A specific beta-glucosidase-aggregating factor is responsible for the beta-glucosidase null phenotype in maize

Area of Science:

• Plant Biochemistry
• Molecular Genetics
• Enzymology

Background:

• Maize (Zea mays L.) beta-glucosidase is crucial for plant development.
• Previous studies identified "null" maize genotypes with significantly reduced enzyme activity.
• The genetic basis for this apparent null phenotype was previously attributed to the beta-glucosidase gene.

Purpose of the Study:

• To investigate the molecular basis of the apparent null phenotype in maize beta-glucosidase.
• To identify the factor responsible for the reduced enzyme activity in the H95 maize genotype.
• To elucidate the mechanism of beta-glucosidase aggregation and insolubility.

Main Methods:

• Enzyme activity assays on wild-type (K55) and null (H95) maize genotypes.
• Native polyacrylamide gel electrophoresis (PAGE) and zymography.
• Extraction using buffers with and without sodium dodecyl sulfate (SDS).
• Characterization of the beta-glucosidase-aggregating factor (BGAF).

Main Results:

• Null genotype extracts exhibited ~10% of wild-type beta-glucosidase activity.
• Zymograms of null genotype showed no activity bands in the resolving gel but a smeared zone in the stacking gel.
• SDS treatment enabled the smeared activity to enter the resolving gel, indicating aggregation.
• A 35-kD protein, BGAF, was identified as responsible for specifically binding and precipitating beta-glucosidase, causing insolubility.

Conclusions:

• The apparent null phenotype in maize beta-glucosidase is not due to a lack of enzyme but its insolubility.
• A specific beta-glucosidase-aggregating factor (BGAF) mediates the formation of large, insoluble quaternary complexes.
• Monogenic inheritance previously reported for null alleles actually refers to the BGAF protein, which is solely responsible for the observed phenotype.