Annexin‐mediated secretory vesicle aggregation in plants

Abstract

The mechanism by which membranes fuse during vesicle‐mediated secretion is of considerable importance for plant cell growth, but remains unknown. We have identified Ca 2+ ‐dependent phospholipid‐binding proteins (annexins) from maize ( Zea mays ), that may play a part in this process. An assay for Ca 2+ ‐dependent binding of annexins to liposomes, revealed that the maize proteins (p23, p33 and p35) and annexins from bovine lung, bind over a similar range of Ca 2+ concentrations. Turbidity assays further revealed that both maize and bovine annexins induced liposome aggregation and that the plant annexins were also effective at aggregating plant secretory vesicles. This aggregation occurred at levels of free Ca 2+ similar to that required for the binding of annexins p33 and p35. We discuss the significance of these results for the plant secretory apparatus.