Changes in Two Forms of Membrane-Associated Cellulase during Ethylene-Induced Abscission

Only one form of membrane-associated cellulase was found previously in the lower petiolar pulvinus of Phaseolus vulgaris (cv Red Kidney). The cellulase has an isoelectric point (pI) of 4.5 (DE Koehler, LN Lewis 1979 Plant Physiol 63: 677-679). This enzyme was detected in abscission zones collected before the onset of abscission (control tissue), and was thought to represent a pre-secretory form of another cellulase, the abscission cellulase, which has a basic pI and is secreted during abscission. We now show that this acidic, membrane-associated cellulase is a glycoprotein, tightly bound to the membrane, with maximum activity at pH 5.1, and that it is not immunologically related to the abscission cellulase. Furthermore, when bean explants are induced to abscise with ethylene, the activity of the acidic cellulase declines rapidly to 50% of control levels in the first day. When abscission is fully developed, the membranes contain a basic form of cellulase with a pI of 8.0 to 9.0 and only trace levels of the acidic cellulase. The basic form is not a high mannose glycoprotein; it has maximum activity in a broad pH range (4.0-8.0) and is antigenically related to the abscission cellulase, which is induced during abscission and transported to the cell wall. Antibody raised against the abscission cellulase recognized two proteins in a crude membrane fraction from abscising tissue. One of those proteins comigrated with the abscission cellulase, and the other was 1 to 2 kilodaltons larger. Thus, during abscission, the acidic membrane-associated cellulase rapidly declines before the appearance of the abscission cellulase. We conclude that there is no conversion from the acidic cellulase to the basic cellulase and suggest that the acidic and basic cellulase isoenzymes are proteins derived from two different genes.