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

Maize (Zea mays L.) beta-glucosidase was extracted from shoots of a wild-type (K55) and a "null" (H95) maize genotype. Enzyme activity assays and electrophoretic data showed that extracts from the null genotype had about 10% of the activity present in the normal genotype. Zymograms of the null genotype were devoid of any activity bands in the resolving gel, but had a smeared zone of activity in the stacking gel after native polyacrylamide gel electrophoresis. When extracts were made with buffers containing 0.5% to 2% sodium dodecyl sulfate, the smeared activity zone entered the resolving gel as a distinct band. These data indicated that the null genotypes have beta-glucosidase activity, but the enzyme occurs as insoluble or poorly soluble large quaternary complexes mediated by a beta-glucosidase-aggregating factor (BGAF). BGAF is a 35-kD protein and binds specifically to beta-glucosidase and renders it insoluble during extraction. BGAF also precipitates beta-glucosidase that is added exogenously to supernatant fluids of the null tissue extracts. The specific beta-glucosidase-aggregating activity of BCAF is unequivocally demonstrated. These data clearly show that the monogenic inheritance reported for the null alleles at the beta-glucosidase gene is actually for the BGAF protein, and BCAF is solely responsible for beta-glucosidase aggregation and insolubility and, thus, the apparent null phenotype.