Rhythmic kinetics of single fusion and fission in a plant cell protoplast.

Plant cells exhibit a lively activity of exocytosis and endocytosis at the plasma membrane. This is required for cell surface area regulation and also for membrane protein recycling. Although these processes are important for growth and development, they are understood purely with respect to their molecular mechanisms, regulation, spatial organization, and kinetics. According to the classical view, secretory vesicles release all their cargo in one step upon fusing and incorporating into the plasma membrane. Recent studies have shown that this model is oversimplifying the reality. Here we use real-time patch-clamp recording to monitor single-vesicle fusion and fission in order to resolve the kinetic properties of these elementary processes. The data show that single vesicles can, in a rhythmic fashion, make and break contact with the plasma membrane of plant protoplasts. Such oscillations are only possible if the two processes are linked by a distinct feedback system. The fact that similar rhythmic fusion/fission activity is also present in constitutive exocytosis in animal cells implies that the underlying mechanisms appear universal in eukaryotic cells.