glucosidase-binding site. Neither the dirigent nor the JRL domains alone (produced separately in E. coli) produced aggregates of Glu1 based on results from pull-down assays. However, gel shift and competitive binding assays indicated that the JRL domain binds beta-glucosidase without causing it to aggregate. These results with those from deletion mutagenesis and replacement of the JRL domain of a BGAF homolog from sorghum, which does not bind Glu1, with that from maize allowed us to conclude that the JRL domain of BGAF is responsible for its lectin and beta-glucosidase binding and aggregating activities.
Highlights
-Glucosidases (-D-glucoside glucohydrolase; EC 3.2.1.21) hydrolyze -glycosidic linkage(s) in alkyl and aryl -D-glucosides, glycoproteins, and glycolipids and that between two glucose residues in -linked oligosaccharides
Since BGAF bound to lactosyl-agarose lectin affinity column and the jacalin-related lectin (JRL) domain competed with intact BGAF for binding sites on -glucosidase, free BGAF and BGAF-glucosidase isozyme 1 (Glu1) complex (PBS extract of H95 tissues contains a considerable amount of soluble BGAF-Glu1 complex) were adsorbed onto a
The null phenotype for -glucosidase in maize is due to aggregation of the enzyme by BGAF, which interacts with maize -glucosidase isozymes Glu1 and Glu2 and forms insoluble complexes [12]
Summary
Materials—The maize null line H95 was grown in a growth chamber at temperatures of 30 °C (night) and 24 °C (day) in darkness. Fractions containing free BGAF and complex were identified by -glucosidase activity (both elute together at 100 mM lactose concentration). To separate free BGAF from the minor BGAF-Glu complex contaminant, the above fractions were pooled, dialyzed, and loaded onto a Sephacryl HR 200 gel filtration column (80 ϫ 1.6 cm) that had been equilibrated with PBS. Hemagglutination activity of native free BGAF, rBGAF, and BGAF-Glu complex was determined by a 2-fold dilution procedure using trypsin-treated rabbit erythrocytes. Gel filtration was carried out on a BioSep-SEC-S 2000 HPLC column (7.8 ϫ 300 mm) equilibrated and developed with PBS at a flow rate of 0.5 ml/min operating at room temperature on a Waters Breeze HPLC system (Waters). Oligonucleotide primers used in PCR to prepare various BGAF cDNA constructs used in this study
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