Inactivation of solubilised fusicoccin-binding sites by endogenous plant hydrolases

The poor stability of crude solutions of fusicoccin-binding sites, prepared from acetonedried microsomal fractions of spinach leaves, results from the attack by endogenous phosphatase and α-mannosidase. The addition of either of these enzymes to solubilised binding sites preincubated with [(3)H]fusicoccin promptly releases most of the bound radioactivity. A satisfactory stabilization of the crude preparations is obtained with fluoride added either during homogenization of the tissue, or immediately after solubilisation. The results indicate that the fusicoccin-binding sites are phosphorylated glycoproteins.