Implications of the galactosidase activity of yieldin in the regulatory mechanism of yield threshold that is fundamental to cell wall extension

To understand the action mechanism of yieldin (YLD) on the regulation of the yield threshold (Y), one of the critical parameters of cell wall extension, YLD was extracted from the cell walls of cowpea (Vigna unguiculata L.) hypocotyls and the hemagglutinin activity (HA) as well as the glycosidase activity of the protein was measured. Sedimentation assays using trypsinated rabbit erythrocytes showed that YLD possessed HA at pH 7. The digestion assays using 4-nitrophenyl (pNP) glycopyranosides as artificial substrates showed that YLD liberated galactose residues from pNP alpha-d-galactopyranoside mainly at pH 4.0, i.e. the pH level where Y was decreased at most. These results show that YLD is a bifunctional protein that switches between the HA and the galactosidase activities depending on the surrounding pH. Since D-galactose at concentration of 0.03 g l^-1 perfectly inhibited the HA, YLD was suggested to associate with galactose residues. However, the galactose application ten times concentrated was necessary to inhibit both the galactosidase activity of YLD and the acid-induced shift of Y regulated by YLD. In addition, the specific inhibitor of alpha-d-galactosidase (deoxygalactonojirimycin) inhibited both the galactosidase activity of YLD and the shift of Y at the same concentration, but not the HA. On the basis of these results, it is suggested the galactosidase activity of YLD plays a central role in the mechanism of Y-regulation at acidic pH.