Transient and permanent fusion of vesicles in Zea mays coleoptile protoplasts measured in the cell-attached configuration.

Exocytosis in protoplasts from Zea mays L. coleoptiles was studied using patch-clamp techniques. Fusion of individual vesicles with the plasma membrane was monitored as a step increase of the membrane capacitance (Cm). Vesicle fusion was observed as (i) An irreversible step increase in Cm. (ii) Occasionally, irreversible Cm steps were preceded by transient changes in Cm, suggesting that the electrical connection between the vesicle with the plasma membrane opens and closes reversibly before full connection is achieved. (iii) Most frequently, however, stepwise transient changes in Cm did not lead to an irreversible Cm step. Within one patch of membrane capacitance steps due to transient and irreversible fusions were of similar amplitude. This suggests that the exocytosis events do not result from the fusion of vesicles with different sizes but are due to kinetically different states in a fusion process of the same vesicle type. The dwell time histogram of the transient fusion events peaked at about 100 msec. Fusion can be described with a circular three-state model for the fusion process of two fused states and one nonfused state. It predicts that energy input is required to drive the system into a prevailing direction.